mc 73-1980

GEOMETRIC DESIGNSTANDARDS

FORRURAL (NON-URBAN)

HIGHWAYS

PublishedbyTHE INDIAN ROADS CONGRESS

Jsmuag*rHouse, ShahjsbsaRod,New DeIhI4lOOll

1990

PriceRs48/-(PlusPa~klng& Postage’~

IRC ~73-I98O

First published October1980.Reprinted : June 1990Reprinted: February,2000

(Rj~I,iiofP~ibUcafionand of Thm~rlal1onare ruerped)

PrintedatDEE KAY PRINTERS,NewDelhi-110015(500copies)

IRC: 73400$

CONTENTS

1. introductIon I

2. Scope .~ 2

3. ClassificatIonof Non-UtbanRoads ., 2

4. Terrain Classification 0 3

~ DesignSpeed ‘~ 3

6 ‘Cross-SectionalElements S

7. DesIgnTraffic and Capacity ... 13

B. SightDIstance .~ 14

9. HorIzontalAlignment .,, 19

10. Verticsl Alignment ... 32

11. Co~ordinatIonof Horizontal and VerticalAlignments ,,, 37

12. Lateral and Vertical CI~arancesat Underpasses ~.. 37

IRC: 73-1980

LIST OF TABLES

Table PageNo.

1, Terrain Classification ,,, 3

2, Design Speeds ,, 4

3, RecommendedLand Width for DifferentClassesof Road $

4. RecommendedStandardsfor Building Linesand ControlLines ,,, 6

5, Width of Roadwayfor Single-i..aneandTwo-LaneRoadsin PlainandRolling Terrain ,,, 8

6, Width of Roadwayfor Single-Laneandtwo-LaneRoadsin MountainousandSteepTerrain ,,, 9

7. Width of Carriageway ,,, 11

8, Camher/CrossfailValues forDifferent RoadSurfaceTypes,,, 12

9, EquivalencyFactorsfor Different Typesof Vehicles ,,, 13

10. Capacityof Different Typesof Roads ,,, 1411, StoppingSight Distancefor VariousSpeeds ,,. 15

12, OvertakingSight Distancefor VariousSpeeds ,,, 1613. IntermediateSight Distancefor Various Speeds ,,, 17

14, Criteriafor MiasuringSight Distance ... 18

15!, Radii beyondwhich Supercievationis not Required ,,, 21

16, Minimum Radii of Horizontal Curves forDifferentTerrain Conditions ,,, 24

17, Minimum TransitionLengthsfor DifferentSpeedsand

Curve Radii .. 26IS, ExtraWidth of Pavementat HorizontalCurves ‘ 2819, Gradientsfor Roadsin Different Terrains ,,, 33

20., Minimum Length of Vertical Curves ,~. 34

LIST OP FIGURES

IR,C: 73-1980

Plate Page

No.

1, SuperelevatlonRatesfor VarloásDeign Speeds

2., SchematicDiagramsShowingDifferent Method!ofAttaining Superelevatlon

3. Length&f SummitCurvefor StoppingSight Distance

4, Length of SummitCurve for intermediateSight Distance

5. Length of Summitcurve for OvertakingSight Distance

6, Lengthof Valley Curve

7, TMSketchesillustratingGood and Bad AlignmentCo-ordlnitlon

Fig. No, Page

7

27

30

31

1, Road Land Boundary,Building Linesand ControlLines

2, Elementsof a Combined CircularandTraniition Curve

3’ Visibility at HorizontalCurves

4, Minimum Set-beck DistanceRequiredat Horizontalcurvesfor Safe Stopping Sight Distance

LIST OF PLATES

39

41

4345

4749

51

IRC: 73-19*0MEMBERS OF THE SPECIFICATIONS & STANDARDS COMMITTEE

1. 3.5. Marya(Chairman)

2. R.P. Slkka(Member-Secretary)

3. QazIMobd. Afral4, R.C. Arora5. R.T. Mr.6. M.K. Cbsuerjee

7. B.C. Chandraeekharan8. M.G. Dandavate9. 3. Datt

10. Dr. H.P. Dhlr

11. Dr. R.K. Ghosh

12, BR. Govind13, iC. Gupta14. S,A. Hoda

15. M.B. Jeyawant16. D.R. Kobli

17. S.B. Kulkarni18. P.K. Lauria19. HC, Maihotra20. M,R. Malya21. 0. Muthachen22. K. Sunder Naik23, K.K. Nambiar

24. T,K. Nalarajan

25, M.D. Patel

26. Satish Prasad

27. S,K. Samaddar28. Dr. O.S.Sahgal29. N. Sen30. D. Ajitha Simha

31. Maj. Geni. 3.5. Soin32. Dr. N.S. Srinivasan

33, Dr. Bh. Subbaraju34. Prof. C,G. Swaminatban35, MIss P.K. Thressla36. The Director

(Prof. G.M. Andavan)

Director General(Road Development)& AddI. Sees’,to the Govt. or India, Ministry of Shipping &Transportchi.r Engineer (Roads), Mlnhtry of Shipping &Tnu*spodDevelopment Commissioner,Jammu & KashmirND,S.E. Part I, NewDelhiSecretaryto the Govt. of Maharashtra, PW & H Deptt,Chief ExecutiveOfficer, West Bengal Industrial infra.Structure DevelopmentCorpn.Chief Engineer, Parnban Bridge Project MadrasEngineer, ConcreteAssociationof IndiaChief Engineer (Rctd.), Greater Kailash, New Delhi-110048Deputy Director & Head, Roads Division, CentralRoad ResearchinstituteDeputy Director& Head Rigid and SemiRigid Pave-mentsDivision, Central ~oad ResearchInstituteDirector of Designs,Engineer-in-Chief’s Branch, AHQEngineer-in-Chief, Haryana P.W.D., B & RProject Manager-cum-Managing Director, Bihar StateBridge Construction Corporation Ltd.Synthetic Asphalts, 24, Carter Road, Bombay-400050Manager, Electronics Data Processing,BharatPetroleum Corporation Ltd.Manager (Asphalt), Indian Oil Corporation Ltd.AddI. Chief Engineer(N.H.), Rajasthan P.W,D.Englneer-in-Chief& Secy.to the Govt., H.P. P.W,D.DevelopmentManager, Gammon India Ltd., BombayPoomkavil House, P.O. Punalur (Kerala)Chief Engineer(R.etd.), Indranagar Bangalore“Ramanalaya”, ii, FirstCrescentPark Road, Gandhi-nagar, Adyar, Madras-600020Deputy Director & Head, Soil Mechanics Division,CentralRoad ResearchInstituteSecretaryto the Govt. of GujaratBuildings andCommunication DepartmentManager, Indian 011 CorporationChief Project Administrator, Hoogbly River BridgeCommissioners,CalcuttaPrincipal,PunjabEngineering College,ChandlgarhChief Engineer (Retd.), 12, Chltranjan Park, NewDelhi-i 10019Director (Civil Engineering), Indian Stan~ardsInsti -tutionDirector General Border RoadsChief Executive, National Traffic Planning & Automa-tion CentreSri Ramapuram,Bhimavaram-534202(A P.)Director1 Central RoadResearchinstituteChief Engineer (Construction), KeralaHighways ResearchStation, Madras

IRC: 73-1980

GEOMETRIC DESIGN STANDARDS FORRURAL (NON-URBAN) HIGHWAYS

I. INTRODUCTION

1.1. “Geometric design” dealswith the visible elements ofa highway. Sound geometric design results in economicalopera-tion of vehiclesand ensuressafety.

1.2. The Specification! and Standards Committee of theIndian Roads Congresshad previously published a few Papers ongeometric aspectsof design. The first Paper entitled: “Horizontaland Transition Curves for Highways” appeared in the I.R.C. Journalin 1947. This wasfollowed by two other Papers on “Sight Distanceand Vertical Curves” in 1950 and 1952 respectively. For manyyears,thesePapers servedas a guide for design of highways in thiscountry. Later, in, 1966, sonic important extracts from thesePapers were published bythe Congressunder tile title “Geometricsof Roads”.

1.3. Fcllowing the adoption of metric system, there was aneed to revise this publication with suitable modifications in thelight of other standards brought out by the LR.C. in the interveningperiod as also more recent practices round the world. To fulfilthis need, a new draft was prepared in the LR.C. Secretariat byL.R. Kaciiyali and A.K. Bhattacharya. This was reviewed andmodified by ~a Working Group set up by the Specifications andStandards Committee consisting of:

Dr. H.P. DhirILP. SlkkaA.K. Bbattacharya

1.4. The modified draft was approved by the Specificationsand Standards Committeein their meeting held on ~6thMay, 1977.It waslater approved by the ExecutiveCommittee through circula-tion and then by the Council of the Indian Roads Con;ress Intheir 93rd meetingheldon the 3rd June, 1978subject to certain modisfications which were left to a Working Group comprising Prof. C.G,Swaminathan, R.C. Singh, Col. Avtar S~ngh,R.P, Sikka and P.C.Bhasin, Secretary IRC, The final modification and editing of the

I

IPC: 73-1980

text wasdonejointly by R.P,Sikka,Member-Secretary,Specificationsand Standards Committee and K. Arunachalam.

