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Content A STUDY OP M ODERN DESIGN STANDARDS IN STREET DESIGN A T h esis P re se n te d to th e F a c u lty of th e School o f E n g in ee rin g The U n iv e rs ity o f S o uthern C a lif o r n ia In P a r t i a l F u lf illm e n t o f th e R equirem ents f o r the Degree M aster of S cien ce In C i v il E n g in e e rin g by Edward Loyd Gorman August 1957 UMI Number: EP41976 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. U M I* Dissertation Publishing UMI EP41976 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 4 8 1 0 6 -1 3 4 6 & '5% &67I This thesis, w ritten by Gorman............................. under the guidance of&&3.JFacuity C om m ittee ^ ) 0 0 and a p p ro v e d by all its m em bers, has been f$&l presented to and accepted by the Sch ool of 5 1 - E ngineering in partial fulfillm ent of the re quirem ents fo r the degree of .....................M astex„or.Sci.enc© ................... ......................I n . . j C i v i l . ^ j g I n © e r i ^ ....................... D ate A u g & s t. 1 < ? 5 . 7 ................. Faculty Committee s K Chairman ................................... TABLE OP CONTENTS CHAPTER PAGE I . THE PROBLEM.................................................................... 1 I I . DESIGN CONTROLS AND CRITERIA .............................. 4 Topography and P h y s ic a l F e a tu re s . . . . 4 T r a f f ic Volume ....................... 5 V eh icle C h a r a c t e r i s t i c s and D esign V eh icle ......................................................... 9 I I I . ELEMENTS OP DESIGN ......................................................12 S topping S ig h t D ista n c e .................................... 12 H o riz o n ta l A lignm ent ........................................... 22 V e r tic a l A lignm ent ..................................... . 38 IV. DESIGN STANDARDS ...........................................................48 W idths o f Improvement and R ig h ts o f Way 48 Grade of S t r e e t s ..............................51 A lignm ent ...............................................................51 S u p e re le v a tio n ............................................52 I n t e r s e c t i o n D esign ........................................... 54 Cross S e c t i o n ...............................................................59 LIST OP TABLES TABLE PAGE I . Minimum S to p p in g S ig h t D ista n c e ...................... 17 I I . M&ximum Degree o f Curve and Minimum R adius D eterm ined For L im itin g Values o f e and f ......................................................... 25 LIST OP FIGURES FIGURE PAGE 1. R e la tio n Between P eak-hour and Average D a ily T r a f f ic V o l u m e s ................................ . . 6 2 . D esign V eh ic les and Minimum Turning P aths 11 3 . S kid R e s is ta n c e o f Pavements Assumed f o r D esign . ......................................................... 16 4. S to pping S ig h t D ista n c e on H o riz o n ta l Curves ........................................ ............................ 21 5. Maximum S afe S ide F r i c t i o n F a c to rs . . . 26 6. R e la tio n of S u p e re le v a tio n (e) Plus S ide F r i c t i o n F a c to r ( f ) to Degree o f Curve f o r D esign Speeds . ....................... 27 7 . Methods of D i s t r i b u t i n g S u p e re le v a tio n . . 29 8 . D esign S u p e re le v a tio n R ates .............................. 32 9. D esign C o n tro ls f o r C re s t V e r tic a l Curves 41 10. D esign C p n tro ls f o r Sag V e r tic a l Curves . 45 11. C u ttin g Curb R etu rn s Out o f a P lane . . . 56 CHAPTER I THE PROBLEM S t r e e t d e sig n i s one of th e o ld e s t b ran ch es o f th e C iv il E n g in e e rin g p r o f e s s io n and in s p i t e o f t h i s f a c t r e l a t i v e l y l i t t l e h as been w r i tte n to g u id e th e young e n g in e e r in th e p r o f e s s io n , o r to a tte m p t to s ta n d a r d iz e th e p r a c t i c e o f th e more ex p e rien ce d e n g in e e rs in the f i e l d . I t is w ith t h i s problem in mind t h a t the fo llo w in g t h e s i s was w r i t t e n . As th e r e have been many volumes w r i t t e n on th e d e sig n o f highways and because o f th e c lo s e r e l a t i o n s h i p | i t was determ ined t h a t a stu d y o f t h i s w e a lth of inform a- ; ti o n would be a l o g i c a l bas&s f o r th e e s ta b lis h m e n t o f any s ta n d a rd s in s t r e e t d e s ig n . A r e c e n t p u b lic a t io n by "The American A s s o c ia tio n o f S ta te Highway O f f ic ia ls " t i t l e d "A P o lic y on G eom etric D esign o f R u ra l Highways” j ! was s e le c te d as a r e f e r e n c e f o r th e s tu d y . T his o u t- I j sta n d in g volume has g a th e re d to g e th e r an amazing amount j i ^ J o f in fo rm a tio n based on e x p e rim en ts, f i e l d t e s t s and ob- j i s e r v a tlo n s , m a th em atical d e r iv a tio n s and s ta n d a rd p r a c t ic e th ro u g h o u t th e n a t io n . I t i s in th e i n t e n t o f t h i s th e s i s i to p r e s e n t a stu d y o f th e b a s ic d e sig n p r i n c i p l e s which I " ' i a re r e a d i l y a d a p tib le to s t r e e t d e sig n as w e ll as h ig h - | way d esig n and th e n to e s t a b l i s h a s e t o f sta n d a rd s which o f n e c e s s ity would be c o n s id e ra b ly low er th an th e s ta n d a rd s used in highway d e s ig n . In co n d u ctin g t h i s stu d y i t was d eterm in ed t h a t an a tte m p t sh o u ld be made to see w hat th e m ajor c i t i e s th ro u g h o u t th e U nited S t a t e s were u sin g f o r s ta n d a r d s . T h erefo re l e t t e r s were w r i t t e n to tw enty-one o f th e l a r g e r c i t i e s in th e U n ited S ta te s p re s e n tin g them w ith th e problem o f th e stu d y and re q u e s tin g they send any in fo rm a tio n o r sta n d a rd s in use in t h e i r r e s p e c tiv e c i t i e s . Nine r e p l i e s were re c e iv e d b u t th e re were only two t h a t had any in fo rm a tio n t h a t could be c o n s tru e d as an a tte m p t to s ta n d a r d iz e th e d e sig n p ro c e d u re . These two c i t i e s had p u b lis h e d i n s t r u c t i o n s on th e s u b d iv is io n j o f la n d i n th e are a and in doing so had s e t some minimum J f sta n d a rd s on w id th o f improvements and d e d ic a tio n o f j i r i g h t s of way. A lso th e maximum and minimum g rad es were ! i lim ite d . The rem aining r e p l i e s were r e s t r i c t e d to an- j j sw ering a s e r i e s o f q u e s tio n s asked in th e l e t t e r based on th e assum ption t h a t p u b lis h e d m a te r ia ls would n o t in many cases be a v a i l a b l e . These answers in d ic a te th e re i s a need f o r sta n d a rd s in s t r e e t d e s ig n . In some la r g e c i t i e s , where i t is n e c e s s a ry to d iv id e th e c i t y i n to d i s t r i c t s , th e s ta n d a rd s vary c o n s id e ra b ly in th e d i f - j f e r e n t d i s t r i c t s . T his i s very u n d e s ira b le from th e 1 s ta n d p o in t o f th e p r i v a t e e n g in e e r who i s a tte m p tin g to j 3 comply w ith th e v a rio u s d e sig n sta n d a rd s in d i f f e r e n t d i s t r i c t s . In view o f th e above survey i t ap p e ars t h a t each c i t y may have in d iv id u a l problem s or la c k of problem s th a t may or may n o t r e q u ir e s ta n d a r d iz a tio n . However, th e re d e f i n i t e l y sh o u ld be s ta n d a rd s f o r th e area and th e y should n o t be l e f t to th e d i s c r e t i o n o f th e i n d i v id u a l d i s t r i c t e n g in e e rs . T h e re fo re , in th e m a te r ia ls to fo llo w an a tte m p t w i l l be made t o j u s t i f y th e b a s ic d e sig n c o n tro ls and to e s t a b l i s h a s e t o f sta n d a rd s t h a t would be a p p lic a b le to i th e Los A ngeles area s p e c i f i c a l l y b u t would apply in ! g e n e ra l to any c i t y . CHAPTER I I DESIGN CONTROLS AND CRITERIA Topography and P h y s ic a l F e a t u r e s . The lo c a tio n o f a s t r e e t and I t s d esig n elem ents a re determ ined by th e topography, p h y s ic a l f e a t u r e s , and la n d use o f th e area tr a v e r s e d . Topography i s a m ajor f a c t o r in th e d e te rm in a tio n o f th e p h y s ic a l l o c a ti o n o f a s t r e e t and g e n e r a lly a f f e c t th e alig n m e n t, g r a d ie n t, s ig h t d is ta n c e , c ro ss s e c tio n and o th e r d e sig n e le m e n ts. S in ce topography and la n d use have such a prounced e f f e c t on s t r e e t d e sig n , in fo rm a tio n re g a rd in g th e s e f e a t u r e s sh o u ld be o b ta in e d e a r l y i n th e s ta g e s of p la n n in g and d e s ig n . A e r ia l su rv ey s g e n e r a lly expe d i t e th e c o l l e c t i o n of th e s e d a t a . Topographic maps j i o f s u i t a b l e s c a le in th e range of one in c h eq u a ls f o r ty j i f e e t to one in c h eq u a ls one hundred f e e t , showing a l l j | p e r tin e n t c u l tu r e , form th e n e c e s s a ry b ase f o r lo c a tio n j I and d e s ig n . In th e p r e p a r a tio n of f i n a l p la n s a s c a le j i | I i I o f one in c h eq u a ls tw enty f e e t o r one in c h eq u a ls f o r t y ! S ! j f e e t i s g e n e r a lly u se d . The to p o g rap h ic maps should be | supplem ented by f u r t h e r d a ta r e g a rd in g su b su rfa c e and | d rain a g e c o n d itio n s , th e s i z e , type and valu e o f b u ild - I in g s , and o th e r developm ents t h a t may a f f e c t or be a f f e c t e d by th e s t r e e t . 5 In fo rm a tio n r e g a rd in g topography and p h y s ic a l f e a t u r e s to g e th e r w ith t r a f f i c d ata form th e m ajor d e sig n c o n t r o ls . A ll o th e r d e s ig n c r i t e r i a depend la r g e ly on th e s e c o n t r o ls . T r a f f ic Volume. The g e n e ra l u n i t o f m easure used In r e p o r tin g th e t r a f f i c on a roadway i s th e an n u al average d a ily t r a f f i c volume, a b b re v ia te d as ADT. I t r e p r e s e n ts th e t o t a l t r a f f i c f o r th e y ea r d iv id e d by 365, or th e average volume p e r day. i T r a f f ic volume d u rin g an i n t e r v a l o f tim e s h o r t e r | j th a n a day more a p p r o p r ia te ly r e f l e c t s th e o p e ra tin g I c o n d itio n s which sh o u ld be used f o r d e s ig n i f t r a f f i c i s to be se rv e d p r o p e r ly . In n e a r ly a l l e a se s a p e r io d o f ! I one hour i s a d e q u a te . The t r a f f i c p a t t e r n on any s t r e e t shows con- j ; s id e r a b le v a r i a t i o n i n t r a f f i c volumes d u rin g th e d i f - ! | f e r e n t hours of th e day, and even a g r e a t e r f l u c t u a t i o n ! | in h o u rly volumes th ro u g h o u t th e y e a r. The h o u rly t r a f f i c used in d e sig n should n o t be exceeded too o f te n or to o i much. On th e o th e r hand, i t should n o t be so h ig h th a t i t t r a f f i c would r a r e l y be g r e a t enough to make f u l l use of i th e r e s u l t i n g f a c i l i t y . j F ig u re I , page 6 shows the r e l a t i o n betw een th e j h ig h e s t h o u rly volumes and ADT on main r u r a l highways ta k en from an a n a ly s is of t r a f f i c count d a ta f o r one HOURLY TRAFFIC A S A PERCENTAGE O F ADT: TW O W A Y VOLUMES 6 3 6 3 2 28 2 4 20 ---------------------------- 0j4 ~ ----- ■«! 