2. SCOPE

2.1. The publication is basedprimarily on existing standardsand reco~nmendationsof the Indian RoadsCongress,with suitablemodifications and additions in the light of current engineering prac-tice. The standards prescribed are essentially advisory in naturebut may be relaxed somewhat in very difficult situationsif conside-redjudicious. Effort in generalshould,however,beto aim at stand-ards higher than the minimum indicated,

2,2. The text dealswith geometric dçsignstandards for ruralhighways5~, i.e. non-urban roads located predominantly in opencountry outside the built-up area. The alignment may howeverpassthrough isolated stretches of’ built-up nature as long as charac-ter of the road as a whole doesnot change. The standard is notapplicable to urban roads or city streets. It is also not applicableto expressways, Geometric design elementsof road intersectionsare not consideredin the standard either,

2.3. The geometric features of a highway except cross•-sectional elements do not lend to stageconstruction. Geometricdefitcienciesare costly and sometimes impossible to rectify later ondue to the subsequent roadside development, Therefore, it isessential that geometric requirementsshouldbe kept in view rightIn the beginning.

3. CLASSIFICATION OP NON-URB AN ROADS3.1. Non-urban roads in India are classifiedinto five cate-

gories:(I) National Highways

(Ii) StateHighways(ill) Major District RoadsiLv) Other District Roads(v) Village Roads

‘5Thcscshould not be confusedwith Rural Roads which refer commonly toOther District RoadsandVillage Roads. While geometric design elementsofRural Roids areduly covered in this publication alongwith roids of highercategory,morecomprehensiveguidanceaboutdifferentfacets of deslgn andconstructionof the Rural Roadscan be had from the IRC SpecialPublicationNo. 20, “Minual on Route Location, Design,ConstructionandMaintenanceof Rural Road~(Other District Roads andVillage Roads)”.

2

IRC: 73-1980

3.2. NationalHighwaysare mainhighways running throughthe lengthand breadthof thecountryconnectingmajor ports,foreignhighways, Statecapitals, largeindustrial and tourist centresetc.

3.3. State Highways are arterial routes of aStatelinkingdistrict headquartersandimportantcities within the State and con-nectingthem withNationalHighways or highwaysof the neighbour-ing States.

3,4. Major District Roads are important roadswithin adistrict servingareasof production and markets, nnd connectingthesewith eachother or with the main highways.

3.5. OtherDistrict Roadsare roads serving rural areas ofproduction andprovidingthem with outlet to marketcentres,taluka/telisii headquarters,block developmentheadquarters,or othermainroads.

3.6. Village Roadsare roadsconnecting villages or groupsof villages with eachother andto the nearestroadof a highercate-gory.

4. TERRAiN CLASSIFICATION

4,1. The geometricdesignof a highway is influenced signi-ficantly by terrain conditions. Economydictateschoice of differentstandardsfor different typesof terrain. Terrain is classified by thegeneralslopeof the countryacross thehighwayalignment,for whichthe criteria given in Table I shouldbe followed. While classifyinga terrain, short isolated stretchesof varying terrain should not betaken into consideration.

TABLE 1. TEMAIN CLA5STFICATION

S. No. Terrain classification Per cent cross slopeof the country

1. Plain 0—102. RoIling 10—253, Mountainous 25—60

4, Steep Greater than60

5. DESIGN SPEED

5.1. Choiceof designspeeddependson the function of theroad as also.terrainconditions. It is the basicparameterwhichdeterminesall other geometric designfeatures. Design speeds forvariousclassesof roadsshould be as given in Table 2.

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5.2. Normally “ruling designspeed” should be the guidingcriterion for correlating the various geometric design features,“Minimum design speed” may, however, be adoptedin sectionswhere site conditions,includingcosts,do not permit a designbasedo~the “ruling designspeed”.

5.3. The designspeed should preferably be uniform alonga given highway. But variations in terrain may make changesinspee4unavoidable, Wherethis is so, it is desirablethat the designspeed should not be changed abruptly, but in a gradual mannerby introducingsuccessivesections of increasing/decreasingdesignspeed so that the roadusers get conditioned to the change bydegrees.

6. CROsS-SECTIONAL ELEMENTS

6.1. Road Land, BuI3ding Lines andControl Lines6.1.1. Road land width (also termedthe right-of-way) is the

land acquired for road purposes.Desirablelandwidth for differentclassesof roadsIs indicatedin Table 3.

TABLE 3, RECOMMENDED LANE) WIDm FOR DIPPSRENTCLASSES OP ROAD

(metres)

S.No,

Roadclassification

Plainand rolling terrain Mountainousandsteepterrain

Openareas Built-up areas

Norma~jRange Normal Range

up areas

Normal Normal

1. National and 45 30.60 30 30-60 24 20StateHighways

2. Major District 25 25-30 20 15.25 18 15Roads

3. OtherDistrict 15 15-25 15 15-20 15 12Roads

4. Village Roads 12 12-18 10 10-15 9 9

6.1.2. In high banks or deepcuts, the laud width should besuitably increased. Similarly, a higher value should b~adoptedin unstable or landslide-prone areas. The need for a wider right-of-way at important road intersections should also be kept in view.

5

IRC: 73-1980

6.1.3. If a road is expected to be upgraded to a higherclassification in the foreseeable future, the land width shouldcorrespond to the latter.

6.1.4. In order to prevehtovercrowding and preservesufficient space for future roadimprovement,it is advisableto laydown restrictions on building activity along the roads. Buildingactivity should not be allowed within a prescribeddistancefromthe road, which is defined by a hypothetical line set back from theroad boundary andcalled the “Building Line”. In addition, itwill be desirable to exercise control on the nature of buildingactivity for a further distancebeyond the building line uptowhatare,known as the “Control Lines”. Building and control linesare illustratedin Fig. 1 with respect to the road centre Iin.e androad boundary.

6.1.5. Recommendedstandardsfor building andcontrol linesaregiven in Table4. For more details about measuresfor preventing

TABLE 4, RECOMMENDED STANDARDS rort BUSLDTNO LINF,s AND CONTROL LiNES

Plain and rolling terrain MountaInou~and

Openareas Built-up areas

Roadclassification Overall Overall Distance Distancebetween

width. width betweenBuild- Building Line androad~vt.’~Cfl between ing Line and boundary (set-back)~Ul - Control road boundary

~ies Lines (set-back)(metres) (metres) (metres) (metres)

2 5~ 6

1. Nationaland 80 150 3.6 3..5 3.5StateHighways

2, Major District 50 100 3-5 3..5 3•.3Roads

3. OtherDistrict 25/30~ 35 3.5 3.5 35Roads

4. Village Roads 25 30 3-5 3-5 3..5

Notes: 1. 1f the land width is equalto the width between buildinglinesindicated in this column, the building lines should be set-back 2.5 m fromthe road land boundary.

2. SeeFig. 1 for positionof building lines,control lines and set-back distancereiativeto the roadcentreline and road landboundary.

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ribbon development along roads, reference may be made toIRC Special Publication No, 15, “Ribbon Development alongHighways and its Prevention”, also IRC 62-1976 “Guidelines forControl of Access on Highways”.

6.2. Roadway Width

6.2.1. Roadway width for single-lane and two-lane rGads Inplain and rolling terrain: The width of roadway for single and two-lane roadsin plain and rolling terrainshould be as given in Table 5.

TABUl 5 WIDTH OF ROADWAY ~OR SINGLE-LANE AND TWO-LANEROADS iN PLAIN AND ROLLING TERRAIN

S. No, Roadclassiikation Roadwaywidth(metres)

1. NatIonal Highways and State(single or two lanes)

Highways12.0

2. MaJor bistrict Roads(single or two lanes) 9.0

3. OtherDistrict Roads(i) single lane(Ii) two lanes

7,59,0

4. Village Roads(singlelane)

75

Note: in caseof State Highways having single-lane pavement, thewidLh Ofroadway might be reduced to 9 m if the possibility of widening thecarriage~ay to two lanes is consideredremote

6.2.2. Width of Roadway for sin%le-lane and two-lane roads Inmountainous and steep terrain: The width of roadway,exclusiveofside drainsand parapets,for singleand two-lane roadsin mounta-inousandsteepterrain shouldbeas indicatedin Table6. In certaincases,passingplacesmay be requiredin addition, seepam 6.2.3.