20 6 0 8 0 1 0 0 140 160 NUMBER OF HOURS IN ONE YEAR WITH HOURLY VOLUME GREATER THAN THAT SHOWN FIGURE I RELATION BETWEEN PEAK-HOUR AND AVERAGE DAILY TRAFFIC VOLUMES MAIN RURAL HIGHWAYS i 7 hundred and s ix ty - s e v e n lo c a tio n s in f o r t y - e i g h t s t a t e s , co v erin g a wide ran g e of volumes and b e in g g e o g ra p h ic a lly r e p r e s e n t a t i v e . The m iddle curve i s th e av erag e f o r a l l lo c a tio n s s tu d ie d and r e p r e s e n ts a highway w ith average f l u c t u a t i o n in t r a f f i c flo w . E xam ination o f th e s e curves le a d s to th e c o n c lu s io n t h a t th e h o u rly t r a f f i c used in d e sig n sh o u ld be th e 3 0 th h ig h e s t h o u rly volume o f th e y e a r, a b b re v ia te d as 30 H.V. On r u r a l ro a d s w ith average f l u c t u a t i o n in t r a f f i c flow 30H.V. i s ab o u t f i f t e e n p e r c e n t o f th e ADT. A f o r tu n a te c h a r a c t e r i s t i c o f 30H.V. i s t h a t , as a p e rc e n ta g e o f ADT i t u s u a lly rem ains su b s t a n t i a l l y unchanged from y ear to y e a r, in s p i t e of s i g n i f i c a n t changes in ADT. Thus th e p e rc e n ta g e o f ADT f o r 30H.V. from c u r r e n t t r a f f i c d ata on a g iv e n f a c i l i t y g e n e r a lly can be used w ith co n fid en ce in com puting th e 3QH.V. from an ADT volume d eterm ined f o r some f u tu r e y e a r. The r e l a t i o n of 30H.V. to ADT may be b ased on e x p e rie n c e w ith o th e r s t r e e t s s e rv in g a re a s w ith s im i l a r la n d -u s e c h a r a c t e r i s t i c s . The 30H.V. c r i t e r i o n a p p lie s in g e n e ra l t o u rban a r e a s , b u t where th e f l u c t u a t i o n i s t r a f f i c flow may be r a d i c a l l y d i f f e r e n t from t h a t on r u r a l h ig h ways, o th e r r e l a t i o n s may have t o be c o n s id e re d . The d i r e c t i o n a l d i s t r i b u t i o n of t r a f f i c d u rin g th e d e sig n h o u r sh o u ld be d e te rm in e d ,by f i e l d m easure ments on th e f a c i l i t y u nder c o n s id e r a tio n , o r on p a r a l l e l 8 and s im ila r f a c i l i t i e s . The d e s ig n h o u rly volume f o r one d i r e c t i o n may be computed by m u ltip ly in g th e two way ADT by th e p e rc e n ta g e t h a t 30H.V i s o f th e two way ADT, and be th e p e rc e n ta g e o f t r a f f i c in th e predom inant d i r e c t i o n d u rin g th e d e sig n h o u r. V e h ic le s o f d i f f e r e n t s iz e and w e ig h ts have d i f - i f e r e n t o p e r a tin g c h a r a c t e r i s t i c s , w hich must be c o n s id e re d in s t r e e t d e s ig n . B esides b e in g h e a v ie r , tru c k s g e n e ra l l y a re slow er and occupy more roadway sp a ce, and co n se- q u e n tly impose a g r e a t e r t r a f f i c lo a d on th e s t r e e t th a n do p a s se n g e r v e h i c le s . For d e sig n o f a p a r t i c u l a r s t r e e t , d a ta on t r a f f i c co m p o sitio n sh o u ld be o b ta in e d by f i e l d s tu d i e s . At im p o rta n t i n t e r s e c t i o n s , p e rc e n ta g e s o f I i tru c k s d u rin g b o th th e A.M. and P.M. peaks sh o u ld be de te rm in e d . V a r ia tio n s i n tr u c k t r a f f i c betw een th e d i f - i i f e r e n t movements a t i n t e r s e c t i o n s may be s u b s t a n t i a l arid i ; a p p r e c ia b ly a f f e c t th e geom etric la y o u t. I : The t r a f f i c p r o je c tio n f a c t o r i s a r a t i o o f \ f u tu r e t r a f f i c to c u r r e n t t r a f f i c . T h is t r a f f i c in c re a s e ! I : j i s due to norm al t r a f f i c grow th, g e n e ra te d t r a f f i c w hich ! j ! | c o n s i s t s o f m otor v e h ic le t r i p s ( o th e r th a n by p u b lic I | t r a n s i t ) t h a t w o u ld ,n o t have been made i f th e new f a c i l i t y i | j had n o t been added, and developm ent t r a f f i c which i s t h a t i ; i | due to improvem ents o f ad jacen t; la n d o v er and above th e ! developm ent which would have ta k e n p la c e had n o t th e new improvement been c o n s tru c te d . P r o je c tio n f a c t o r s t h a t would apply to a m a jo rity o f m ajor s t r e e t s should be b ased on p a s t re c o rd s a v a i l a b l e . A p e rio d o f tw enty y e a rs i s n orm ally used as a b a s is f o r d e s ig n . S p e c if ic v alu es cannot be c i t e d f o r t r a f f i c p r o j e c t i o n f a c t o r s f o r use in d e sig n where no a n a ly s is is made of f a c t o r s of 1 t r a f f i c grow th. O bviously such f a c t o r s w i l l vary w idely j i between d i f f e r e n t f a c i l i t i e s . P r o je c tio n f a c t o r s t h a t j j would apply to a m a jo rity o f r u r a l highway improvements i today f o r a tw enty y e a r p e rio d are p ro b a b ly in th e range o f one and a h a l f to two and a h a l f . T r a f f ic f o r e c a s ts in y ea rs to come are a p t to show d i f f e r e n t v a lu e s . Much ! I r e s e a r c h rem ains to be done to determ ine s p e c i f i c g uide J v alu es f o r f u tu r e t r a f f i c . But even w ith r e s e a r c h d ata a v a ila b le a th o rough knowledge o f l o c a l c o n d itio n s and 5 sound judgement i s e s s e n t i a l f o r p r o je c tin g t r a f f i c to j i f u tu r e y e a r s . V eh icle C h a r a c te r i s tic s and D esign V e h ic le . The p h y s ic a l c h a r a c t e r i s t i c s o f V eh icles and th e p ro p o rtio n s j of v a rio u s s iz e v e h ic le s u sin g th e s t r e e t s a re p o s it iv e c o n tro ls in g eo m etric d e s ig n . A d e sig n v e h ic le i s a s e le c te d m otor v e h ic le whose w eig h t, dim en sio n s, and j i ! o p e ra tin g c h a r a c t e r i s t i c s a re used t o e s t a b l i s h d e sig n c o n tro ls to accommodate v e h ic le s o f a d e s ig n a te d ty p e . 10 For purpose o f d e s ig n , th e d e sig n v e h ic le sh o u ld be one w ith dim ensions and minimum tu r n in g r a d iu s l a r g e r th an alm ost a l l v e h ic le s in i t s c l a s s . S a tis f y in g th e s e r e quirem ents a re th e v a rio u s d e s ig n v e h ic le s w ith th e dim ensions and tu r n in g c h a r a c t e r i s t i c s shown in F ig u re 2, page 11, 0ESI6N VEHICLES A N D MINIM U M TURNING PATHS PASSENGER VEHICLE 43* SEMITRAILER COMBINATION / s t N N l\\ * \ S , I I ' H AO • l I , I I __L i i m n> 50' SEMITRAI LER COMBINATION SINGLE UNIT TRUCK OR BUS / 4 - IT CHAPTER I I I ELEMENTS OP DESIGN S to p p in g S ig h t D is ta n c e . A b i l i t y to se e ahead i s of th e utm ost im portance in th e s a f e and e f f i c i e n t o p e r a tio n o f a s t r e e t . The p a th and speed of m otor v e h ic le s a r e s u b je c t to th e c o n tr o l o f d r iv e r s whose t r a i n i n g l a r g e l y i s e le m e n ta ry . I f s a f e t y Is t o be b u i l t i n t o s t r e e t s th e d e s ig n e r m ust p ro v id e s i g h t d is ta n c e of s u f f i c i e n t le n g th i n w hich d r iv e r s can c o n tr o l th e speed o f t h e i r v e h ic le s so as to av o id s t r i k i n g an u n ex p ected o b s ta c le on th e t r a v e le d way. Minimum sto p p in g s i g h t d is ta n c e i s th e sum o f two d is t a n c e s : one, th e d is ta n c e j i tr a v e r s e d by a v e h ic le from th e i n s t a n t th e d r i v e r s ig h t s ; an o b je c t, f o r w hich a s to p i s n e c e s s a ry , t o th e i n s t a n t j th e b rak es a re a p p lie d ; and th e o th e r , th e d is ta n c e j r e q u ir e d to s to p th e v e h ic le a f t e r th e b rak e a p p l ic a tio n b e g in s . ! i I Many t e s t a have been made to d eterm in e th e b ra k e ! r e a c t i o n tim e o f m otor v e h ic le o p e r a to r s ; t h a t i s , th e | tim e r e q u ir e d to a p p ly th e b r a k e s . The av e ra g e b rak e i r e a c t i o n tim e i s ab o u t one h a l f seco n d . F o r s a f e t y a r e a c t i o n tim e t h a t i s s u f f i c i e n t f o r m ost o p e r a to r s , l r a t h e r th a n the av e ra g e o p e r a to r , sh o u ld be used in any i d e te rm in a tio n o f minimum s i g h t d is t a n c e . A b rak e r e a c tio n ! 1 3 tim e o f a f u l l second i s commonly used in d e s ig n . P e rc e p tio n tim e i s th e tim e r e q u ir e d f o r m otor v e h ic le o p e ra to rs to come to th e r e a l i z a t i o n t h a t th e b ra k e s m ust be a p p lie d . I t i s th e tim e la p s e from th e i n s t a n t an o b je c t i s v i s i b l e to th e d r iv e r to th e i n s t a n t he r e a l i z e s t h a t th e o b je c t i s i n h i s p a th and t h a t a sto p m ust be made. In d e te rm in a tio n o f s ig h t d is ta n c e f o r d e s ig n , th e p e r c e p tio n tim e v a lu e should be l a r g e r th a n th e averag e f o r a l l d r i v e r s under norm al c o n d itio n s . I t sh o u ld be la r g e enough to in c lu d e th e tim e ta k e n by i j n e a r ly a l l d r iv e r s under most d r iv in g c o n d itio n s . For such use in d e s ig n i t i s assumed t h a t th e p e r c e p tio n tim e i s l|r seco n d s, and th e t o t a l p e rc e p tio n and b rak e r e a c t i o n tim e i s 2^- seconds. A v a ila b le r e f e r e n c e s do n o t j u s t i f y ! ! ! j d i s t i n c t i o n o v e r th e ran g e in d e s ig n speed. The d is ta n c e I tr a v e r s e d d u rin g p e rc e p tio n and b rak e r e a c tio n tim e a re J shown in T able I page 17. The approxim ate b ra k in g d is ta n c e o f a v e h ic le on l e v e l roadway may be d eterm in ed by th e use o f th e ; s ta n d a rd fo rm u la: D = v £ 3 0f d = b ra k in g d is ta n c e i n f e e t . V * I n i t i a l speed in M.P.H. f - c o e f f i c i e n t o f f r i c t i o n between t i r e s and roadway. 