6.2.3. Passing places for roads in mountainous and steepterrain: Passingplacesor lay-byes should be provided on singlelane roads in mountainous and steep terrain to cater to thefollowing requirements:

(a) To facilitate crossing of vehicles approaching from oppositedirection; and

(b) To tow asidea disabled vehicle so that it does not obstructthetraffic,

8

IRC : 73-1980

TABLE 6. Winm OF ROADWAY FOR SiNGLE-LANE AND TWO-LANEROADS IN MOUNTAINOUS AND STEEP TERRA?N

S. No.) Roadc1assificat~on Roadway width(metres)

1. National Highwaysand State Highways

(1) singlelane(ii) two lanes

6.258.8

2. Major District Roads and OtherDistrict Roads (single lane) 4.75

3. Village Roads (single lane) 4.0

Notes: (I) The roadway widths given above are exclusive o(parapets(usualwidth 0.6 m) andsidedrains(usualwidth 0.6 m).

(2) The roadway widths for Village Roadsareon thebasisof asinglelanecarriageway of 3 m. If a higher pavementwidth is adopted,the roadwaywidth should beincreasedcorrespondingly.

(3) In hardrock stretches,or unstablelocationswhereexcessivecuttingmight leadto slope failure, width of roadway may be reducedby0.8 m on two-laneroadsand0,4 m in othercases. However, wheresuchstretchesoccurin continuous long length, reductionin road~way width should not be effected unless requisite passing placesvide pam 6.2,3 areprovided.

(4) On horizontal curves, the roadway width should be increasedcorrespondingto the extra widening of carriagewayfor curvaturevide pam 9.6.

(5) On roadssubjectto heavy snowfall,whereregularsnowclearanceisdoneoverlong perioth to keepthe road opento traffic, roadwaywidth may be increasedby 1,5 m for MDR5, ODRs,andVRs.

Passingplaces are not necessaryon two-laneNationalandState Highwayshaving roadwaywidth in accordancewith Table 6.But on singlelane sections having narrower roadway, it may bedesirableto provide somepassingplacesdependingon actualneeds.On otherroads,theseshouldbe provided in generalat the rate of2-3 per kilometre. Their exact location should be judiciously deter-mined taking into considerationthe available extrawidth on curvesand visibility.

Normally the passing places/lay-byes should be 3.75m wide,30 m long on the inside edge (i.e. towards the carriagewayside),and 20 m long on the fartherside.

9

IRC : 73-1980

6.2.4, Roadwaywidth for multi-lane highways: For multi-Janehighways,roadwaywidth should be adequatefor the requisitenumber of traffic lanes, besides shoulders and central median,Width of shouldersshould in genera]be 2.5 metres. For width ofcarriagewayandmedian, referencemay be madeto paras6.4 and6.6 respectively.

6.3 RoadwayWidth at Cross-DrainageStructures

63.1. General: Cross-drainagestructures are difficult towiden at a later stage. As such, the roadway width for themshould be decidedverycarefully at the planning stage itself. Theminimum recommendedvaluesin this regard are given in paras63,2 and6.3.3. For roadsbeing built to lower standardsinitiallyfor some reason, or those which are expected to be upgradcd/widened in the foreseeablefuture, it will be desirable to go in for ahigher roadway width at the cross-drainage structures right in thebeginning.

6,3.2. Culverts (upto 6 m span): In plain and rolling terrain,the overall width on culverts (measuredfrom outsideto outsideof theparapet walls) should equal the normal roadway width given inTable 5. In mountainousor steepterrain, the clear roadwaywidthavailableon the culverts (measuredfrom inside to Insideofparapetwalls or kerbs)shouldbe as below:

All roadsotherthan Village Roads ., As given in Table6

Village Roadsminimum ... As given In Table 6

desirable ... 4.25 m

6.3.3. BrIdges (greater than 6 m span): At bridges, the clearwidth of roadway between kerbs should be as under:

Single-lanebridge .,. 4.25 m

Two-lane bridge .,, 7.5 m

Multi-lane bridge ... 3.5 m per lane plus

0.5 mfor each carriageway

At causewaysand submersiblebridges,the minimumwidth ofroadway (between kerbs) shouldbe 7.5 m, unless the width isspecially reduced by the competent authority.

Where a footpath ig provided for the use of pedestrians,itswidth should not be less than 1.5 m.

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IRC : 73-1980

6.4. Width of Carriageway

6.4.1. The standardwidth of carriagewayshall be as indicat-ed in Table 7. The total width should be determinedin relationto the design traffic and capacityof the roadway,seeSection 7.

TABLE 7, WIDTH OF CARRIAGEWAY

Width of carriageway (metres)

Singlelane

Two laneraised

s withoutkerbs

Two Iaraised

nes withkerbs

Multi-lanwidth

e pavements,per lane

3.75~~ 7.0 7.5 3.5

Notes: I, **On Village Roads, the carriageway width may be restricted to3.0 m normally. Widths greaterthan 3.0 m may howeverbeadoptedjudiciously, dependingon the typeand intensityof traffic, cost andrelated factors.

2. Excepton important trunk routes, an inteimediatecarrEagewaywidth of 5.5 metres may also be adopted instead of regulartwolanes if the sameis consideredadvantageous.

6.4.2. Wherethe carriageway width changes,e.g. from singlelaneto two lanesor two lanes to four lanes,the transitionshouldbe effectedthrougha taperof 1 in 15 to I in 20.

6.5. Shoulder Width

The width ofshoulders for each class of highway can hedirectlyobtainedusingTables5, 6 and 7. Shoulder width will heone-halfthe difference betweenthe roadway width (Table 5 or 6)and carriagewaywidth (Table7).

6.6. MedIan Width

6.6.1. Mediansshould be as wide as possible,but their widthis oftenrestrictedby economicconsiderations~Minimum desirablewidth of medianson rural highways is 5 metres,but this could bereduced to 3 metres where land is restricted. On long bridges andviaducts,the width of medianmay bereducedto 1.5 meters, but inany casethis should not be less than 1.2 m.

6.6.2. As far as possible, the medianshouldbe of uniformwidth in a particular section of the highway. However, wherechangesare unavoidable,atransition of tin iS to I in 20 mustbeprovided.

11

IRC 73-1980

6.6,3. In rolling and hilly country, the medianwidth will bedictatedby topographyandthe individual carriagewayscould be atdifferent levels.

6.7, PavementCamber or Crossfall

6.7.1. The camber or crossfallon straightsectionsof roadsshould be as recommendedin Table 8 for varioustypesof surfaces.Fora given surface type, the steepervalues in the Tablemay beadoptedin areas having high intensity of’ rainfall and the lowervalueswherethe intensityof rainfall is low.

TABLr 8. CAMIiER/CROSSFALL VA1,urs ron flTrrERn~.rrROAD SURFACE T~res

SNo.

1,

SLIrfACC type Camber/crossfall

High typebituminoussurfacingor cementconcrete

1.7-2.0per cent(1 in 60 to 1 in 50)

2. Thin bituminoussurfacing 2.0-2.5per cent(1 in 50 to 1 in 40)

3. Water bound macadam,gravel 2.5-3.0 percent(1 in 40 to 1 in 33)

4. Earth 3.0-4.0per cent(1 in 33 to 1 in25)

6.7.2. Generally, undivided roads on straightsshould beprovided with a crown in the middleand surface on either sidesloping towardsthe edge. Howeveron hill roads this may not bepossiblein every situation, particularly in reacheswith a windingalignment where straightsectionsare few and far between. In suchcases,discretionmaybe exercisedand instead of normal camberthe carriageway may be given a uni-directionalcrossfall towardsthehill side having regard to factorssuchasthe directionof supereleva-tion at the flanking horizontalcurves,easeof drainage, problemoferosionof the down.’hill faceetc.

6.7.3. On divided roads, i.e. dual carriagewayshaving amedian,it is usualto havea uni-directionalcrossfallfor eachcarri-agewaysloping towardsthe outeredge.

6.8. Crossfall for Shoulders

6.8.1. The crossfall for earth shouldersshould beat least0.5 p~rcent steeper than the slopeof the pavementsubject to a.minimumof 3 per cent.

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IRC: 73-1980

6.8.2. If the shouldersarepaved,a crossfallappropriate tothe typeof surfaceshouldbe selectedwith referenceto Table 8.

6.8.3. On superelevatedsections, the shouldersshouldnor-mally havethe samecrossfallas the pavement.

7. DESIGN TRAPFIC AND CAPACITY

7.1. The width oç carriageway should be sufficient for thedesigntra1~c,i.e. traffic expected on the roadin the design year.Design traffic will dependon the rate of growth of traffic, tWedesignperiod,importanceof roadin the system,nature of roadsidedevelopmentetc. For making capacity computationsunder mixedtraffi,ó conditions, the different typesof vehiclesshouldbe conver-tedto a common unit known as ‘passengercar unit’ by multiplyingtheir number with relevant equivalencyfactors. Tentativevalues~f equivalency factors are given in Table 9. These aremeantforuse in open sections in plain terrain away from intersections. Formoredetails in this respect,referencemay be madeto IRC:64-1976“TentativeGuidelineson Capacityof Roadsin Rural Areas,”

TAILS 9. EQUTYALENCY FACTOR9 ma DtEEERENT Ty~isor V~tHtcLrs

S.No. ( Vehicle type ( Equivalency factor

1. Passengercar, tempo,auto-rickshaw,or agriculturaltractor

10

2. Cycle, motorcycle or scooter 0.5

3. Truck, bus,oragricultural tractbr-trailerunit

3.0

4. Cycle rIckshaw 1.5

5. Morse-drawnvehicle 4.0

6. Bullock cart*S 8,0

‘For smallerbullock-carts,a valueor 6 wIll be appropriate.

7,2. For purposesof design,the capacity of different types ofroads may be taken asgiven in Table 10.

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IRC : 73-1980

TABLE 10. CAPACITY OF DIFFERENT Types OF ROADS

S. No. Typeof roadCapacity

(Passengercarunits perday in both directions)

1. Single-lane roadshavinga 3,75 m wide earn-agcwaywith normalearthen shoulders 1,000

2, Single-laneroadshaving a 3.75m wide carri-ageway with adequately designed hardshoulders1.0 m wide 2,500

3. Two-laneroadshavinga 7 m wide carriage-way with normalearthenshoulders 10,000

4. Roadsof intermediatewidth, i.e having acarriageway of 5.5 metres with normalearthenshoulders 5,000

Note: Capacityof highwayshavinga dual carriagewaywill dependon factorslike thedirectionalsplit of traffic, degreeof accesscontrol,compositionof traffic etc. Depending on the actual conditions, capacityof a 4-lane divided highway could be upto 20,000-30,000pcus.

7.3 The standardsin Table 10 are applicable wherethe visibi-lity is unrestricted and there are no lateral obstructions within1.75 m from the edgeof pavement. Thesealso presumethat only anominal amount of animal drawn vehicles(say 5-10 percent) arepresentin the traffic streamduringthe peakhour. For more details,referencemaybe madeto !RC:64-1976.

8. SIGHT DISTANCE

8.1. General8.1.1. Visibility is an important requirementfor the safety of

travel on highways. For this, it is necessary that sight distanceofadequatelength should be available in different situationsto permitdrivers enoughtime anddistanceto control their vehicles so thatthereareno unwarrantedaccidents.

8.1.2, Threetypesof sight distancel~*are relevantinsofar asthe designof summit vertical curves and visibility at the horizontalcurves: StoppingSight Distance; Overtaking Sight Distance; andIntermediateSight Distance. Standardsfor thesearegiven in paras8.2 tO 8.4; andthe generalprinciplesof their application in para8.5.Criteria for measurementof the sight distnncesare set forth inpara 8.6. Application of the sight distancerequirements at horizon-tal cu~vesis discussedin para9.7.

~These aredealtwith in greaterdetail in IRC~66-i976“RecommendedPracticefor Sight Distanceon Rural Highways”.

14

..—~.-—. .. ‘I ..~._• —..—.-. V

IRC: 73-1980

8.1.3. For valley curves, the design is governed by nightvisibility which is reckoned in terms of the Headlight Sight Distance.This is the distance ahead of the vehicle illuminated by the head-

lights which is within the view of the driver. Standards for head-light sightdistancearegiven in para 8,7.

8.2. StoppingSight Distance

8.2.1. Stopping sight distance is the cleardistanceaheadneededby a driver to bring his vehicle to a stop before meeting astationaryobjectin his path. Minimum stopping sight distanceisgiven by the sum of: (1) distance travelled during the perceptionand brakereactiontime and (ii) the braking distance. Minimumdesignvaluesof stoppingdistancefor different vehicle speeds areshownin Table 11. Thesearebasedon perceptionandbrake-reac-tion time of 2.5 secondsandcoefficient of longitudinal friction vary-ing from 0.40 at 20 km/hto 0.35 at 100 km/h. For application ofTable 11,the speedchosenshouldbe the sameas the design speedof the road.

TABLE 11. STOPPING SIGHT DISTANCE FOR VARIOUS SPEEDS

Speed Braking

Distance Coefficient Distance RoundedV

(km/h)Time, t(sec.)

(metres)d1=0.278V:

of longitu-dinal fric-tion (f)

(metres)d ~“~ 254f

Calculatedvaluesd~±d5

off valuesfor

design

20253045608090

120180

20 2.525 2.530 2.540 2.550 2.560 2.565 2.580 2.5

100 2.5

14 0.4018 0.4021 0.4028 0.3835 0.3742 0.3645 0.3656 0.3570 0.35

4 186 249 30

17 4527 6239 8146 9172 118

112 182

8.3. OvertakIngSight Distance

8,3.1. Overtaking sight distance is the minimum sight distance

that should be available to a driver on a two-way road to enable15

IRC 73-1980

him to overtake another vehicle safely. Optimum condition fordesign is one in which the overtakingdriver canfollow the vehicleaheadfor a short time while he assesseshis chancesfor overtaking,pulls out his vehicle,overtakesthe other vehicle atdesign speedofthe highway,and returnsto his own sideof the roadbefore meetinganyoncomingvehiclefrom the opposite direction travelling at thesamespeed.

8.3,2. Designvaluesfor overtakingsight distance are givenin Table 12. Thesearebased on a time componentof 9 to 14secondsfor the actualovertakingmanoeuvre depending on designspeed,increasedby about2/3rd to take into account the distancetravelled by a vehiclefrom the oppositedirection during the sametime.

TABLE 12. OVERTAJaNG SIGHT DISTANCE POR VARiOUS SPEEDS

40 9 6 15 165

7 23550 10 17

60 10.8 7.2 18 300

65 11.5 7.5 19 340

80 12.5 8.5 21 470

100 14 9 23 640

8.4. IntermediateSight Distance

8.4.1. Intermediatesight distanceis definedas twice the safestoppingsight distance. it is the experiencethat intermediatesightdistanceaffords reasonableopportunitiesto driversto overtakewithcaution.

8.4.2. Designvaluesof intermediatesight distancefor differ-ent speedsaregiven in Table 13.

16

IRC: 73-1980

TABLE 13. INTERMEDIATE Siotit DISTANCE FOR VARIOUS SPEEDS

Speedkm/h

Intermediatesightdistance(metres)

20 4025 5030 6035 8040 9050 12060 16065 18080 240

100 360

8.5. ApplIcation of Sight DistanceStandards

Single/two-laneroads

8.5.1. Normally the attemptshould beto provide overtakingsight distancein as much length of the road as possible. Wherethis is not feasible, intermediate sight distance, which affordsreasonableopportunitiesfor overtaking,shouldbe adoptedas thenext bestalternative,In no casehowevershouldthe visibility corres-pond to lçss than the safe stopping distance which is the basicminimum for any road.

8.5.2. No hard and fast rule can be laid down for theapplicationof overtakin~sightdistance since this will depend onsite cot~ditions,economicsetc. It will begood,engineeringpracticehoweverto useovertakingsight distance in the case of followingsituations:

(I) Straight sections of road with Isolated overbrldges or summItvertical cun’es where the provision of overtaking sight distancewould convenientlyresult In unobstructedvisibility over a longlength of the road;and

(11) relativelyeasysectionsof terrainadjacentto long reachesaffordingno opportunItyfor overtaking at all, e.g. on either sidc of a wIndingroadIn hlllyjrolllng terrain.

Divided highways8.5.3, On divided highways, i.e. dual carriagewayshavinga

centralmedian,the design shouldcorrespondat least to stopping

17

1RC~13-1980

sight distance vide Table II. It will be desirable, though, foroperationalconvenienceandbetter appearanceof the highway todesignfor somewhatmoreliberal values,say upto twice the valuesgiven in Table~Il,

Undividedfour-lane highways

8.5.4. On undivided 4-lane highways there are sufficientopportunities for overtaking within one half of the carriageway, andthereshouldbe no needto crossthe centreline unless the capacityof the road is grossly deficient. Such roads may, therefore, bedesigned on the lines of divided highways, i.e. vide para 8.5.3.

8.6. CriterIa for Measuring Sight Distance

Criteria for measuring the different types of sight distancediscussedaboveare given in Table 14.