14 In t h i s form ula f o r b ra k in g d is ta n c e th e " f ” f a c t o r used i s an o v e r a l l o r a s in g le v a lu e t h a t i s r e p r e s e n ta ti v e f o r th e whole of th e speed change. Meas urem ents show t h a t " f ” i s n o t th e same f o r a l l sp eed s. I t d e c re a se s a s th e i n i t i a l speed in c r e a s e s . I t v a r ie s c o n s id e ra b ly due to many p h y s ic a l elem ents such a s a i r p r e s s u r e of t i r e s , ty p e o f t i r e s , amount o f t i r e tr e a d , type and c o n d itio n o f th e pavement s u r f a c e , th e p rese n ce o f m o is tu re , mud, snow, o r i c e , and e t c . F u r th e r , and o f g r e a t e r s ig n i f ic a n c e , i n sto p s from h ig h speeds th e a c t u a l b ra k in g d is ta n c e i s dependant upon th e v e h ic le b ra k in g system r a t h e r th a n th e sk id d in g o f th e t i r e on th e pavem ent. The f r i c t i o n f a c t o r does n o t e n t e r a | fo rm u la, based on b ra k in g t e s t s d a ta , where th e b ra k in g d is ta n c e i s e x p re sse d in term s o f th e speed o n ly a t th e ! tim e o f b rake a p p l i c a t i o n . The form ula f o r a curve t | r e l a t i o n from such t e s t d a ta , shows t h a t th e b ra k in g t | d is ta n c e v a r ie s w ith th e speed to a power g r e a t e r th a n i | th e sq u a re , ap p ro ac h in g a cube. I f , however, " f" i s | computed f o r each t e s t from th e s ta n d a rd fo rm u la, I p i d * V_ , i t w i l l r e p r e s e n t th e e q u iv a le n t c o n s ta n t I 3 Of j f r i c t i o n f a c t o r . i Because o f th e low er c o e f f i c i e n t s o f f r i c t i o n j ! on wet pavem ents a s compared to d ry , th e wet c o n d itio n j governs in d e te rm in in g sto p p in g d is ta n c e s f o r use in 15 d e s ig n . The c o e f f i c i e n t o f f r i c t i o n f o r d esig n c r i t e r i a should n o t o nly r e p r e s e n t w et pavem ents in good c o n d itio n h u t a ls o s u rfa c e s th ro u g h o u t t h e i r u s e f u l l i f e . The v a lu e sh o u ld encompass n e a rly a l l s i g n i f i c a n t pavement s u rfa c e s and th e l i k e l y f i e l d c o n d itio n s . They should he such a s to he sa fe f o r worn t i r e s , as w e ll as f o r new t i r e s , and f o r n e a r ly a l l ty p e s o f tr e a d s and t i r e co m p o sitio n . The f r i c t i o n f a c t o r sh o u ld s a f e ly encompasd th e d if f e r e n c e s in v e h ic le s and d r i v e r b ra k in g from d i f f e r e n t sp e ed s. On th e o th e r hand, th e v alu es need n o t j be so low a s to he s u i t a b l e f o r o b s o le s c e n t or b le e d in g j s u rfa c e s o r f o r pavem ents under ic y c o n d itio n s . P r e f e r a b ly , th e llf H v a lu e s f o r d e s ig n should be n e a r ly a l l { I j | in c lu s iv e , r a t h e r th a n a v e ra g e; a v a ila b le d a ta a re n o t > ( ■ I f u l l y d e t a i l e d o v er th e range f o r a l l th e s e v a r i a b l e s , | i and c o n c lu sio n s m ust be made i n term s o f th e s a f e s t | i • , i j r e p o r te d average v a lu e s . The low er curve in F ig u re 3 , j ; page 16, g iv e s th e nf n v alu es assumed f o r c a lc u l a ti o n s ! | o f d e sig n sto p p in g d is ta n c e s , re c o g n iz in g th e s e f a c t o r s . The speed f o r wet c o n d itio n s i s c o n s id e re d to be ap p ro x - | j i i | im a te ly e i g h ty - f iv e to n in e t y - f i v e p e r c e n t o f d esig n j j speed which i s i n d ic a t iv e o f th e top speed when pavem ents ! i | a re d ry . These a d ju s te d speeds on w et pavem ents a re n o t j ; | a g r e a t d e a l low er th an d e sig n speed and, w ith b ra k in g d is ta n c e s d eterm in ed f o r them th e g r e a t e r p ro p o rtio n o f I { i 1 I f =ASSUMED COEFFICIENT OF FRICTION FOR USE IN STANDARD STOPPING DISTANCE FORMULA C O * ;o j» m t o o > co e 2 PI O * n o o p i co © C O H > z o P I o n T J > < PI z PI z H C O n o c 3 9 P I C O i n ■ o m ro o o n < K l X m P o > k> o i o i 0 1 o * o O I o 0> o o H o > TABLE I MINIMUM STOPPING SIGHT DISTANCE DESIGN SPEED MPH ASSUMED SPEE.D FOR condition MPH PERCEPTION-REACTION COEFFICIENT OF FRICTION BRAKING DISTANCE ON LEVEL FEET STOPPING SIGHT DISTANCE TIME SEC DISTANCE FEET COMPUTED FEET ROU NDED FOR DESIGN FE ET Dl■SIGN CRITERIA WET PAVE MENTS 20 19 2.5 70 0 . 4 0 30 100 120 . 25 24 2.5 88 0 3 8 51 139 160 3 0 28 2.5 103 0 3 6 73 1 76 2 0 0 3 5 32 2.5 1 17 0 3 4 5 99 216 2 4 0 4 0 36 2.5 132 0 3 131 263 275 45 4 0 2 5 147 0 3 2 167 314 3 15 50 4 4 2 5 161 031 2 0 8 3 6 9 3 50 5 5 4 8 25 176 0 3 0 5 252 4 2 8 410 60 52 25 19 1 0 3 0 301 492 475 COf IPARATl VE VAt.UES DRY •A VEMENT S' 20 2 0 25 73 0 6 6 20 93 25 2 5 25 9 2 0 6 4 33 125 30 3 0 25 110 0 6 2 4 8 158 35 35 25 128 061 67 195 4 0 4 0 25 147 0 6 0 89 2 36 4 5 4 5 2 5 165 0 5 9 114 2 79 5 0 50 25 183 0 5 8 144 3 27 5 5 5 5 2 5 2 0 2 0 5 7 177 3 7 9 6 0 6 0 2 5 2 2 0 0 5 6 214 4 3 4 18 t r a f f i e which, i s t r a v e l l i n g a t y e t low er speeds w i l l en jo y a s u b s t a n t i a l a d d i t i o n a l s a f e ty f a c t o r . These speeds a re g iv e n in th e second column o f Table I , page 17, When a roadway i s on a g ra d e , th e s ta n d a rd form ula f o r b ra k in g d is ta n c e i s : d * V2 , ,t , in which G i s th e 30{f*G) p e r c e n t o f g rade d iv id e d by 100, and th e o th e r term s a re as p r e v io u s ly s t a t e d . On n e a rly a l l s t r e e t s th e i g rad e i s tr a v e r s e d by t r a f f i c in b o th d ir e c tio n s b u t th e s i g h t d is ta n c e g e n e r a lly i s d i f f e r e n t i n each d i r e c t i o n s , p a r t i c u l a r l y on s t r a i g h t ro a d s i n r o l l i n g t e r r a i n . As a g e n e r a l r u l e th e s i g h t d is ta n c e a v a ila b le on down g ra d e s i s l a r g e r th a n th o se on u p g rad es, more o r l e s s a u to m a tic a lly p ro v id in g th e n e c e s s a ry c o r r e c tio n s f o r g ra d e . T his may e x p la in why most d e sig n o f f ic e s u t i l i z e s to p p in g s i g h t d is ta n c e s as d eterm in ed f o r f l a t c o n d i tio n s w ith o u t re g a rd to grade c o r r e c t io n s . E x cep tio n s a re one way ro ad s on a d iv id e d roadway w ith In d e p e n d e n tly d esig n ed p r o f i l e s f o r th e two roadw ays, f o r w hich th e s e p a r a te grade c o r r e c tio n s a re in o rd e r and th e r e f i n e ments in d esig n i s in k eeping w ith th e o v e r a l l sta n d a rd s u se d . The minimum sto p p in g s i g h t d is ta n c e s , as d e r iv e d , d i r e c t l y r e f l e c t p a sse n g e r c a r o p e r a tio n and th e y m ight be q u e s tio n e d f o r use in d e sig n f o r tr u c k o p e r a tio n . 19 Trucks as a w hole, e s p e c i a l l y th e l a r g e r and h e a v ie r u n its , r e q u ir e a lo n g e r s to p p in g d is ta n c e from a g iv e n speed th an do p a sse n g e r v e h i c le s . However, two f a c t o r s te n d to b a la n c e th e a d d itio n a l b ra k in g le n g th f o r tru c k s f o r g iv e n d e sig n speeds w ith t h a t f o r p a s se n g e r c a r s . F i r s t , on a v e r t i c a l s i g h t o b s tr u c tio n th e tru c k o p e ra to r i s a b le to see s u b s t a n t i a l l y f a r t h e r due to h is h ig h e r p o s it io n in the v e h ic le . Second, in n e a rly a l l cases tru c k s t r a v e l slo w er th a n p a s se n g e r v e h ic le s , e i t h e r th ro u g h r e g u la tio n o r by o p e ra to rs c h o ic e . S e p a ra te s to p p in g s i g h t d is ta n c e s f o r tru c k s and p a s se n g e r c a r s , th e r e f o r e , are n o t used in i I d e s ig n s ta n d a r d s . There i s one s i t u a t i o n t h a t sh o u ld be t r e a t e d w ith c a u tio n and ev ery e f f o r t made to p ro v id e s to p p in g s i g h t ! d is ta n c e g r e a t e r th a n the minimum d e sig n v a lu e . This i s j i th e ca se of h o r iz o n ta l s i g h t r e s t r i c t i o n o c c u rrin g on j ! ; down-grades p a r t i c u l a r l y a t th e end o f long dow n-grades, j j ! The g r e a t e r h e ig h t of eye of th e tr u c k o p e ra to r i s of [ ! l i t t l e v a lu e , even when th e h o r iz o n ta l s i g h t o b s tr u c tio n i I i s a c u t s lo p e , and on long dow n-grades tru c k s speeds ; \ i j may te n d to approach th o se of c a r s . j S ig h t d is ta n c e along a roadway i s m easured from | j th e d r iv e r s eye to some o b je c t on th e tr a v e le d way when j i t f i r s t comes i n t o view . D if f e r e n t elem ents a re i n - ! ! volved when s i g h t d is ta n c e i s c o n t r o ll e d by v e r t i c a l l 20 alig n m en t and by h o r iz o n ta l a lig n m e n t. The eyes of the averag e d r iv e r in a p a sse n g e r v e h ic le a re c o n s id e re d to be 4 .5 f e e t above th e ro ad s u r f a c e . Minimum s to p p in g s i g h t d is ta n c e i s based on th e d is ta n c e r e q u ir e d t o sto p w ith s a f e ty from th e i n s t a n t a s t a t i o n a r y o b je c t in th e same la n e becomes v i s i b l e . On c r e s t v e r t i c a l curv es t h i s s i g h t d is ta n c e i s lim ite d by some p o in t on th e roadway s u r f a c e . On h o r iz o n ta l curves i t i s lim ite d by a l a t e r a l I o b s tr u c tio n beyond th e roadw ay, as a c u t s lo p e , clump o f j I t r e e s , b rid g e abutm ent, e t c . A h e ig h t o f o b je c t o f fo u r ! I J in ch es i s assumed f o r m easuring s ig h t d is ta n c e oh c r e s t j p r o f i l e s . | i j For g e n e ra l use i n d e sig n of h o r iz o n ta l curves | i | th e s i g h t l i n e is a chord of th e curve and th e minimum ! s to p p in g s i g h t d is ta n c e i s m easured along the c e n t e r l i n e j i s ; o f th e in s id e la n e around the c u rv e . F ig u re 4, page 21, j i s a d e sig n c h a r t showing th e r e q u ir e d m iddle o r d in a te j f o r c l e a r s i g h t a re a s a t v a rio u s r a d i i of curve t o s a t i s f y sto p p in g s i g h t d is ta n c e re q u ire m e n ts . T his c h a r t and th e form ulas shown th e re o n ap p ly o n ly to c i r c u l a r curv es ' lo n g e r th a n th e s i g h t d is ta n c e f o r the d e sig n sp e ed . For j \ : any d e s ig n speed th e r e l a t i o n betw een th e degree o f curve { ! j i and m iddle o r d in a te i s a s t r a i g h t l i n e . H o riz o n ta l s i g h t | i i • r e s t r i c t i o n s o f te n occur in t e r r a i n c o n d itio n s where th e re i i 1 j is a c u t slo p e on th e in s id e o f th e c u rv e . For sto p p in g J 21 vers m = r ( vers. 2 8.6 5 s) 5 730 200 r - m 28.65 50 I V = DESIGh S PEED IN o = MAX. D WHEN e = S= STOPPING SIGHT Dl MPH 0.06 STANCE 125 I 45 ui 4 0 150 U i u. ‘ { I I 35 L U z u_ ui 30 ui 200 i u o . U I o tr Ui 300 £ j Li. < / ) 5 o 400 < U J 500 i 600 i i 800 1000 s^ JL < t I 5 30 25 0 5 I 0 20 I m = MIDDLE ORDINATE.’ CENTERLINE INSIDE LANE TO SIGHT OBSTRUCTION - FEET FIGURE 4 STOPPING SIGHT DISTANCE ON HORIZONTAL CURVES 22 s i g h t d is ta n c e h e ig h t c r i t e r i a of 4 .5 f o o t h e ig h t o f eye and 4 in c h o b je c t, a h e ig h t of 2 .5 f e e t can be used to approxim ate t h a t a t th e mid p o in t o f th e s i g h t l i n e where th e c u t slo p e u s u a lly o b s tr u c t s s i g h t . ■H orizontal A lig n m en t. For b a lan ce in d e s ig n a l l g eo m etric sh o u ld , as f a r as eco n o m ically f e a s i b l e , be d eterm in ed to p ro v id e s a f e o p e r a tio n a t a speed l i k e l y u nder th e g e n e ra l c o n d itio n f o r th e roadway. When a v e h ic le moves i n a c i r c u l a r p a th i t i s fo rc e d r a d i a l l y outw ard by c e n t r i f u g a l fo rc e whieh is c o u n te r b a la n c e d by th e v e h ic le w eight component due to th e roadway s u p e re le v a tio n and by th e s id e f r i c t i o n betw een t i r e s and s u r f a c in g . The b a s ic form ula f o r v e h ic le o p e ra tio n on a curve i s : e + f - 0.067V2 vf__ H 15R where e = r a t e o f roadway s u p e re le v a tio n f o o t p e r f o o t f » s id e f r i c t i o n f a c t o r V s v e h ic le speed in M.P.H. R = ra d iu s of curve in f e e t . When a v e h ic le t r a v e l s a t a c o n s ta n t speed on a i I curve s u p e re le v a te d so t h a t th e " f" v a lu e i s z e ro , th e 23 c e n t r i f u g a l f o rc e i s b a la n c e d by th e w eig h t component o f the v e h ic le and no s te e r in g e f f o r t i s n eeded. In a re a s s u b je c t to snow and ic e th e r a t e of s u p e r e le v a tio n cannot be g r e a t e r than t h a t on w hich v e h ic le s s ta n d in g or t r a v e l l i n g slo w ly would s l i d e down th e c ro ss slo p e when th e pavement i s ic y . When t r a v e l l i n g slo w ly around a curve w ith a h ig h s u p e re le v a tio n r a t e th e v e h ic le is I h e ld in a p ro p e r p a th o nly when th e d r iv e r s t e e r s up th e j slo p e o r a g a in s t th e d i r e c t i o n o f th e h o r iz o n ta l cu rv a- j i t u r e . Such a h ig h r a t e o f s u p e r e le v a tio n i s u n d e s ira b le i on high-volum e ro a d s , as in urban or suburban a r e a s , i where th e re a re numerous tim es when th e v e h ic le speeds a re red u ced c o n s id e ra b ly because of th e h ig h t r a f f i c j volum e. I J W ith th e s e f a c t o r s , which p la c e l i m i t s on th e ! < * ; s u p e re le v a tio n r a t e , and w ith wide v a r i a t i o n in v e h ic le | : speeds a t any one p o in t on a roadw ay, th e r e u s u a lly i s an unbalanced f o r c e as a v e h ic le o p e ra te s around a c u rv e . f T his fo rc e r e s u l t s i n t i r e s id e t h r u s t w hich i s c o u n te r b alan ced by f r i c t i o n between t i r e s and s u r f a c e ; th e c o e f- I ! | f i c i e n t o f f r i c t i o n i s th e f r i c t i o n f o r c e d iv id e d by the I I component o f th e w eig h t p e rp e n d ic u la r t o th e pavem ent. I T his c o e f f i c i e n t i s commonly r e f e r r e d to as th e ’’s id e I i f r i c t i o n f a c t o r . ” The upper l i m i t o f t h i s f a c t o r is t h a t 1 a t whieh th e t i r e i s sk id d in g or a t th e p o i n t of impending' i ! 24 s k id . Sine© cu rv es a re d e sig n e d to a v o id s k id d in g con d i t i o n s w ith a m argin o f s a f e t y , th e H f M v a lu e s sh o u ld he b u t a f r a c t i o n o f th e c o e f f i c i e n t of f r i c t i o n a t im pending s k id . Based on numerous t e s t s and o b s e r v a tio n s , i t i s concluded t h a t th e s a f e s id e f r i c t i o n f a c t o r s f o r use in curve d e s ig n sh o u ld v ary d i r e c t l y w ith th e d e sig n speed from 0.16 a t 30 l .P .H . to 0.12 a t 70 M.P.H. F ig u re 5, page 25, shows th e r e l a t i o n betw een speed and s id e f r i c t i o n f a c t o r on cu rv es a t i n t e r s e c t i o n s . For a g iv e n speed th e maximum s u p e r e le v a tio n r a t e and assum ption f o r maximum s id e f r i c t i o n f a c t o r in | co m b in atio n d eterm in e th e r a d iu s r e q u ir e d . A maximum | i r a t e o f 0.06 f o o t p e r f o o t i s commonly used f o r s t r e e t ! ■ d e s ig n . The minimum s a fe r a d i u s , R, can be c a lc u l a te d d i r e c t l y from th e s ta n d a r d curve fo rm u la: 15(©+ f ) | U sing th e maximum v a lu e s f o r " e B and nf w as above i | p r e s e n te d , th e minimum r a d iu s f o r th e d e s ig n a te d d e s ig n j i | I speeds a re l i s t e d in T able I I , page 2 6 . The r e l a t i o n of i I th e s e v a lu e s In term s o f d eg ree o f curve and (e + f ) a re j i j shown i n F ig u re 6 , page 27. Minimum r a d iu s a re c a lc u l a t e d from M f ” v a lu e s based on o p en-road c o n d itio n s . I i ; { F or curve d e s ig n i t i s n e c e s s a ry to d eterm in e i i s u p e r e le v a tio n r a t e s as a p p lic a b le over th e ran g e o f j f i f T AB LE It MAXIMUM DEGREE OF CURVE AND MINIMUM RADIUS DETERMINED FOR LIMITING VALUES OF e AND f DESIGN MAXIMUM MAXIMUM TOTAL MINIMUM MAX. DEGREE SPEED e f (e + f) RADIUS OF CURVE MPH FEET DEGREES 2 0 0.0 2 0.27 0.2 9 9 2 6 2.2 2 5 0.0 2 0.2 3 0.2 5 150 3 8.2 3 0 0.0 4 0.2 0 0.24 231 2 4.8 3 5 0.0 4 0.18 0.22 314 18.2 4 0 0.06 0.16 0.2 2 4 2 6 1 3,4 4 5 0.06 0.15 0.21 6 4 3 8.9 50 0.06 0.14 0.20 8 3 3 6.9 5 5 0.06 0.135 0,195 10 3 4 5.5 6 0 0 .0 6 0.1 3 0.19 126 3 4.5 SPEED- MPH 6 0 33 t s s r r ^ o f ~ 5 0 4 0 3 0 & -fS 4 * s> f& a / /sr * ^ < £ > s ~ S C * t r V < S 7 / 7 . 2 0 0 0 0.2 0.3 0.4 0.5 0.6 0.7 ' / ’"* SIDE FRICTION FACTOR FIGURE 5 MAXIMUM SAFE SIDE. FRICTION FACTORS & + P* RATE O F SUPERELEVATION PLUS SIDE FRICTION FA C TO R R - RADIUS OF C U R V E - F E E T o o o o § 0 0 0 o o o o o o o 10 ■ o C O 2 4 20 I 6 I 2 7 ' 7 _ -------7 " ,m r a id /iu 7 5 £ = g » .< g < S <S= < £ > . ---------------- -------------- - J T - ISf j J /r* ‘7 1 / 7 _ - — / - I s f j k l $ / k0/ / j „ V v 7 7 y ' ■ $ / / z 7 4 / / 0/ / » / / i / l * \ M \ 7 W W f t p ' BASED f VARIE DN OPEN 1 S FROM 0. 0.13 AT tOAD CON IT AT 2 0 A 60 MPH DITI0NS 1 PH TO --- 1 ----1 ----L __1 __1 __1 __ .... X ....A .....1 ■ „ , - I I I ---1 ----1.....1 ...----1---1 ----L. — 1 —1 — I____ 1 __ 1 __I__----1 ----1 ----L. __1__1__1__ -1--- D = DEGREE OF CURVE FIGURE 6 RELATION OF SUPERELEVATION (e) PLUS SIDE FRICTION FACTOR (Z1 ) TO DEGREE OF CURVE _F0R. DESIGN SPE EDS_________ to -0 28 c u rv a tu re used f o r each d e s ig n sp e ed . One extrem e o f t h i s range i s th e maximum s u p e r e le v a tio n , e s ta b li s h e d by p r a c t i c a l c o n s id e r a tio n s and used to d eterm in e th e minimum ra d iu s f o r each sp e ed ; The o th e r extrem e i s z e ro , no s u p e r e le v a tio n b ein g needed on ta n g e n t ro a d ways. For r a d i i between th e s e extrem es and f o r a g iv e n d e s ig n sp eed , th e s u p e re le v a tio n sh o u ld be d i s t r i b u t e d i in such manner t h a t th e re i s a l o g i c a l r e l a t i o n between th e s id e f r i c t i o n f a c t o r and th e a p p lie d s u p e re le v a tio n i r a t e . i i I I F or a g iv e n d e s ig n speed th e r e a re fo u r l o g i c a l | methods f o r d i s t r i b u t i o n o f (e) o v er the a p p lic a b le ran g e j ! of c u rv e s . These methods fo llo w and the r e s u l t i n g r e l a t i o n s a re i l l u s t r a t e d in F ig u re 7, page 29; 1 . S u p e re le v a tio n r a t e i s d i r e c t l y p r o p o r tio n a l to th e r a d iu s , t h a t i s , a s t r a i g h t l i n e i r e l a t i o n between D - 0 and D = maximum. j I 2 . S u p e re le v a tio n r a t e i s such t h a t a v e h ic le t r a v e l l i n g a t d e s ig n speed has a l l c e n t r i - I I f u g a l f o rc e c o u n te r a c te d by s u p e r e le v a tio n | on cu rv es up to t h a t r e q u ir in g th e maximum i t j " e , ” and maximum " e ” p ro v id ed on a l l s h a rp e r 1 j c u r v e s . ! | 3 . Same as method two e x c e p t b ased on averag e j ru n n in g speed (th e d is ta n c e d iv id e d by th e = SUPERELEVATION RATE R= RADI US OF C U R V E - F E E T .24 .06 .0 4 NG Z5 "AT DESIGN SPEI'D CORItESPON .0 2 .04 COR RESPOIIOING / AT RUNNING SPEED 20 22 2 4 2 6 2 8 3 0 OEGREE OF CURVE & FIGURE 7 METHODS OF DISTRIBUTING SUPERELEVATION to SIDE FRICTION FACTOR 30 punning tim e ). 4 . S u p e r e le v a tio n r a t e in a e u r v i l i n e a r r e l a t i o n w ith degree of c u rv e , w ith v a lu e s betw een th o se of methods one and two. Method one m ight appear to be an i d e a l means of d i s t r i b u t i n g th e s id e f r i c t i o n f a c t o r , b u t i t s su c c e ss depends upon each v e h ic le in th e t r a f f i c stre a m m ain ta in in g a c o n s ta n t speed r e g a r d le s s o f w hether t r a v e l i s upon a ta n g e n t, a curve o f in te rm e d ia te r a d i i o r one j w ith th e minimum r a d iu s f o r t h a t d e s ig n sp e ed . W hile j i J u niform speed i s th e aim o f most d r iv e r s and can be had f on w e ll d esig n ed roadways when volumes a re n o t heavy, th e re i s a tendency f o r some d r iv e r s to t r a v e l f a s t e r upon ta n g e n ts and th e f l a t t e r curv es th a n upon th e i » s h a rp e r c u rv e s . This p o in ts to th e d e s i r a b i l i t y o f i i p ro v id in g s u p e r e le v a tio n r a t e s f o r in te rm e d ia te curves ! i j somewhat in ex cess of t h a t which r e s u l t s from method one. i I ‘ | In method two, no f r i c t i o n i s r e q u ir e d on f l a t curv es w ith l e s s th a n maximum s u p e r e le v a tio n f o r v e h ic le s t r a v e l l i n g a t th e d e sig n sp eed , and f r i c t i o n in c r e a s e s j r a p id ly as curv es w ith maximum s u p e re le v a tio n grow j s h a r p e r . F u r th e r , f o r v e h ic le s t r a v e l l i n g a t averag e j ! ru n n in g sp eed , t h i s method r e s u l t s i n n e g a tiv e f r i c t i o n ! i S I f o r cu rv es from very f l a t up to a b o u t th e m iddle of th e ra n g e , above w hich c u rv a tu re th e f r i c t i o n in c re a s e s 31 r a p id ly up to th e maximum. This marked d if f e r e n c e in f r i c t i o n r e q u ir e d f o r d i f f e r e n t cu rv es i s n o t l o g i c a l 1 and r e s u l t s in e r r a t i c d r iv in g , e i t h e r a t d e s ig n speed o r averag e ru n n in g speed. Method th re e h as th e same d isa d v a n ta g e as method two, b u t th e y ap p ly to a l e s s e r d e g re e . To f a v o r th e o v e rd riv in g c h a r a c t e r i s t i c s t h a t o ccu r on f l a t to in te rm e d ia te curves i t i s d e s ir a b le t h a t the s u p e r e le v a tio n approach th a t o b ta in e d by method t h r e e . I t i s a ls o d e s ir a b le to fa v o r method one which avoids a s u b s t a n t i a l p a r t o f th e ran g e o f curv es w ith | maximum s u p e r e le v a tio n . Using method f o u r , a curved l i n e ! ! j i as shown w ith in th e t r i a n g u l a r w orking range betw een j curv es one and th r e e in F ig u re 7 , page 29, r e p r e s e n ts a | i j s u p e r e le v a tio n d i s t r i b u t i o n re a s o n a b ly s a t i s f y i n g b oth j ! a s p e c ts . On t r i a l s o f v a rio u s c u r v i l i n e a r r e l a t i o n s i i j w ith in th e c o n tr o l t r i a n g l e , i t was concluded t h a t a * j p a r a b o lic form (w ith th e h o r iz o n ta l d is ta n c e g o v ern in g ) ! r e p r e s e n ts a p r a c t i c a l d i s t r i b u t i o n over th e ran g e o f c u r v a tu r e . F ig u re 8 , page 32, g iv e s s u p e re le v a tio n I r a t e s f o r a l l d e sig n speeds f o r th e c o n d itio n where th e j maximum s u p e r e le v a tio n r a t e i s 0 .0 6 f o o t p e r f o o t , and I f v a lu e s a re b ased on open-road c o n d itio n s . j j i S u p e r e le v a tio n ru n o ff i s th