TABLE 14. CRITERIA FOR MEASURING S1GIIT DISTANCE

No. , Sight distance Driver’s eye He ht of

1. Safestoppingsight distance i.2rn 0.15 m

2. Intermediatesight distance 1.2 m I. 2 m

3. Overtaking~ightdistance 1.2 rn 1. 2 m

8.7. HeadlIghtSight Distanceat Valley Curves

8.7.1. During day time, visibility is not a problem on valleycurves. Howeverfor night travel the designmust’ ensure that theroadwayaheadis illuminated by vehicle headlights to a sufficientlengthenablingthe vehicle to braketo a stop if necessary. Thisdistance,called the headlightsight distance,should at least equalthe safestoppingsight distancegiven in Table 11.

8.7.2. In designingvalley curves, the following criteria ofmeasurementshould be followed as regards the“headiight sightdistance:

(i) height of headlight aboveroadsurfaceis 0.75 m;(ii) theuseful beamof headlightis upto one degreeupwards from the

gradeof the road; and(iii) the height of object is nil.

18

IRC: 73-1980

9. HORiZONTAL ALIGNMENT

9.!. General

9.1.1. Uniformity of designstandardsisone of the essentialrequirementsof aroadalignment. In a given section, there mustbe consistentapplicationof a designelement to avoid unexpectedsituationsbeing createdfor the drivers, For instance,a short sharpcurve in an otherwisegood alignmentis boundto act as an acci-dent-pronespot if the designeris not vigilant. Similarly, any un-necessarybreakin horizontal alignmentatcross-drainagestructuresshouldbe avoided.

9.1.2. As a generalrule, the horizontal alignment should befluent andblend well with the surroundingtopography. A flowingline which conforms to natural contours is aestheticallyprefer-able to onewith long tangents slashing through the terrain. Thiswould not only help in limiting the damageto the environment butalso assistin preservationof naturalslopesandplant growth. Dueconsiderationshouldalso be given to the conservation of existingfeatures. This aspectis dealtwith at length in IRC Special Publi-cation No. 21-1979“Manual on Landscapingof Roads”.

9.1.3. Long tangentsectionsexceeding3 km in length shouldbe avoidedas far as possible. A curvilinear alignment with longcurves is betterfrom the pointof safetyand aesthetics.’

9.1.4. As a normal rule, sharpcurvesshouldnot be introducedat the end of long tangentssincethesecanbe extremelyhazardous.

9.1.5. Shortcurvesgive appearanceof kinks, particularly forsmall deflection angles, and should be avoided. The curves shouldbe sufficiently long and have suitable transitions to providepleasing appearance. Curve length shouldbe at least 150 metresfor a deflection angle of 5 degrees,andthis shouldbe increasedby30 metres for each one degree decreasein the deflection angle.For deflection angles less thanonedegree,no curve is requiredtobe designed.

9.1.6. Reversecurvesmaybe neededin difficult terrain. Itshould be ensuredthat there is sufficient length betweenthe twocurvesfor introductionof requisitetransitioncurves.

9.1.7. Curvesin the samedirectionsCparatedby short tan-gents,knownasbroken-backc~cves,shouldbe avoided as far aspossiblein the interestof aesth*esand safety and replaced by asingle curve, if this is not feasible,atangentlength corresponding

19

IRC 73-1980

to 10 secondstravel time mustat leastbe ensuredbetween the twocurves.

9.1,8. Compoundcurvesmay be usedin difficult topographybut only whenit is imponible to fit in a single circular curve. Toensuresafeand smoothtransitionfrom onecurveto the other, theradiusof the flatter curve should not be disproportional to theradiusof the sharpercurve. A ratio of 1.5 : I should be consideredthe limiting value.

9.1.9. To avoid distortions in appearance,the horizontalalignmentshould be co-ordinated carefully with the longitudinalprofile, keeping in mind that the road is a three-dimensionalentity anddoesnot consistsimply of aplan andL-section. Require-mentsin this regardare discussedin Section 11.

9.1.10. The siting of thebridgesandthe location of the ap-proachesshould beproperlyco-ordinatedkeepingin view theoveralltechnicalfeasibility, economy,fluencyof alignment and aesthetics.The following criteria maybe followed in general:

(i) For majorbridgesabove300 metresspan,propersiting ofthe bridge shouldbe the principal consideration and theapproachalignment matchedwith the same;

(ii) For smallbridgesless than 60 metresspan,fluencyof thealignment should governthe choiceof the bridge location;and

(iii) For spans between 60 and 300 metres, the designershould usehis discretionkeepingin view the importanceof the road,overall economicconsiderationsandaesthe-tics.

9.2. HorIzontalCones

9.2.1. In general1horizontalcurves should consist of acir-cularportion flanked by spiral transitions at both ends. Designspeed,superelevationand coefficientof sidefriction affect the designof circular curves. Length of transition curve is determined onthe basisof rateof changeof centrifugalaccelerationor the rate ofchangeof supérelevatlon.

9.3. Superelevation

9.3.1. DesIgn values: Superelevationrequiredon horizontalcurves should be calculated from the following formula. Thisassumesthat centrifugal force correspondingtq three-fourth the

20

IRC : 73-1980

designspeedis balancedby superelevationand rest cotrnterac-ted by sidefriction:

where

vse 2~rr’

e = superelevationin metreper metre,V = speedin km/h, andR = radiusin metres

Superelevationobtained from the above expression shouldhoweverbe kept limited to the following values:

(a) In plain and rolling terrain(b) In snow-bound areas(c) In hilly areas not bound by snow

Plate I indicates the superelevation for various design speeds

on this basis.

9.3.2, RadIi beyond which no iuperelevatlon Is required;Whenthe valueof the superelevationobtained vide para 9.3.1 i~lessthan the road camber,the normal camberedsection shouldbecontinuedon the curvedportionwithout providing any supereleva-tion. Table IS showsthe radii of horizontal curves for differentcamber rates beyond which superelevationwill not be required.

TASLE 15. RADII BEYoND WHICH SUPERELEVATXON IS NOT REQUIEm

Designspeed(km/h) 4 per cent

RadiuS(metres)for camberof

3 per cent 2.5per cent 2 per cent 1.7 per cent

20 50 60 70 90 10025 70 90 110 140 15030 100 130 160 200 24O35 140 180 220 270 32040 180 240 280 350 42050 280 370 430 550 63065 470 620 750 950 110080 700 950 1100 1400 1700

100 1100 1500 1800 2200 2600

7 per Cent7 percent

10 per cent

21

IRC 73-1980

9.3.3. Methods of attaining superelevatlon: The normalcamberedsectionof the road is changedinto superelevated sectionin two stages. First stageis the removal of adverse camber inouterhalf of the pavement. In the secondstage, superelevationisgraduallybuilt up over the full width of the carriageway so thatrequiredsuperelevationis available at the beginning of the circularcurve. Therearethreedifferentmethodsfor attaining the supere-levation~(i) revolvingpavementaboutthe centreline; (ii) revolvingpavementaboutthe inner edge;and (iii) revolving pavementaboutthe outer edge. Plate2 illustratesthesemethodsdiagrammatically.The small cross-sectionsat the bottomof eachdiagramindicatethepavementcrossslopecondition at differentpoints.

Eachof the abovemethodsis applicableunder differentcon-ditions. Method (i) which involves least distortion Qf the pavementwill be found suitable in mostof the situationswhere there are nophysical controls, and may be. adopted in the normal course.Method (ii) is preferablewherethe lower edge profile is a majorcontrol, e.g. on accountof drainage. Where overall appearanceisthe criterion, method(iii) is preferablesince the outer edge profilewhich is mostnoticeableto driversis not distorted.

The superelevationshouldbe attained graduallyover the fulllen~thof the transitioncurve so that the design superelevationisavailableat the starting point of the circular portion. Sketchesin Plate2 havebeendrawnon this basis. In caseswheretransitioncurvecannotfor somereasonbe provided, two-third superelevationmaybe attainedon the straightsectionbeforestart of the circularcurve andthe balanceone-third on the curve,

In developingthe required superelevation,it sho~sldbe en-suredthat the longitudinal slopeof the pavementedgecomparedtothe centre-line(i.e. the rate of change of superelevation)is notsteeperthan 1 in 150 for roadsin plain androlling terrain, andI in60 in mountainousandsteepterrain.

When cross-drainagestructuresfall on a horizontal curve,their deckshouldbesuperelevatedin thesamemannera~describedabove.

9.4. RadiI of Horizontal Curves

9.4.1. On ahorizontalcurve, the centrifugal force is balanc-ed by the combinedeffectsof superelevationand side friction. The

22

IRC: 73-1980

basicequationfor this conditionof equilibrium is:

V2

V2

or = in (e±f)

wherev = vehiclespeedin metreper second

V = vehiclespeedin km/h

g accelerationdueto gravity in metre persee2

e = superelevation ratio in metre per metre

f = coefficient of side friction betweenvehicle tyres andpavement(takenas 0.15)

ft = radiusin metres

Basedon this equation and the maximum permissible values ofsuperelevationgiven in para9.3.1. radii for horizontalcurvescorres-ponding to ruling minimum and absoluteminimum design speedsareshown in Table 16.

9.4.2. On new roads,horizontalcurvesshould be designedtohavethe largestpracticableradius, generallymorethan the valuescorrespondingto the ruling designspeed(seeTable 16). However,absoluteminimum valuesbasedon minimum design speed (Table16)might be resortedto if economics of construction or the siteconditions so dictate. While improvifl existing roads, curveshavingradii correspondingto absolute minimum standardsmaynot be flattenedunlessit is necessarytorealignthe road for someotherreasons.

9.5. TransitIonCurves

9.5.1. Transitioncurvesare necessaryfor a vehicle to havesmooth entry from a straight section into acircular curve. Thetransition curves also improve aestheticappearance of the roadbesidespermitting gradul application of the superelevation andextrawidening of carriagewayneeded at ,the horizontal curves,Spiral curveshouldbe usedfor this purpose.

9.5.2. Minimum ‘length of the transition curve should bedeterminedfrom the following two considerationsandthe largerofthe two valuesadoptedfor design.

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24

IRC: 73-1980

(i) The rateof changeof centrifugalaccelerationshould notcausediscomfortto drivers. Fromthis consideration,thelengthof transitioncurve is given by:

0.0215 V’L,— CR

whereL1 = lengthof transitionin metresV = speedin km/hft = radiusof circular curvein metres

80 (subjectto amaximumof 0.8 and75+V minimum of 0.5)

(ii) The rate of changeof superelevation(i.e. the longitádinalgrade developed at the pavementedge compared tothroughgradealongthecentreline) shouldbe such as notto causediscomfort to travellers or to make the roadappearunsightly. I~ateof change should not be steeperthan I in 150 for roads in plain androlling terrain, andI in 60 in mountainous/steepterrain. The formulaeforminimumlength of transitionon this basisare:

For Plain andRolling Terrain:

2.7V’

R

For Mountainousand SteepTerrain:

1.ov~R

9.5.3. Having regard to the aboveconsiderations,the mini-mum transition lengthsfor differentspeedsandcurveradii arC givenin Table 17.

9.5.4. The elementsof a combined circular and transitioncurvesareillustrated in Fig. 2. For derivingvaluesof the ii~dividuaLelementslike shift, tangentdistance,apexdistanceetc.and workingout coordinatesto lay the curvesin the field, it is convenientto usecurvetables. For this, referencemaybemadeto IRC: 38 “DesignTablesfor Horizontal Curves for Highways”.

9.6, Widening of Carriageway on Curves9.6.1. At sharphorizontalcurves, it is necessary to widen the

carriagewayto providefor safepassageof vehicles. The wideningrequired has two components: (i) mechanical widening to compen-

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27

IR’C 73-1980

satethe extra width occupied by a vehicle on the curve due totracking of the rear wheels,and (ii) psychologicalwidening’’to per’~.mit ‘easy crossing~ofvehicles sincevehicles in a lane tend to wandermoreon a curvethan on a straight reach,

~ On two-lane or wider roa’ds it is necessarythat boththe abovecomponentsshouldhe fully catered forso’ that the lateralclearancebetweenvehicleson curves is maintained equal to theclearanceavailableon straight$, Position of ‘tingle-lane roadshow-everis somewhat different,since during crossing manoeuvresouterwheelsof vehicles have in any caseto usethe shoulderswhetheronthe straight or on thecurve. It is thereforesufficienton single-laneroads ifonly the ‘mechanical,component ofwidening is taken intoaccount.

9,6,3, Basedon the aboveconsiderations,the extrawidth ofcarriagewayto he providedat horizontalcurveson single and two-lane roadsis given in Table 18, For ‘multi-laneroads, the pavementwideningmay be calculatedby adding half the widening for two~lane roads toeachlane.

‘T*nr iS, EXTRA WIDTH OF PAVEMENT AT HoRizoNTAL Cuives

Radius ofcurve(m)

tipto 20 211o40 41 to60

61 to100

101300

to Above300

Extrawidth (in)

Two-lane 1.5 1~5 1,2 0,9 0,6 NilSingle-lane 0,9 0,6 0,6 Nil Nil Nil

9,64. The widening should he effected by increasing thewidth at an approximately uniformrate alongthe transition curve~~The extra‘width should he continued over the full length of thecircular curve, On curves havingno transition, widening shouldhe achievedin the sameway as the superelevationi.e~two-’thirdbeing attainedon th’e straight sectionbeforestartof the curve andone-thirdon the curve,

9.6.5. The wideningshouldbe applied equallyon both sidesof the carriageway,exceptthat on bill roadsit will he preferableifthe entire widening is done only on the inside. Similarly, thewideningshouldhe provided only on the inside when the curveis plain circular and hasno transition~~

28

IRC 73-1980

9.6~6. The extra widening may be attained by means ofoffsets radial to the centreline. It should be ensured that thepavementedgelines are smoothandthere is no apparentkink.

9.7, Set-backDl stanceat Horizontal Curves9.7,1. Requisitesight distance should be avaiLableacrossthe

inside of horizontal curves. Lack of visibility in the lateraldirec-tion may arise dueto obstructionslike walls, cut slopes,buildings,woodedareas,high farm cropsetc. Distancefrom the road centreline within which the obstructionsshould be clearedto ensuretheneededvisibility, i.e. the “set-back distance”, can be calculatedvide proceduredescribed in para9.7,2. But in certaincases,dueto variations in alignment, road cross-section,and the type andlocationof obstructions,it may become necessaryto resortto fieldmeasurementsto determinethe limits of clearance.

9.7.2. The set-backdistance is calculatedfrom the followingequation(seeFig. 3 for definitions):

in = ,R—(R---n)Cos9

where 0 = i~(E—n) radians;

in = the minimum set-backdistanceto sight obstructioninmetres(measuredfrom the centreline of the road);

R = radiusat centreline of the roadin metres;it = distance between the centre line of the roadandthe

centreline of the inside lane in metres;andS = sight dlitancein metres

In the aboveequation,sight distanceis measuredalongthe middleof inner lane. On single-lane roads,sight distance is measuredalong centreline of the roadand‘n’ is takenas zero.

9.7.3. Based on the aboveequation,designchartsfor set-,back distance correspondingto the safe stopping sight distancearegiven in Fig. 4,

9,7.4. Set-back distancefor overtakingor Intermediatesightdistance can be computedsimilarly but the clearancerequired isusuallytoo large to be economically feasibleexcepton very fiatcurves.

9,7.5, Whenthereis acut slopeon the insideof the horizontalcurve, the averageheightof sight line can be usedas an approxima-tion for deciding the extent of’ clearance, Fot stopping sight

29

IRC

:73-1980

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31

IRC : 73-1980

distance,which is the bare minimumrequirement for design, theaverageheight may be takenas 0.7 m. Cut slopesshouldbekeptlower thanthis heightat the linedemarcatingthe set-back distanceenvelope,either by cutting backthe slopeor benchingsuitably~Inthe case of intermediate orovertakingsight distance,heightof sightline abovethe groundshouldbe takenas 1.2 m.

9,7.6. Wherehorizontalandsummit vertical curves overlap,the design should provide for the requiredsight distanceboth inthe vertical direction along the pavement and in the horizontaldirection on the inside of the curve,

9.8. Hair-PIn BendsIn hilly areas it may becomedifficult to avoid bendswhere

direction of the road reverses. Design criteria for such bends,commonly known as thehair-pin bends,are dealt with in para 10.5.

10. VERTICAL ALIGNMENT

10,1. General10.1,1. The verticalalignmentshould provide for a smooth

longitudinal profile consistent withcategory of the roadand layof the terrain. Grade changesshould not be too frequentas tocause kinks and visual discontinuitiesin the profile. Desirably,thereshould be no changein gradewithin a distanceof 150 m.

10,1,2~ A short valley curvewithin an otherwisecontinuousprofile is undesirable since this tends to distort the perspectiveview andcanbe hazardous.

10.1.3. Broken-back grade lines, i.e. two vertical curvesin the same direction separatedby a short tangent, shouldbeavoideddue to poor appearance andpreferablyreplacedby a singlelong curve.

10.1.4. Decksof small cross-drainage structures,(i.e. culvertsand minor bridges)shouldfollow the sameprofile as the flankingroad section, withoutany breakin the gradeline.

10,1.5. The longitundinalprofile shouldbe co-ordinatedsuit-ably with the horizontalalignment. This is discussedin Section II.

10.2. GradIents10.2.1. Gradesshould be carefully selected kecpingin view

the design speed, terrain conditionsandnature oftraffic expected

32

IRC: 73-1980

on the road. It is difficult andcostly to flatten the gradientslater.