e g e n e ra l term denoting! th e change in c ro s s s e c tio n from a norm al crown s e c tio n SUPERELEVATION RATE R -RA DIU S OF C U R V E - F E E T o o o o o o o ° ° O O O O O O o o O o> ® r- ip in ^ c m 0 6 0 5 0 4 03 02 n < D O l 0 10 5 15 20 25 D* DEGREE OF CURVE FIGURE 8 DESIGN SUPERELEVATION RATES 33 to th e f u l l y s u p e re le v a te d s e c tio n , or v ic e v e r s a . To meet th e re q u ire m e n ts o f com fort and s a f e t y , t h e s u p e r e l e v a t io n ru n o ff sh o u ld he e f f e c t e d u n ifo rm ly over a le n g th adequate f o r th e l i k e l y t r a v e l sp e e d s. To be p le a s in g in appearance th e r u n o f f pavem ent edges sh o u ld n o t be d i s t o r t e d as th e d r iv e r views them . C u rre n t p r a c t i c e in d ic a t e s t h a t f o r appearance and co m fo rt th e le n g th o f s u p e re le v a tio n ru n o ff sh o u ld be such t h a t th e lo n g it u d in a l slo p e (edge compared to c e n te r l i n e of a two la n e roadway) o f one in two hundred i s n o t exceed ed . C onsidered w ith p la n e s e c t i o n s , t h i s can be s t a t e d t h a t th e d if f e r e n c e in lo n g it u d in a l g r a d ie n t betw een an o u ts id e edge o f a two la n e pavement and i t s c e n te r l i n e p r o f i l e s h a l l n o t exceed one h a l f p e r cent! | In a n o th e r so u rce th e same one in two hundred is I * j used f o r a d e s ig n speed o f f i f t y M.P;H. and h ig h e r . j | Where th e d e sig n speed i s t h i r t y and f o r t y M .P.H., r e l - i I | a t iv e slo p e s of 1 :1 5 0 and 1:175 r e s p e c t i v e l y , are u se d . Review o f e u r r e n t d e s ig n p r a c t i c e shows use of minimum r u n o f f le n g th s in th e range o f 100 t o 200 f e e t fo r speeds i ' I from t h i r t y to se v e n ty M.P.H. r e s p e c t i v e l y , r e g a r d le s s j i j of w idth and s u p e r e le v a tio n . j The le n g th o f r u n o f f a p p lic a b le to pavements j * I * w id er th a n two la n e s i s s u b je c t to th e same t h e o r e t i c a l d e r iv a tio n as f o r two la n e s . W hile le n g th s of th is o rd e r | 34 may be c o n s id e re d d e s i r a b l e , i t f r e q u e n tly i s n o t f e a s i b l e to su p p ly le n g th s b ased on such d i r e c t r a t i o s . On a p u r e ly e m p iric a l b a s is i t i s concluded t h a t minimum d e s ig n s u p e re le v a tio n ru n o ff le n g th s f o r pavem ents w id er th a n two la n e s sh o u ld be as fo llo w s : 3 la n e roadways 1 .2 tim es le n g th f o r a 2 la n e roadway 4 la n e roadway 1 .5 tim es le n g th f o r a 2 la n e roadway 6 la n e roadway 2 .0 tim es le n g th f o r a 2 I la n e roadway. Common p r a c t i c e in d ic a t e s t h a t ap p ro x im a te ly ! I ! j s e v e n ty - f iv e p e r c e n t o f th e le n g th o f r u n o f f p r e f e r a b ly ! | sh o u ld be lo c a te d on th e ta n g e n t a t c u rv e s . j | Changes i n c ro s s slo p e should be e f f e c t e d w ith j j edge o f pavement p r o f i l e s which are rounded to smooth ! I j flo w in g l i n e s . Three s p e c i f i c methods o f p r o f i l e d e sig n I ! in, a t t a i n i n g s u p e r e le v a tio n a re p r a c t ic e d ; nam ely (1) j r e v o lv in g th e pavem ent ab o u t th e c e n te r l i n e p r o f i l e , (2> r e v o lv in g th e pavement ab o u t th e in s id e edge p r o f i l e , j (3) r e v o lv in g th e pavement ab o u t th e o u ts id e edge p r o f i l e . In an o v e r - a l l se n se th e method of r o t a t i o n a b o u t th e ! ■ c e n te r l i n e u s u a lly i s m ost a d a p ta b le . j ! I A eommon d e s ig n p ro ced u re i s one in which th e i ru n o ff p r o f i l e s are d eterm in ed g r a p h ic a lly . The method i s e s s e n t i a l l y one o f ”s p lin e l i n e d ev e lo p m e n t.” In t h i s method th e c e n te r l i n e i s p l o t t e d on a v e r t i c a l s c a le of one in c h eq u a ls two f e e t or one in c h eq u a ls one f o o t and a h o r i z o n t a l s c a le In th e range o f one In ch e q u a ls tw enty f e e t . S u p e re le v a tio n c o n tr o l p o in ts are p l o t t e d In th e form o f brake p o i n t s . Then by means o f a s p li n e smooth flo w in g l i n e s a re drawn t o approxim ate th e s t r a i g h t l i n e c o n t r o l s . Once th e edge p r o f i l e s a re drawn i n th e p ro p e r r e l a t i o n to one a n o th e r, e le v a tio n s can be r e a d f o r s t a t i o n s as n e c e s s a ry f o r c o n s tru c t io n c o n t r o l. In a d d i tio n to th e s p e c i f i c d e s ig n elem ents f o r h o r iz o n ta l alig n m en t d is c u s s e d p r e v io u s ly , a number o f g e n e ra l c o n tr o ls a re re c o g n iz e d In p r a c t i c e as fo llo w s : 1. The c u rv a tu re o f a lig n m e n t should be k e p t to a minimum, b u t sh o u ld be c o n s is te n t w ith th e topography. W inding alig n m en t composed of s h o r t c u rv e s , sh o u ld be av o id ed sin c e i t u s u a lly i s a cause o f e r r a t i c o p e r a tio n . 2 . In alig n m e n t p r e d ic a te d on a g iv e n speed, use o f th e minimum r a d iu s f o r t h a t speed sh o u ld be avoided w herever p o s s i b le . The d e s ig n e r sh o u ld a tte m p t to use g e n e ra lly f l a t c u rv e s , r e t a i n i n g th e minimum f o r th e most c r i t i c a l c o n d itio n . The c e n t r a l a n g le sh o u ld be ab so rb ed in th e lo n g e s t p o s s ib le c u rv e . C o n s is te n t a lig n m e n t always sh o u ld be so u g h t. Sharp curv es sh o u ld n o t be In tro d u c e d a t . t h e ends o f lo n g ta n g e n ts . Sudden changes from a re a s o f easy c u rv a tu re t o a re a s o f sh a rp c u rv a tu re sh o u ld be a v o id e d . Where sh a rp c u rv a tu re m ust be in tro d u c e d i t sh o u ld be approached where p o s s i b l e , by s u c c e s s iv e ly \ s h a rp e r cu rv es from th e g e n e r a lly easy c u r v a tu r e . j i i F or sm a ll d e f l e c t i o n a n g le s , curves sh o u ld I I be s u f f i c i e n t l y long t o av o id th e ap p earan ce ! I o f a k in k . j C a u tio n sh o u ld be e x e r c is e d In th e use o f i compound c i r c u l a r c u r v e s . P r e f e r a b ly t h e i r use sh o u ld be av o id ed where curv es a re s h a rp . i i Where topography makes t h e i r use n e c e s s a ry , j i i th e r a d iu s of th e f l a t t e r c i r c u l a r a rc (R^) | sh o u ld be n o t more th a n f i f t y p e r c e n t J g r e a t e r th a n th e r a d iu s o f th e s h a rp e r i i c i r c u l a r a rc (Rg) or (R^) sh o u ld n o t exceed ! | 1 .5 (Rg)* ^ s e v e r a l s te p compound curve on j t h i s b a s is i s s u i t a b l e as a form o f t r a n s i t i o n j 37 to s h a rp c u rv e s . Any a b ru p t r e v e r s a l in alig n m e n t sh o u ld be a v o id e d . Such a change makes i t d i f f i c u l t f o r a d r iv e r to keep i n h is own la n e . A lso i t i s d i f f i c u l t to s u p e r e le v a te b o th curv es a d e q u a te ly , and hazardous and e r r a t i c o p era t i o n may r e s u l t . A r e v e r s a l in alig n m en t can be d e sig n e d s u i t a b l y by in c lu d in g a s u f f i c i e n t le n g th of ta n g e n t betw een th e two curv es f o r s u p e r e le v a tio n r u n o f f . j The b ro k en -b ack arrangem ent of cu rv es ( f l a t - back, or s h o r t ta n g e n t betw een two curv es in I th e same d ir e c t i o n ) sh o u ld be av o id e d . Such j { alig n m en t i s hazardous in t h a t m ost d r iv e r s ! do n o t e x p e c t su cceed in g curv es t o be i n j th e same d i r e c t i o n , th e p re p o n d e ra n t con- ; d i t i o n o f su cceed in g cu rv es in o p p o s ite d i r e c t i o n s d ev elo p in g a su b co n scio u s h a b i t i n d r iv e r s to fo llo w them. A lso , b rocken-back i a lig n m e n t i s n o t p le a s in g i n a p p e a ra n c e . ! Use o f compound curve a lig n m e n t, w herein | th e r e i s some degree of c o n tin u o u s s u p e r- j e l e v a t i o h , i s p r e f e r a b le f o r such c o n d itio n s . j 58 V e r tic a l A lig n m en t. V e r t i c a l curv es sh o u ld be sim ple in a p p l i c a t i o n and sh o u ld r e s u l t i n a d e s ig n w hich i s s a f e , co m fo rtab le i n o p e r a tio n , p le a s in g i n appearance and adequate f o r d ra in a g e . The m ajor c o n tr o l f o r s a f e o p e ra tio n on c r e s t v e r t i c a l cu rv es i s th e p r o v is io n o f ample s i g h t d is ta n c e f o r th e d e s ig n sp e e d . Minimum s to p p in g s i g h t d is ta n c e o r g r e a t e r v a lu e sh o u ld be p ro v id e d in a l l c a s e s . C o n s id e ra tio n o f com fort r e q u ir e t h a t v e h ic u la r r a t e o f change o f g rad e be k e p t w ith in t o l e r a b l e l i m i t s . T his i s m ost im p o rta n t i n sa g v e r t i c a l curv es where g r a v i t a t i o n a l and c e n t r i f u g a l f o r c e s a c t i n th e same d i r e c t i o n . A ppearance sh o u ld a l s o be c o n s id e re d . A long curve has a more p le a s in g ap p earan ce th a n a s h o r t one w hich may g iv e th e appearance of a sudden b re a k in th e p r o f i l e due to th e e f f e c t of f o r e s h o r te n in g . I ! D rainage problem s may a r i s e on sag v e r t i c a l cu rv es b ecau se o f th e d e p o s it of sed im en t in th e g u t t e r s i f f a s t flo w in g w a te r i s n o t c a r r i e d o f f b e fo re i t slow s down where th e g rade le v e l s o f f . i | V e r tic a l cu rv es can be c i r c u l a r , sim ple p a r a b o lic , I o r cu b ic p a r a b o lic c u rv e s . The sim p le p a r a b o lie g e n e r a l- i J l y i s th e p r e f e r a b le curve in s t r e e t p r o f i l e d e sig n and i i t c l o s e ly appro x im ates th e c i r c u l a r e u rv e . W ith t h i s 39 cu rv e th e v e r t i c a l o f f s e t s from th e ta n g e n t v ary as th e sq u a re o f th e h o r iz o n ta l d is ta n c e from th e cu rv ed end ( p o in t o f ta n g e n c y ). E le v a tio n s a lo n g th e cu rv e a re c a lc u l a te d sim ply as p r o p o rtio n s o f th e v e r t i c a l o f f s e t a t th e v e r t i c a l p o in t o f i n t e r s e c t i o n w hich i s . 800 The r a t e o f change o f grade to s u c c e s s iv e p o in ts on th e 1 cu rv e i s a c o n s ta n t amount f o r e q u a l in c re m e n ts o f h o r iz o n ta l d is ta n c e and eq u a ls th e a lg e b r a ic d if f e r e n c e betw een i n t e r s e c t i n g ta n g e n t g rad es d iv id e d by th e le n g th of curve in f e e t , o r A in p e rc e n t p e r f o o t . The I ; l | r e c i p r o c a l i s th e h o r iz o n ta l d is ta n c e i n f e e t r e q u ir e d I j to e f f e c t a one p e r c e n t change in g r a d ie n t and i s , th e r e - ! T i f o r e , a m easure o f c u r v a tu r e . The e x p re s s io n ±1, term ed j i K i s used h e r e in to d e s ig n a te cu rv es n e c e s s a r y f o r a ! t ; g iv en speed. A second use o f th e K v a lu e i s th e d e te rm i- ! i j n a t io n o f th e h o r iz o n ta l d is ta n c e from th e b e g in n in g o f i ! th e v e r t i c a l cu rv e to th e apex o f c r e s t v e r t i c a l cu rv es j f ; o r to th e low p o in t o f sag v e r t i c a l c u rv e s . The p o in t j ' ! i where th e slo p e e q u a ls z e ro occu rs a t a d is ta n c e from j ! I j th e b e g in n in g o f th e v e r t i c a l curve e q u a l t o K tim e s th e j | ap p ro ach g r a d ie n t. » | Minimum le n g th o f c r e s t v e r t i c a l cu rv e s as d eterm in ed by s ig h t d is ta n c e re q u ire m e n ts g e n e r a lly a r e i j s a t i s f a c t o r y from th e s ta n d p o in t of s a f e t y , co m fo rt and | a p p e a ra n c e . B asic fo im u la s f o r le n g th o f p a r a b o lic 40 v e r t i c a l cu rv es In term s o f a lg e b r a ic d if f e r e n c e i n g rade and s i g h t d is ta n c e a re s AS^ When S i s l e s s th an L, L = —_—. . ■ ( i ) 100 ( V2hg)2 vx/ When S i s g r e a t e r th a n L, L = 2 S - 200( ) 2 (2) A Where L = le n g th o f v e r t i c a l curve i n f e e t S = s i g h t d is ta n c e i n f e e t A - a lg e b r a ic d if f e r e n c e i n g rad es in p e r c e n t h^ = h e ig h t o f eye above roadway s u r fa c e i n f e e t h_ = h e ig h t o f o b je c t above roadway s u r f a c e in f e e t . i When th e h e ig h t o f eye and th e h e ig h t o f o b je c t j i a r e 4 .5 f e e t and fo u r in c h e s r e s p e c t i v e l y , a s used f o r ! s to p p in g s i g h t d is t a n c e , th e above fo rm u la s becomes j When S i s l e s s th a n L, L = (3) j When S i s g r e a t e r th a n L, L = 23 - (4) A The l i g h t s o l i d l i n e s i n F ig u re 9 , page 41 , show th e r e q u ir e d le n g th s o f v e r t i c a l cu rv es from fo rm u las (3) and (4) f o r th e d i f f e r e n t d e sig n sp e ed s. - a lg eb r a ic d iff e r e n c e in g r a d e s * p e r c e n t 200 400 600 800 1000 1200 1400 L= MINIMUM LENGTH OF VERTICAL CURVE- FEET FIGURE 9 DESIGN CONTROLS FOR CREST VERTICAL CURVES 1600 1800 2000 42 The s h o r t d ash curve a t th e low er l e f t , c r o s s in g th e s e l i n e s , i n d ic a t e s where S = L. To th e r i g h t o f th e S = L l i n e , th e v alu e o f K, o r l e n g th o f v e r t i c a l p e r p e r c e n t change i n A, i s a sim ple and c o n v e n ie n t ex p r e s s io n o f th e d e s ig n c o n t r o l. F o r ea ch d e sig n speed t h i s s in g le v a lu e i s a p o s i t i v e w hole number t h a t i s in d i c a t i v e o f th e r a t e o f v e r t i c a l c u r v a tu r e . The d e s ig n c o n tr o l i n term s o f K c o v e rs a l l co m b in atio n s o f A and L f o r any one d e s ig n speed; th u s A and L need n o t ! j be in d ic a te d s e p a r a te ly i n d e s ig n v a lu e t a b u l a t i o n . I t f a c i l i t a t e s th e s e l e c t i o n o f d e s ig n c u rv es s in c e th e r e q u ir e d le n g th o f curve in f e e t i s e q u a l to K tim es th e a lg e b r a ic d if f e r e n c e in g rad es in p e r c e n t, L = KA. i | C o n v ersely , ch e ck in g p la n s i s s im p lif ie d by com paring | a l l c u rv es w ith th e d e s ig n K v a lu e . | Where S i s g r e a t e r th a n L, low er l e f t i n F ig u re I 9 , page 41, th e l i g h t s o li d l i n e s bend to th e l e f t and t j f o r sm a ll v a lu e s o f A, th e r e q u ir e d le n g th s a r e zero j b ecause th e s ig h t l i n e p a s se s o ver th e apex. T h is r e l a t i o n does n o t r e p r e s e n t c u r r e n t d e s ig n p r a c t i c e . To re c o g n iz e th e d i s t i n c t i o n i n d e s ig n speed and to approx- i im ate th e ran g e o f c u r r e n t p r a c t i c e , minimum le n g th s o f v e r t i c a l c u rv es a r e e x p re sse d as a b o u t th r e e tim es th e d e sig n speed L m in. = 3V. These te rm in a l a d ju stm e n ts show a s th e heavy v e r t i c a l l i n e s a t th e low er l e f t i n F ig u re 9 , page 41, I The K v a lu e s d e riv e d f o r th e case where S i s l e s s th a n L a ls o can be used w ith o u t s i g n i f i c a n t e r r o r where S i s g r e a t e r th a n L a s can be seen by in s p e c tin g i F ig u re 9 , page 41. The minimum lo n g i t u d i n a l grade f o r d ra in a g e i s ab o u t 0 .3 5 p e r c e n t. T here i s a l e v e l p o in t on a c r e s t v e r t i c a l c u rv e , b u t no d i f f i c u l t y i s e x p e rie n c e d i f th e cu rv e i s sh a rp enough so t h a t th e minimum g rade o f 0 .3 5 p e r c e n t i s reac h ed a t a p o in t f i f t y f e e t from th e c r e s t . T his c o rre sp o n d s to an A v a lu e p e r s t a t i o n o f 0 .7 f o r w hich K s MlO s 143 f e e t p e r p e r c e n t change in A. T his 0 .7 • l i n e i s p l o t t e d in F ig u re 9, page 41, a s th e d ra in a g e i • maximum. i ! When a v e h ic le t r a v e r s e s a sag v e r t i c a l curve i j a t n ig h t th e p o r tio n o f th e roadway l i g h t e d ahead i s dependant upon th e p o s i t i o n o f th e h e a d lig h t and th e j d i r e c t i o n o f th e l i g h t beam. G en eral use i s b e in g g iv e n i to a h e a d lig h t h e ig h t o f 2 .5 f e e t and a one d eg ree up ward d iv e rg e n c e o f th e l i g h t beam from th e lo n g i t u d i n a l a x is o f th e v e h i c le . The upward sp re a d o f th e l i g h t i p ro v id e s some a d d i t i o n a l v i s i b l e le n g th but t h i s i s g e n e r a lly ig n o re d . The fo llo w in g form ula shows th e S, ; L, and A r e l a t i o n , u s in g S a s th e d is ta n c e betw een th e v e h ic le and p o in t where th e one d eg ree an g le o f l i g h t 44 ra y i n t e r s e c t s th e s u rfa c e o f th e roadw ay: When S i s l e s s th a n L, L = — _______ ( 5 ) 5 0 0 + 3 . 5S When S i s g r e a t e r th a n L, L = 2 S - .3«5§. ( 6 ) A Where L = le n g th o f sag v e r t i c a l curve in f e e t S = l i g h t beam d is ta n c e in f e e t r A - a lg e b r a ic d if f e r e n c e in g ra d e s , p e r c e n t. F o r o v e r a l l s a f e t y , a sag v e r t i c a l curve should be lo n g enough so t h a t th e l i g h t beam d is ta n c e i s n e a r ly th e same as th e sto p p in g s ig h t d is t a n c e . A cc o rd in g ly i t i s p e r t i n e n t t o use s to p p in g s i g h t d is ta n c e f o r d i f e r r e n t d e sig n speeds as th e S v a lu e s in th e above fo rm u la s. The le n g th s o f sag v e r t i c a l c u rv e s b ased ori th e above fo rm u las a r e shown in F ig u re 10, page 45 by l i g h t s o l i d l i n e s and th e rounded v a lu e s f o r use i n d e sig n a r e shown w ith heavy l i n e s . A minimum le n g th o f th re e tim es th e d e s ig n speed i s a ls o used f o r sag v e r t i c a l c u rv e s . In a d d i tio n to th e above s p e c i f i c c o n t r o l s , th e r e ! a re s e v e r a l g e n e r a l c o n t r o ls t h a t sh o u ld be c o n s id e re d in d e s ig n . (1) A broken back grade l i n e (two v e r t i c a l c u rv e s in th e same d i r e c t i o n s e p a ra te d by a s h o rt s e c tio n o f ta n g e n t g rad e) I 8 1 7 16 I 5 I 4 13 12 11 10 9 8 7 6 5 4 3 2 I 0 200 4 00 600 800 1000 1200 1400 1600 1800 2000 L= MINIMUM LENGTH OF VERTICAL CURVE- FEET FIGURE 10 | Cfl ! DESIGN CONTROLS FOR SAG VERTICAL CURVES ! 46 g e n e r a lly sh o u ld be a v o id e d . (2) At i n t e r s e c t i o n s on roadways w ith m oderate to s te e p g ra d e s , i t i s d e s ir a b le to red u ce th e g r a d ie n t th ro u g h th e i n t e r s e c t i o n . Such a change i s b e n e f i c i a l f o r a l l v e h ic le s making tu r n s and s e rv e s to red u ce th e p o t e n t i a l h a z a r d s . (5) C u rv atu re and g rad es sh o u ld be in p ro p e r b a la n c e . T angent alig n m e n t o r f l a t c u r - i v a tu re a t th e expense o f s te e p o r lo n g j g ra d e s , and e x c e s s iv e c u rv a tu re w ith f l a t j I c u rv e s , a re b o th poor d e s ig n s . A l o g i c a l j d e s ig n i s a compromise betw een th e two, ■ i w hich o f f e r s th e m ost in s a f e t y , and p l e a s in g ap p earan ce w ith in th e p r a c t i c a l l i m i t s j i o f t e r r a i n and a r e a tr a v e r s e d . j i (4) Sharp h o r iz o n ta l c u r v a tu re sh o u ld n o t be i ( in tro d u c e d a t o r n e a r th e to p o f a p r o - j nounced c r e s t v e r t i c a l c u rv e . The h az ard o f t h i s arrangem ent i s av o id ed i f th e h o r iz o n ta l curve i s made lo n g e r th a n th e I v e r t i c a l c u rv e . j (5) Sharp c u rv a tu re sh o u ld n o t be in tro d u c e d J a t o r n e a r th e low p o in t o f a pronounced sa g v e r t i c a l c u rv e . Because th e ro ad ahead i s f o r e s h o rte n e d any b u t f l a t h o r iz o n ta l c u r v a tu re assumes an u n d e s ir a b le d i s t o r t e d ap p e a ra n c e . H o riz o n ta l c u rv a tu re and p r o f i l e sh o u ld be made as f l a t as p o s s i b le a t i n t e r s e c t i o n s where s i g h t d is ta n c e i s im p o rta n t and ve h i c l e s may have to slow down o r s to p . CHAPTER IV DESIGN STANDARDS The p re c e d in g c h a p te r s c l e a r l y e s t a b l i s h th e need f o r d e s ig n s ta n d a rd s b ased on th e a n t i c i p a t e d speed o f th e v e h ic le s t h a t w i l l u t i l i z e th e im provem ent. The h o r iz o n ta l and v e r t i c a l alig n m e n t should p ro v id e f o r s a fe and c o m fo rta b le use o f th e Improvement w h ile t r a v e l l i n g a t th e d e s ig n speed. The fo llo w in g s ta n d a rd s s h a l l be b ased on th e above c r i t e r i a . I i W idths o f Improvement and R ig h ts o f Way. On ! m inor s t r e e t s w hich a re c o n s tr u c te d to se rv e n o t more i th a n tw e n ty -f iv e l o t s on a dead end s t r e e t , c u l- d e - s a c j i i | s t r e e t s , o r a c ro s s s t r e e t t h a t i s d e sig n e d to p ro v id e I : ! a c c e ss f o r th e a b u t tin g p r o p e r ty o n ly , a minimum w id th j o f tw e n ty -e ig h t f e e t o f improvement sh o u ld be r e q u ir e d . ! ' 1 . j T his w id th w i l l a llo w two p a s se n g e r c a rs to m eet and p a ss ] j ; j w ith com fort when a v e h ic le i s p ark ed on one s id e o n ly . j i I The minimum r i g h t o f way f o r t h i s improvement sh o u ld be j f o r t y f e e t . T h is w id th w i l l p ro v id e s i x f e e t from curb 1 i to p ro p e r ty l i n e , w hich i s th e minimum r e q u ir e d f o r th e n e c e s s a ry u t i l i t i e s on one s id e and a f iv e and one h a l f j j f o o t sid e w a lk on a t l e a s t one s id e a d ja c e n t to p r o p e r ty ! | l i n e . In l e v e l a r e a s where la n d i s n o t as sc a rc e a s i n 49 h i l l s i d e developm ent, a d e s ir a b le w id th o f improvement would be t h i r t y - s i x f e e t in a r i g h t of way f i f t y - s i x f e e t w ide. T his w i l l p ro v id e room f o r th e n e c e s s a ry u t i l i t i e s and a fo u r f o o t sid e w alk on each s id e a d ja c e