10.2.2. Recommendedgradientsfor different classesof terrainaregiven in Table 19,

TABLE 19. GRADIENTS FOR ROADS IN Du’raaan’rTERRAINS

S,No,

Terrain Rulinggradient

Limitinggradient

Exceptionalgradient

1, Plain or rolling 3.3per cent(1 in 30)

S per cent(1 in 20~

6.7 percent(1 in IS)

2, Mountainousterrain,andsteep terrain having ele-vation more than 3,000mabove the mean sealevel

5 per cent(1 in 20)

6 percent(1 in 16.7)

1 per cent(1 in 14.3)

3, Steep terrain upto 3,000mheight above mean sealevel

6 percent(tin 16.7)

7 percent(1 in 14.3)

S per cent(1 in 12.5)

10.2.3. Gradientsupto the ‘ruling gradient’ may be usedasa matterof coursein design.Howeverin special situationssuch asisolated over-bridges in flat country or roads carrying a largevolume of slow movingtraffic, it will be desirableto adopta flattergradientof 2 per centfrom the angleof aesthetics,trafficoperations,and safety.

10.24. The ‘limiting gradients’maybe usedwherethe topo-graphyof a placecompelsthis course or where the adoption ofgentlergradientswould add enormouslyto the cost. In suchcases,the length of continuous grade steeper than the ruling gradientshoijld be as shortas possible.

10.2.5. ‘Exceptionalgradients’aremeantto be adoptedonlyin very difficult situationsand for shortlengthsnot exceeding100 mat astretch. In mountainousandsteepterrain, successivestretchesof exceptionalgradientsmustbe separatedby a minimum length of100 m havinggentlergradient(i.e. limiting gradientor flatter).

10,2.6. The rise in elevationovera lengthof 2 km shall notexceed100 m’in mountainousterrain and 120 m in steepterrain.

102,7. MinImum gradientsfor drainage: On unkerbedpave-ments in embankment,near-level grades are not objectionablewhenthe pavementhas sufficient camberto drain the storm water

33

IRC: 73-1980

laterally. HOwever,in cut sectionsor wherethe pavementis provi-ded with kerbs,it is necessarythat the road should have somegradient forefficient drainage. Desirableminimum gradient forthis purposeis 0.5 per centif the sidedrains are lined and 1.0 percent if theseareunlined.

10.2.8. Grade compensation atcurves on hill roads: At hori-zontal curves,thegradientsshould be easedby an amount knownas the ‘grade compensation’whichis intended to offset the extratractiveeffort involvedat curves. This shouldbe calculated fromthe following formula:

30+RGrade compensation(percent) = —

subjectto a maximumof 75/R whereR is the radius ofthe curvein metres.

Since grade compensation is not necessary for gradients flatter than4 percent,whenapplyinggradecompensationcorrection, thegrad-ientsneed not be easedbeyond4 percent.

10.3. VertIcal Curves10.3,1. Vertical curvesare introduced for smooth transition

at grade changes., Convex vertical curves are known as summitcurvesandconcaveverticalcurvesas valley or sag curves. Boththeseshouldbe designedas square parabolas,

10.3.2. The lengthof the verticalcurvesis controlledby sightdistancerequirements,but curveswith greater length are aestheti-cally better.

10.3.3. Curvesshouldbe provided at all gradechangesexce-eding thoseindicated in Table 20. For satisfactory appearance,the minimumlengthshould beas shown in the Table,

TABLE 20. MINIMUM LENGTh 01 VERTICAL CURVES

Designspeed(km/h)

Maximumcent) not

gradechanrequiring a

curve

ge (pervertical

Minimum length ofvertical curve

(metres)

Upto ~35.40

1.51.2

1520

50 1.0 3065 0.8 4080 0.6 50

100 0,5 60

34

4-

mC:73-1980

10.4. SusnmttCurves:10.4.1. The length of summit curves is governed by the

choiceof sight distance. The length is calculatedon the basisofthe following formulae:

(a) For safe stopping sight distance

Case(1) When the length of the curve exceedsthe requiredsight distance,i.e. L is greater than S

NS’L~-~--

whcre N — deviation angle, i.e. the algebraicdifferencebetweenthetwo grades

L = length of parabolic verticalcurvein metres

S = sight distancein metres

Case(U) When the length of the curve Is less than therequiredsight distance, I.e. L Is lessthan S

L = 25—

(b) For Inter,*.dlate or overtaking sight distance

Case(1) When the length of the curve exceeds the requiredsight distance,La. L is greaterthan S

NS5

Case(II) When the length of the curve Is less..than~therequiredsight distance, I.e. L ii less thanS

10.4.2. The lengthof summit curvefor various casesmentionedabove can be read from Plates 3, 4 and 5. In these Plates, valueof the ordinate “M” to the curve from the intersection point ofgrade lines is also shown.

10.5. Valley Curves

10.5,1. The lengthof’valley curvesshould be such that for nighttravel, the headlightbeamdistanceis equal to the. stopping sight

35

.-.--—-~-—.—. .- . ________

IRC: 73-1980

distance. The lengthof curvemay be calculated asunder:

Case(I) Whenthe length o the curve exceeds the required

sight distance, i.e. L Is greater than S

Case(Ii) When the length of the curve is lessthan the requiredsight distance,i.e. L is lessthan S

1.50 ±0.035SL—25— N -,

In both cases

N= deviationangle, i.e. the algebraicdifferencebetweenthetwo grades

L = length o(parabolie vertical curve in metres

S = stoppingsight distance in metres

10.5.2. Length of valley curvefor variousgradedifferencesisgiven in graphical form in Plate6.

10.6. DesignCriteria for Hair-Pin Bends

10.6.1. Hair-pin bends, where unavoidable, may be designedeither as acircular curvewith transitionat each end, oras a com-pound circular curve. The following criteria should be followednormally for their design:

(a) Minimum designspeed ... 20 km/h(b) Minimum roadway width st apex

(I) National/State Highways

(Ii) MajorDistrict RoadsandOther District Roads

(iii) Village Roads(c) Minimum radlul for the inner curve(d) Minimum length of transitioncurve

(e) GradientMaximumMinimumSuperelevation

11.5m for double-lane9.0at for single-lane

lin4O (2.5 percent)

I in 200 (0.5 percent)

(f~ .,. linlOflOpercent)10.6.2. Inner and outer edges of the roadway should be

concentricwith respect to centre line of’ the pavement. Where a

36

P/5’L — 1.50 + 0.035S

7.5 at

6.5at

14,Om15.Om

IRC 73-1980

numberof bair-pinbendshaveto be introduced,,a minimuminter-veningdistanceof 60 m shouldbe ~rovkIed betweenthe successivebendsto enablethe driver to negotiatethealignmentsmoothly.

10.6,3. Widening of hair~pinbends subsequentlyis a difficultand costly process. Moreover, gradients tend to become sharperas generallywideningcanbe achievedonly by cuttingthe hill side.Thesepointsshould be kept in view at the planning stage,especiallyif aseriesof hair-pinbendsis involved.

10,6.4. At hair-pin bends, preferably the full roadway widthshould be surfaced.

11. CO-ORDINATION OP HORIZONTAL ANDVERTICAL ALIGNMENTS

11.1. The overall appearanceof a highway canbe enhancedconsiderably by judicious combination of the horizontaland verticalalignments. Plan and profile of theroad should not be designedindependently but in unison soas to producean a~propciate three-dimensionaleffect. Proper co-ordination in this respectwill ensuresafety, improve utility of the highway and contribute to overallaesthetics,

11.2. The degree of curvature should be in properbalancewith the gradients. Straight alignment or flat horizontal curves at the

expense of steep or long grades, or excessivecurvature in a roadwith flat grades,do not constitute balanced designs and should beavoided.

11.3. Vertical curvature superimposedupon horizontal cur-vature givesa pleasingeffect. As suchthe vertical and horizontal-curves should coincide as far as,possible and their lengthshould bemore or lessequal. If this is difficult for any reason,the horizontalcurve should be somewhatlonger than the verticalcurve.

11.4. Sharp horizontal curves should be avoided at or nearthe apex of pronounced summitjsag vertical curves from safetyconsiderations.

11.5. P1a~e7 illustrates sometypical casesof good and badalignment co-ordination.

12. LATERAL AND VERTICAL CLEARANCES

AT UNDERPASSES12.1. LateralClearance

12.1.1, De~irabIythe full ro*dway’wldth at the approachesshould be carried through the underpass. This implies that the

37

1R~C:73-1980

minimum lateral clearance(i.e. the distancebetweenthe extremeedgeof’ the carriageway andthe face of nearestsupport,whether asolid abutment, pier or column)shouldequal the normalshoulderwidth.