n t to p ro p e rty l i n e w ith a f i v e and one h a l f f o o t p la n te d a re a betw een th e curb and sid e w a lk . The t h i r t y - s i x f o o t improvement w i l l allo w two v e h ic le s to m eet and p ass w ith a v e h ic le p a rk e d on each s id e o f th e s t r e e t . C o lle c to r s t r e e t s , which p ro v id e a c c e ss to a number of c ro ss s t r e e t s o r dead end s t r e e t s , sh o u ld be I improved w ith a maximum w id th o f t h i r t y - s i x f e e t in a j f o r t y - e i g h t f o o t r i g h t o f way. A more d e s ir a b le im prove- ! ment i f f e a s i b l e , would be a f o r t y f o o t improvement in a j t s i x t y f o o t r i g h t o f way. The above w id th s a re based on j s e rv in g o n ly th e s t r e e t s b ein g c o l l e c t e d and i f i t appears} I l i k e l y th e s t r e e t may develop i n t o a th o ro u g h fa re o r a s h o r t c u t f o r a la r g e number o f v e h ic le s t r a v e l l i n g from one p o p u la tio n c e n te r to a n o th e r, th e s t r e e t sh o u ld be j developed as a seco n d ary or m ajor s t r e e t . A lso , i f th e s t r e e t i s o f s u f f i c i e n t le n g th , i t may be n e c e s s a ry to t add a d d i t i o n a l la n e s i n th e low er re a c h e s o f th e c o lle c to r ! i s t r e e t , as t r a f f i c a c c u m u late s, p r i o r to i t s ju n c tio n w ith a seco n d ary o r m ajor s t r e e t . Secondary s t r e e t s , which s e rv e as co n n e c tin g I a r t e r i e s f o r c o l l e c t o r s t r e e t s and p ro v id e a th o ro u g h - j f a r e f o r v e h ic le s t r a v e l l i n g from one p o p u la tio n c e n te r t o a n o th e r , sh o u ld be improved w ith a minimum w id th of s i x t y - f o u r f e e t in a n in e ty f o o t r i g h t o f way. T his w id th w i l l p ro v id e f o r a f i v e f o o t sid e w alk on each s id e a d ja c e n t to p ro p e rty l i n e w ith a seven and one h a l f f o o t p la n tin g s t r i p betw een th e curb and sid e w a lk . A s i x t y - fo u r f o o t improvement w i l l p ro v id e f o r two tw elv e f o o t t r a f f i c la n e s and one e ig h t f o o t p a rk in g la n e i n each d i r e c t i o n . A more d e s ir a b le improvement would be an e ig h ty f o o t improvem ent i n a hundred and te n f o o t r i g h t o f way. T his w id th w i l l p ro v id e f o r two t r a f f i c la n e s and a p a rk in g la n e in each d i r e c t i o n w ith a f o u r te e n f o o t wide median to accommodate l e f t tu r n in g v e h i c le s . This w id th a ls o p ro v id e s f o r th r e e e le v e n f o o t t r a f f i c la n e s j i ! i n each d i r e c t i o n d u rin g peak h o u rs o f t r a f f i c by ' p r o h i b i t i n g p a rk in g d u rin g th e s e i n t e r v a l s on th e t h i r t y - th r e e f o o t roadw ays. Major s t r e e t s sh o u ld be improved to a minimum { I w id th o f one hundred and two f e e t in a hundred and t h i r t y - ! two f o o t r i g h t o f way. T his w i l l p ro v id e f o r t h r e e J i tw elve f o o t la n e s o f t r a f f i c in each d i r e c t i o n w ith an j j e i g h t f o o t p a r a l l e l p a rk in g la n e on each s id e w ith a f o u rte e n f o o t m edian f o r l e f t tu r n movements. A lso , i t w i l l p ro v id e f o r a sid e w a lk f i f t e e n f e e t wide on each ; i l s id e in b u s in e s s a r e a s . ! 51 Grade of S t r e e t s . On m inor s t r e e t s , th e maximum grade sh o u ld n o t exceed f i f t e e n p e r c e n t and a d e s ir a b le l i m i t i s te n p e r c e n t. On c o l l e c t o r s t r e e t s th e maximum I g rad e s h o u ld n o t exceed te n p e r c e n t and a d e s ir a b le l i m i t i s s i x p e r c e n t. On m ajor and se co n d ary s t r e e t s , th e maximum g rad e sh o u ld n o t exceed s i x p e r c e n t, keep ing in mind t h a t l e s s e r r a t e s o f g rad e a re more d e s ir a b le b o th f o r co m fo rt and s a f e t y . The minimum g rad e on a l l s t r e e t s should be a t l e a s t one h a l f o f one p e r c e n t w ith an a b s o lu te minimum j o f one q u a r te r o f one p e r c e n t. This a b s o lu te minimum i i i I i ! of one q u a r te r p e r c e n t g rad e sh o u ld be av o id ed a t a l l j i ! tim es i f f e a s i b l e . j 1 i I i i A lig n m en t. Minor s t r e e t s should be d esig n ed j based on a d e s ig n speed o f tw e n ty -fiv e m ile s p e r h o u r, i ; | ; i f ec o n o m ic ally f e a s i b l e , and a d e s ig n speed of tw enty ; j • m ile s p e r h o u r sh o u ld be a minimum d e s ig n sp eed . The ' minimum c e n te r l i n e r a d iu s sh o u ld be n o t l e s s th a n one hundred f e e t and sh o u ld be made as la r g e as eco n o m ic ally f e a s i b l e . Hie minimum d is ta n c e betw een r e v e r s e curves i sh o u ld be a t l e a s t f i f t y f e e t , w ith a d e s i r a b l e minimum ; i J o f s e v e n ty - f iv e f e e t . j C o lle c to r s t r e e t s sh o u ld be d e sig n e d b ased on a j d e s ig n speed o f t h i r t y - f i v e m ile s p e r hour w ith a minimum j 52 d e s ig n speed o f t h i r t y m ile s p e r h o u r. The ta n g e n t d is ta n c e betw een curve r e v e r s a l s sh o u ld be a minimum o f one hundred f e e t w ith a d e s i r a b l e minimum o f one hundred and f i f t y f e e t . The minimum r a d iu s sh o u ld n o t be l e s s th a n two hundred f e e t and a d e s ir a b le minimum, i f f e a s ib le , would be a r a d iu s o f th r e e hundred f e e t . Secondary and m ajor s t r e e t s sh o u ld be d esig n ed w ith a d e s ig n speed o f f o r t y - f i v e m ile s p e r hour and a minimum d e s ig n speed o f f o r t y m ile s p e r h o u r. The minimum ta n g e n t d is ta n c e betw een curve r e v e r s a l s sh o u ld j I be a t l e a s t two hundred f e e t w ith a d e s ir a b le minimum j I j o f th r e e hundred f e e t . The minimum r a d iu s o f c u rv a tu re J 1 i S sh o u ld be a t l e a s t f i v e hundred f e e t w ith a l l cu rv es ! I j d e sig n e d w ith as la r g e a r a d iu s as I s ec o n o m ic ally i j f e a s i b l e . ! ! | S u p e r e le v a tio n . Minor s t r e e t s a re u s u a lly j 1 d e sig n e d w ith no s u p e r e le v a tio n or lim ite d to a maximum ! o f two p e r c e n t. A norm al crown s e c t i o n of o n e-h u n d red th o f a f o o t p e r f o o t of w id th betw een th e l i p s o f the | f c o n c re te g u t t e r s w i l l p ro v id e s a t i s f a c t o r y c o n tr o l f o r I I I i * lawn w a te r d ra in a g e . A maximum s u p e r e le v a tio n o f two \ p e r c e n t w i l l p ro v id e a r e l a t i v e l y le v e l s e c tio n on th e o u ts id e o f th e curve and a p p ro x im a te ly fo u r p e r c e n t f c r o s s - f a l l on th e in s id e o f th e c u rv e . T his sm a ll amount I 53 o f s u p e r e le v a tio n w i l l u s u a lly improve th e appearance as w e ll as ease th e o p e r a tio n o f v e h ic le s n e g o t ia t in g th e s e c u rv e s , as th e y q u ite o f te n have a r a d iu s s m a lle r th a n i s r e q u ir e d f o r a d e sig n speed o f tw e n ty -f iv e m iles p e r h o u r. C o lle c to r s t r e e t s sh o u ld be d e sig n e d w ith s u p e r e l e v a t io n up to a maximum of fo u r p e r c e n t. S p e c ia l care sh o u ld be ta k e n to p ro v id e s u f f i c i e n t w a te r c a rr y in g c a p a c ity on th e o u ts id e o f a l l cu rv es to c a rr y th e lawn J w a te r which may be e x p e c te d . The norm al crown s e c t i o n \ sh o u ld be removed i n th e curve alig n m e n t and a t r a n s i t i o n j j from f u l l cfcown to z e ro crown i n th e ta n g e n t alig n m e n t ; i p re c e d in g and fo llo w in g th e c u rv e . Where ta n g e n ts j betw een r e v e r s e curv es a re s h o r t a p o r tio n o f th e curve i i alig n m e n t may be r e q u ir e d to o b ta in th e maximum s u p e r- ■ j e l e v a t io n and remove th e crown s e c t i o n . I f t r a n s i t i o n i s e c tio n s a re s h o r t e r th a n th e d e s ir a b le le n g th s m entioned j J p r e v io u s ly in th e s ta n d a rd s on alig n m e n t, s p e c i a l c a re ! ! w i l l be r e q u ir e d in d e s ig n in g th e to p o f curve g rad e in th e t r a n s i t i o n o r e x c e s s iv e g rad e changes w i l l be e v id e n t, I , : Top o f curb p r o f i l e s are u s u a lly d eterm in ed by th e s p lin e : m ethod. C o n tro l p o in ts a r e p l o t t e d and a smooth curve j s p lin e d th ro u g h them . E le v a tio n s a re p ic k e d o f f th e s p li n e curve t o p ro v id e th e n e c e s s a ry e o n tr o l f o r th e top o f curb p r o f i l e . 54 Major and secondary s t r e e t s sh o u ld be d esig n ed w ith s u p e r e le v a tio n up to a maximum o f s i x p e r c e n t. The same p r e c a u tio n s f o r c o n tr o l of lawn w a te r a re n e c e s s a ry as on c o l l e c t o r s t r e e t s . The crown s e c tio n s sh o u ld a ls o be removed in th e curved alig n m e n t w ith s u i t a b l e t r a n s i t i o n s on th e ta n g e n t alig n m e n t p re c e d in g and fo llo w in g i th e c u rv e . S p e c ia l c a re i s r e q u ir e d to a s s u re a smooth g rad e f o r th e top of curbs in th e t r a n s i t i o n s e c t i o n s . Tangent alig n m e n t of a l l c la s s e s o f s t r e e t s a re n o rm ally d esig n ed w ith z e ro c r o s s - f a l l e x c ep t p re c e d in g c u rv e s as p r e v io u s ly m en tio n ed . A c r o s s - f a l l o f one p e r c e n t i s n o t r e a d i l y a p p a re n t and w i l l n o t a d v e rs e ly j a f f e c t th e r i d i n g q u a l i t i e s o r a p p e a ra n c e , th e re fo r© i f I | J topography o r o th e r c o n tr o ls i n d i c a t e th e d e s i r a b i l i t y j ! j o f d e s ig n in g on t h i s b a s i s , i t i s c o n s id e re d a s a t i s f a c - | ! I . , | to r y d e s ig n . T his minimum c r o s s - f a l l can be u t i l i z e d | a t i n t e r s e c t i o n s to improve t h e i r ap p earan ce when an j i i n t e r s e c t i n g s t r e e t i s ap p ro ach in g a t a s te e p r a t e of g r a d e . I n t e r s e c t i o n D e sig n . An e x c e p tio n to th e one p e r c e n t c r o s s - f a l l w i l l be r e q u ir e d in th e d e sig n o f i i n t e r s e c t i o n s where th e s id e s t r e e t i s r o t a t e d to con- i I | form to th e g rad e o f th e m ajor s t r e e t . S p e c ia l ca re w i l l I be r e q u ir e d to acco m p lish t h i s r o t a t i o n in th e d is ta n c e 55 a v a i l a b l e . A common s ta n d a rd i s to allow a maximum c r o s s - f a l l a t th e b e g in n in g o f the curb r e t u r n s o f th e m inor s t r e e t o f two t h i r d s th e r a t e of g rad e on th e m ajor s t r e e t . The minimum r a t e o f g rad e o f th e m inor s t r e e t a t a norm al i n t e r s e c t i o n w i l l occur on th e h ig h s id e o f th e s t r e e t betw een th e flow l i n e o f th e m ajor s t r e e t and th e b e g in n in g o f th e curb r e tu r n s on th e m inor s t r e e t . This r a t e of g rad e sh o u ld n o t be l e s s th a n one p e r c e n t u n le s s th e ap p ro ach in g g rad e on th e m inor s t r e e t j Is le s s th a n one p e r c e n t. The maximum r a t e o f g ra d e , c o n v e rs e ly , w i l l be on th e low s id e of th e m inor s t r e e t betw een th e same l i m i t s as f o r th e minimum g ra d e . The i ! maximum d e s ir a b le g rad e alo n g th e curb l i n e produced on i th e low s id e sh o u ld n o t exceed f i v e p e r c e n t. The max- j < . i i | imum r a t e o f g rade on c e n te r l i n e betw een th e same l i m i t s ; i ; ! sh o u ld n o t exceed fo u r p e r c e n t. There sh o u ld be no gradei I changes betw een th e flo w l i n e o f the m ajor s t r e e t and th e ' j b e g in n in g o f th e curb r e tu r n s on th e m inor s t r e e t on any o f th e th re e l i n e s m e n tio n e d . In some c a ses i t may be n e c e s s a ry to d e c re a se th e norm al crown s e c tio n a t th e b e g in n in g of th e curb r e t u r n s on th e m inor s t r e e t . Curb r e tu r n s on a l l c l a s s i f i c a t i o n s o f s t r e e t s sh o u ld be based on th e r a t e s o f g rad e on the i n t e r s e c t i n g j i s t r e e t s . A common method I s c a l l e d ” c u t t i n g out o f th e I p l a n e . ” This method i s i l l u s t r a t e d i n F ig u re 11, page 56. 