12,1.2. On lower categoryroadsin hill areashavingcompara-tively narrowshoulders,it will be desirableto increasethe roadwaywidth at underpassesto a certainextentkeepingin view para 6.3.andthe principlesset forth in IRC:54-1974 “Lateral and VerticalClearancesat Underpassesfor VehicularTraffic”

12.1.3. For desirablelateral clearancesat dual carriagewayroads,referencemaybe madeto IRC:54-1974.

12.2. VertIcal Clearance

12.2.1. Vertical clearanceatunderpassesshouldbe minimum5 metresafter making dueallowancefor any future raising!streng-theningof the underpassroadway.

38

0.10 PLATE 1

SI-apwa0-SaaSaSa0-Sa

a0I-.

SaSaSa.a

0.03

0.01

0.07

0-OR

0.05

0.04

0.05

0.0*

a~

0

ITI1 \ 0-I~MAXIMUM SUPERELEVATION FOR MOUNTAINOUS

AND STEEP TERRAIN NOT BOUND 5Y SNOW.-\ .

I !

— ~_

~

t•t 0. 07, MAXIMUM SUPERELEVATION FOR PLAIN

ANO ROLLING TERRAIN, AND MOUNTAINOUS AND

STEEP TERRAIN ROUND DY SNOW.~1

~J

-

.

--- 1~ ,

~.

~

‘•._‘

.

vt

- SUPWHEAt e’ SUPERELEVATION

N0 SUPERELEVATION NEED RE PROvIDED IF Tn SUPERELEvaTION

CALCULATED IS LESS THAN THE NORMAL PAVEMEST CAMP.-

-

0 4C’D ac: Sc: ::: 230 CC aoc ieoo ,ooo aaoo a4fl

SUPERELEVATION RATESFOR VARIOUS DESIGNSPEEDS

NADIUC IN uETflI

pNORMAL.....~J L TRANSITION CURVE ~- FULLY SUPERELEVATEDCAMBER CIRCULAR CURVE

OJ’E~!Dfl EvEL~

C a~:c PPVEMENT REVOLED ABOUT OUTER ‘flnC

INNER EDGE OF PAVEMENT

LEGENDCROSS fiCtIoN At AL-NORMAL CAMBER

CROSS SECTION At fl—ADVERSE CAMBER REMOVED

CROSS SECTION AT CC—SUPERELEVAtION EQUAL TO CAMBER

CROSS SECTION AT D~FULL SUPENELEVATION ACHIEVED

NOTE

THE RATE OF CHANGE OF SUPERELEVATION (LONGITUDINAL SLOPEOF EDGE COMPARED TO CENTRE LINE) SHOULD BE MINIMUM I IN ISOFOR ROADS IN PLAIN AND ROLLING TERRAIN AND I IN 40 INMOUNTAINOUS AND STEEPTERRAIN THE ACTUAL RATE USED WILLDETERMINE THE DISTANCES AS~BC AND CD

b ‘ tRANSITION CURVE FULLY :UPUELEVAtED

OUTER EDGE&OF PAVEMENT

CENTRELINE OF PAVEMENT

— ~ — — — — INNER EDGE OF PAVEMENT

Si. CENTRE LINE LEVEL 7

PLATE?

A B C 0NORMAL..,4 I TRANSITION URVE ...L FULLY BUPERELEVATEDICAMBER T CIRCULAR CURVE .1

I I

CENTRELINI OF PAVEMENT

__t ~~ffiUEO4ELEVEL~

(b) PAVEMENT REVOLVED ABOUT INNER EDGE

N(0) PAVEMENT REVOLVED ABOUT CENTRELINE

SCHEMATIC DL6~GRAMSSHOVVIJSaQ nIFFERENT METHODS 0 ATTAINING SUPERELEVTION

PLATE 3

tS2aSU3US

0

SSaS-‘I

see

t00

400

500

too

00

.rrnTh~t \ 7vv =____~—\ —4--~\ \

~H t;±\JH~

~1tw:Io~

~--

- .Jr:__ -~

— - - TIm,-

—*J~=

-liii:- ~ —~-- ~—~! -.

— — — ~r b-...

— -____ - - -

—I,

N 11L, IL’S)

(LC5)

WNUE LW LENGTh OF IUN*T CURVE

5- STOflING SIGH? DISTANcE

N’ DEVIATION ANGLE

N- ORDINATE TO SUN*T CURVEFROM THE INTERSEOTION POINTOF GRADE LINES

NOTE )- FOR MINIMUM LENGTH OF 0~VE.

SEE TAbLE CO

LENGTH OF SUMMIT CURVE

FOR STOPPINGSIGHTDISTANCE

PLATE 4

NB2

(L’S)

L1 tS~t (L.cS)

MIt

WHERE L’ LENGTH OF SUMMIT CURVE

B- INTERMEDIATE SIGHT DISTANCE

HI DEVIATION ANGLE

MI ORDINATE TO SUMMIT CURVEFROM THE INTERsECTION POINTOF GRADE LINES

NOTE~FOR MINIMUM LENGTH OF CURVESEE TABLE 20

LENGTH OF SUMMIT CURVE FORINTERMEDIATE SIGHT DISTANCE

350

1*00

I 050

900

I.’ S0~

ISO

BOO

L Si

450

BOO

ISO

0

OIVIATION ~MGLt — N

NSt0-SaS

S

aU

U.0

S0-SSS

ZOOC

“nfl

-

I$OO-~-—---

~00iT!

‘

~1~ ~ I /

-\iV\-\ ~

\!A\\ -~-_(/1 --___

~_____-, 7---Thli~~

:\I~ ~j -~

\ .

g~ \L\’~’\1. \\\ / /

I 000

$00

$00

400

*00

PLATE 5

-

N

IL’~4 1L’~S)

4LC5)

WHERE L’ LENGTh OF SUMMIT CURVE

5’ OVERTAKING SIGHT DISTANCE

M’OEV1AT4ON ANGLE

M’ ORDINATE TO SUMMIT CuRVEFROM THE INTERSECTIONPOINT OF GRADE LINES

HOTE:~ FOR MINiMUM LENGTH OF CURVE,SEE TABLE t0~

LCFV-cml OF SUMM~.TCURVE FORr-~E~TnwINe s ~T S’SNCE

~Y’k~’1\~\x~”~~JIiei~/YA\,X\~< ~>-~ -

I’~’V \ ~ <~ -~-

I V\~V5e~-~K

~

/ -1%r~TI~CT~t~-— —t -~ — — 1~—-

00 0.02 0.04

PLATE 6

SSUI-S

S

SaU

S

LENGTH OFA -v~~Lcy ~

PLATE 7

GOOD DESIGN FORM UNDESIRABLE DESIGN FORM

(0) VERTICES OF HORIZONTAL AND VERT1CALPLAN CURVES COINCIDE. VERTICAL CURVE

KEPT WITHIN HORIZONTAL CURVE.

BRINGS OUT * VERY PLEASING

PROFILE APPEARANCE.

VERTICAL CURVE PRECEDESPLAN HORIZONTAL CURVE. HORIZONTAL

CURVE LOOKS LIKE A SHARP

KINK. POOR APPEARANCE.

PROFILE

PLAN Ib) SAME AS (a) BUT INVOLVING A SERIES OFCURVES VERTICES OF HORIZONTAL AND

VERTICAL CURVES COINCIDE, PRODUCING

PROFILE A VERY PLEASING APPEARANCE.

PLAN HAZARDOUS LEVEL CROSSING(OR ROAD INTERSECTION) AND SHARP

HORIZONTAL CURVE ARE OBSCURED

FROM DRIVER’S VIEW BY SUMMIT

PROFILE CURVE. DANGEROUS SITUATION.

IC) SIMILAR TO (a) BUT ONE PHASEPLAN SKIPPED IN THE HORIZONTAL PLANE.

~ VERTICES OF CURVES STILL COINCIDE.

A SATISFACTORY APPEARANCE RESULTS.

PROFILE

PLAN HORIZONTAL CURVE IS HIDDENFROM DRIVER’S VIEW, CAUSING A

DISJOINTED EFFECT

PROFILE

PERSPECTIVE

COMPAT~LEWfTH TIlE HORIZONTAL YE URVE ~:A ~TTHE VER~CAL

~ CURVE PRODUCES A SMOOTH FLOWING THOUGH THERE IS NO OISCONTI—

ALIGNMENT AND A PLEASING THREE NUITY IN PLAN OR PROFILE

DIMENSIONAL VIEW. PR F SINGLY, THREE DIMENSIONAL VIEWPROFILE 0 ILE == IS POOR. I

PERSPECTIVE ~ PERSPECTIVE -SKETCHESILLUSTRATING GOODAND BAD ALIGNMENTCOORDINATION