00-101 100.25 100.30 99.63 9 9 .3 4 100.15 100.07 99.10 99 50 99.44 FIGURE II CUTTING CURB RETURNS OUT OF A PLANE 9 8.26 57 T his method n o rm ally p ro v id e s a s a t i s f a c t o r y v e r t i c a l alig n m en t and p ro p e r c o n tr o l o f d ra in a g e . Curb p r o f i l e s sh o u ld always be p l o t t e d to an e x a g g e ra te d v e r t i c a l s c a le and a d ju s te d as r e q u ir e d to p ro v id e a smooth p r o f i l e as w e ll as p ro p e r c o n tr o l of d ra in a g e . This method i s v ery s a t i s f a c t o r y f o r th e d e s ig n and c o n tr o l r e q u ir e d i n con s t r u c t i n g th e c o n c re te s p a n d re l area in c ro ss g u t t e r c o n s tr u c tio n . The s p a n d re l a re a i s th e area betw een th e curb fa c e and th e s e m i-ta n g e n ts f o r th e curb r e t u r n . i i The r a d iu s o f th e cu rb r e tu r n s sh o u ld be based on the ty p e of v e h ic le u sin g th e i n t e r s e c t i o n . A minimum ra d iu s of tw e n ty -fiv e f e e t i s d e s ir a b le f o r p a s s e n g e r i c a rs on r e s i d e n t i a l i n t e r s e c t i o n s . I f b u ses a re ex p ected ,! a minimum r a d iu s of t h i r t y i f i v e f e e t i s d e s i r a b l e . I , i j The g rade of s t r e e t s a t i n t e r s e c t i o n s sh o u ld be j as low as th e topography w i l l p e r m it. A d e s ir a b le max- j imum r a t e o f g rad e a t an i n t e r s e c t i o n w ith a m inor s t r e e t J i s s i x p e r c e n t. However, c a re m ust be ta k e n t o p ro v id e p ro p e r le n g th s of v e r t i c a l cu rv es on th e c o l l e c t o r s t r e e t to a v o id th e ap p earan ce o f a bench a t th e i n t e r s e c t i o n . I I n t e r s e c t i o n s sh o u ld be d e sig n e d to p ro v id e as much s i g h t d is ta n c e as i s ec o n o m ic ally f e a s i b l e . The c o n s tr u c tio n of an i n t e r s e c t i o n on th e in s id e o f a h o r iz o n ta l curve w ith a s h o r t r a d iu s and a la r g e d e l ta i s h azardous as ap p ro ac h in g v e h ic le s can n o t be r e a d i l y 58 s e e n . The e l e v a t i o n o f b u ild in g s i t e s a t i n t e r s e c t i o n s sh o u ld be c o n t r o ll e d so t h a t th e l i n e o f s i g h t i s n o t o b s tr u c te d . The l o c a t i o n o f houses on c o rn e r l o t s sh o u ld be c o n tr o lle d as w e ll as th e h e ig h t o f f e n c e s , sh ru b b ery and t r e e s . I n t e r s e c t i o n s , p r e f e r a b ly , sh o u ld be lo c a te d on ta n g e n t alig n m e n t o r on th e o u ts id e o f a c u rv e . I f i t i s n e c e s s a r y , b ecause o f to p o g ra p h ic or o th e r c o n t r o l s , to lo c a te an i n t e r s e c t i o n on th e in s id e o f a c u rv e , th e curve sh o u ld be as f l a t as p o s s i b l e . A minimum s i g h t d is ta n c e sh o u ld be p ro v id e d a t s to p s t r e e t to allo w a v e h ic le t o s t a r t from a dead s to p and e n t e r th e main s t r e e t w ith a c o m fo rta b le m argin o f s a f e t y b e f o re a c a r , w hich i s v i s i b l e to th e sto p p e d v e h i c le , can t r a v e r s e j th e rem a in in g d is t a n c e to th e i n t e r s e c t i o n . This s ig h t d is ta n c e w i l l depend on th e d e sig n speed o f th e main I s t r e e t . I f th e main s t r e e t i s d e sig n e d w ith p ro p e r | s i g h t d is ta n c e i n mind and th e c o rn e r l o t s a re p r o p e r ly i c o n t r o l l e d , a s a t i s f a c t o r y c o n d itio n w i l l r e s u l t . T his | c o n tr o l a t i n t e r s e c t i o n i s e s p e c i a l l y im p o rta n t in th e d e s ig n o f h i l l s i d e s t r e e t s . V e r tic a l alig n m e n t i s a ls o v ery im p o rta n t a t i n t e r s e c t i o n s and c a re sh o u ld be ta k e n to see t h a t i n t e r s e c t i o n s f a l l v ery c lo s e to th e summit o f c r e s t v e r t i c a l c u rv e s . 59 Cross S e c t io n , S t r e e t s sh o u ld always be g rad ed f o r th e f u l l w id th o f th e d e d ic a tio n w ith cu t and f i l l s lo p e s behind th e p r o p e r ty l i n e s f a r enough to in s u re a s t a b l e improvem ent and p ro p e r s i g h t d is t a n c e . When th e e l e v a t i o n o f th e s t r e e t i s c o n s id e ra b ly above the e l e v a t io n o f th e a b u ttin g l o t s , p r o t e c t i o n sh o u ld be p ro v id e d f o r p e d e s tr ia n s a l i g h t i n g from v e h i c le s , as w e ll as f o r th e low l o t s from th e d an g er o f a v e h ic le jumping th e curb and c ra s h in g o n to p r i v a t e p r o p e r ty . P e d e s tr ia n p r o t e c t i o n can be p ro v id e d by fe n c in g w ith a s u i t a b l e m a te r ia l such as c h a in l i n k fe n c in g , o r by th e i n s t a l l a t i o n o f hand r a i l s or g u ard r a i l s . Guard r a i l s sh o u ld be p ro v id e d on th e o u ts id e o f a l l cu rv es and a p p ro x im a te ly one hundred f e e t o f th e a d jo in in g ta n g e n t alig n m e n t when th e a b u ttin g l o t s a re below th e s t r e e t l e v e l , A s t e e l r a i l i n g mounted on s t e e l p o s ts s e t in c o n c re te sh o u ld be u se d . Wooden r a i l and p o s ts do n o t p ro v id e th e p r o t e c t i o n r e q u ir e d . S idew alks sh o u ld be p ro v id e d in a l l r e s i d e n t i a l a re a s as w e ll as a l l com m ercial a re a s on a t l e a s t one s id e and p r e f e r r a b l y on b o th s id e s o f th e s t r e e t . S id e walks p ro v id e c o n tr o l f o r th e g rade o f drivew ays and d isc o u ra g e th e la n d sc a p in g o f th e d e d ic a te d s t r e e t as though i t were p r i v a t e p r o p e r ty . S idew alks sh o u ld be c o n s tru c te d w ith th e back o f th e w alk on p r o p e rty l i n e . 6 0 S idew alks w i l l a ls o h e lp to p re s e rv e s i g h t d is ta n c e on th e in s id e o f c u rv e s . I n a d d itio n to th e above, s id e w alks a re e s s e n t i a l to p ro v id e a p la c e f o r c h i ld r e n to p la y as w e ll as s e rv in g as a f a c i l i t y f o r p e d e s t r i a n t r a f f i c . A ll t r e e s o r sh ru b b e ry p la n tin g sh o u ld be c o n t r o ll e d by th e c i t y w ith s p e c i a l c a re b e in g tak en t o p ro v id e th e minimum sto p p in g s i g h t d is ta n c e on a l l cu rv es and i n t e r s e c t i o n s . The type o f tr e e s and sh ru b b e ry sh o u ld be c o n t r o ll e d to a s s u re t h a t th e r o o ts w i l l n o t damage th e im provem ents. T rees sh o u ld n o t be p la n te d betw een th e curb and sid e w a lk when t h i s d is ta n c e i s l e s s th a n f i v e f e e t . A ll t r e e s sh o u ld have p r o p e r ly d esig n ed j t r e e w e lls to encourage deep r o o t g ro w th . i C oncrete g u t t e r s sh o u ld be p ro v id e d on a l l s tr e e ts j r e g a r d le s s of g ra d e . An a b s o lu te minimum w id th o f g u t t e r ! ! i s one f o o t w ith a d e s i r a b l e minimum of two f e e t . A t ! i n t e r s e c t i o n s and on th e o u ts id e o f s u p e re le v a te d c u rv e s , a d d i t i o n a l w id th s o f g u t t e r w i l l be r e q u i r e d . P l a t s t r e e t g ra d e s w i l l r e q u ir e w id er g u t t e r s th a n s te e p e r g ra d e s as th e c a p a c ity w i l l be l e s s and th e w ate r w i l l have a ten d en cy to sp re a d o u t o n to th e a s p h a lt p av in g a d ja c e n t to th e c o n c re te g u t t e r . C o n crete c ro s s g u t t e r sh o u ld be p ro v id e d a t a l l s t r e e t i n t e r s e c t i o n s where w a te r i s allo w ed to flow a c ro s s th e i n t e r s e c t i o n . The c o n s ta n t 1 61 d e t i o r a t i o n o f th e a s p h a lt s u r fa c e from lawn w a te r i s one o f th e m ost im p o rta n t re a so n s f o r t h e i r c o n s tr u c tio n . I n r e c e n t y e a rs , a s l o t t e d g u t t e r a p p ro x im a te ly two in c h e s deep and two in c h es wide c o n s tr u c te d w ith a rounded bottom has been u t i l i z e d v ery e f f e c t i v e l y . This p e rm its w a te r to c ro s s th e i n t e r s e c t i o n and 'allow s v e h ic le s to p a s s th ro u g h th e i n t e r s e c t i o n w ith o u t any c o n ta c t w ith th e flo w in g w a te r. I n norm al c ro s s g u t t e r s th e v e h ic le w i l l s p la s h w ate r on th e a d ja c e n t a s p h a lt pavement as w e ll as c a r r y a sm a ll amount on th e t i r e s . Both o f th e s e i r r i t a t i o n s may seem s l i g h t b u t they d e f i n i t e l y c o n tr ib u te to th e d e t i o r a t i o n o f th e a s p h a lt pavem ent u n le s s e x tra wide c o n c re te g u t t e r s a re u se d . i I The prim e d isa d v a n ta g e o f a s l o t t e d g u t t e r i s th e h a z a rd s I j in v o lv e d from a p e d e s tr ia n tu r n in g h i s an k le In them o r i ! a c h i ld on a b ic y c le ru n n in g a w heel in t o i t and c a u sin g j i an a c c id e n t. However, th e y can be used e f f e c t i v e l y j I where p e d e s tr ia n t r a f f i c i s n o t a n t i c i p a t e d in any g r e a t j numbers and where th e norm al p a th o f b ie y c le s would be f a t r i g h t a n g le s to th e s l o t t e d g u t t e r . C o n d itio n s s h o u ld 1 | ! j be th o ro u g h ly in v e s t i g a t e d b e fo re in c lu d in g them i n th e j i | d e s ig n o f any improvement as th e c i t y co u ld be in v o lv e d j j I i i in a la w s u it f o r damage c la im s . I I j i • ! I U niversity of S o u th e rn -California Library

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