Geometric designs for Safe Highways Dr. Manoj M. Asst. Professor Department of Civil Engineering IIT Delhi WORKSHOP-CUM-TRAINING PROGRAMME ON ROAD SAFETY 17th 21st September 2018
Outline Introduction Cross section elements Horizontal Alignment Vertical Alignment 2
Introduction 3
Introduction 4
Introduction Deals with the dimensions and layout - alignment, sight distance and intersection Objective is to provide optimum efficiency with maximum safety at reasonable cost Main Design elements: Cross section elements Sight distance considerations Horizontal alignment Vertical alignment Intersection elements 5
Introduction Maximize the comfort and economy of facilities Efficiency in traffic operation Safety at reasonable cost Environmental impacts 6
Design Controls and Criteria Design Speed Topography Traffic factors Design hour volume and capacity Environmental and other factors 7
Design Controls and Criteria Design Speed o Most important factor o Affected by topography & road type o Influences all geometric elements of roads Topography o Plain; Rolling; Mountainous; Steep terrains o Speed and cross slope governs the design of elements Traffic Factors o Vehicular and Human Characteristics o Design vehicle speed, dimension, weight and acceleration, o Physical, mental and psychological characteristics driver Design hourly volume and capacity o Knowledge of Peak and off-peak hour volume Environmental and other factors o Aesthetics, landscaping, pollution, etc. 8
Terrain and Speed IRC 73-1980 9
Highway Cross Sectional Elements Pavement Surface Characteristics o Friction Tyre and road surface speed, acceleration, sight distance, curve design o o o o o o o Pavement types cement concrete, bituminous, WBM Roughness of pavement Condition of pavement wet/dry, mud/oil spilled Tyre condition Speed of vehicles Load and tyre pressure Temperature, etc. Longitudinal friction 0.35-0.40; Transverse 0.15 10
Highway Cross Sectional Elements Pavement Surface Characteristics o Unevenness Influences operating speed geometric standards Wear tear; accidents; operating cost; Low unevenness index 150 250 cm/km (high speed highways) o Light Reflecting Characteristics Night visibility wet conditions Light coloured night condition (rainy); strain and glare (day) 11
Highway Cross Sectional Elements Cross Slope / Camber o Slope provided in the transverse direction to drain off the rain water Provided by raising the carriage way with respect to the edges Depends on type of the pavement surface & amount of rainfall IRC 73-1980 12
Highway Cross Sectional Elements Width of Pavement or Carriageway o Depends on the width of traffic lane and number of lanes o Carriageway intended for one line of traffic movement is traffic lane o Lane width = vehicle width (2.44m )+ side clearance (0.625) IRC 73-1980 13
Highway Cross Sectional Elements Traffic Separators / Medians o To prevent head on collision between vehicles moving in opposite directions on adjacent lanes o Pavement markings, medians, dividing islands, etc. o 5.0 m for rural highways (3.0 m land restriction) o Long bridges 1.2 to 1.5 m o Transition 1 in 15 to 1 in 20 o Urban roads: (absolute min width 1.2 m; desirable 5.0 m) o 1.2 m for pedestrian refuge o 4.0-7.5 m for protection of vehicles making right turn o 9.0 to 12.0 m for protection of vehicles crossing at grade 14
Highway Cross Sectional Elements Kerb o Indicates the boundary between the pavement and shoulder Barrier, Semi-barrier, and Mountable o Barrier Built-up areas adjacent to footpaths with considerable pedestrian traffic o Semi-barrier periphery of the roadway where pedestrian traffic is light and a barrier could tend to reduce traffic capacity o Mountable Within the roadway at channelization schemes, medians, outer separators and raised medians on bridges 15
Cross Sectional Elements Kerb Source: IRC 86-1983 16
Highway Cross Sectional Elements o Road Margins Shoulders Emergency lane / service lanes (min 2.5 m width) Parking lanes for kerb parking (min 3.0 m width) Lay-byes to stop and clear off the carriageway Busbays 75m away from intersections Frontage roads access to properties Driveways connect commercial establishments Cycle tracks min 2 m width; 1 m for additional lane Footpath when vehicular and pedestrian volume is high (1.5 min ) 17
Highway Cross Sectional Elements 18
Highway Cross Sectional Elements o Width of roadway or formation sum of width of carriageway; separators (if provided) & shoulders IRC 73-1980 19
Highway Cross Sectional Elements Right of Way Source: IRC 86-1983 20
Sight Distance Sight Distance o Sight distance available from a point is the actual distance along the road surface which a driver from a specified height above the carriage way has visibility of stationary or moving objects o The length of road visible to the driver at any instance Should Satisfy: o Length of road visible ahead to stop the vehicle o Safely overtake at reasonable intervals o Control vehicle and avoid collision at uncontrolled intersection 21
Sight Distance Stopping Sight Distance The minimum sight distance available on a highway at any spot Depends on: o Features of the road ahead o Height of the driver s eye above the road surface (1.2 m) o Height of the object above the road surface (0.15 m) Stopping depends on: Total reaction time of the driver Speed of vehicle Efficiency of brakes Frictional resistance between the road and tyres Gradient, if any 22
Sight Distance Total Reaction Time o o o The time taken from the instant the object is visible to the driver to the instant the brakes are effectively applied Total reaction time = perception time + brake reaction time Total reaction time = 2.5 sec Speed of vehicle o Higher the speed, longer the stopping sight distance Efficiency of brakes o o 100% braking efficiency skidding Braking force should not exceed friction Frictional/skid Resistance o Depends on road and tyre o f = 0.35 to 0.40 23
Sight Distance Stopping Sight Distance 0.278Vt + V 2 254(f ± 0.01G) IRC 73-1980 24
Sight Distance Overtaking Sight Distance The minimum distance open to the vision of the driver of a vehicle intending to overtake the slow moving vehicle with safety against the traffic in opposite direction 25
Sight Distance Overtaking Sight Distance Optimum condition is one in which the overtaking driver can follow the vehicle ahead for a short time while he assess his chances of overtaking o Assumptions: o The vehicle being overtaking is travelling at a uniform speed which is 16kmph less than the design speed of the road o The overtaking vehicle follows the vehicle ahead for a short while to perceive the clear road ahead o Overtaking is done by accelerating rapidly to the design speed and is considered completed when the vehicle returns to its own side of the road o Overtaking once began is finished in the face of an oncoming vehicle travelling at design speed in such a way that the latter arrives alongside the former just at the completion of maneuver 26
Sight Distance Overtaking Sight Distance o Overtaking maneuver 8 to 14 seconds o One third of the total time is spent following the vehicle to be overtaken o The opposing vehicle s travel distance in 2/3 of the total time is added. IRC 73-1980 27
Sight Distance Intermediate Sight Distance o Sections of roads where the customary overtaking sight distance cannot be provided should be designed as far as possible for intermediate sight distance. o It is twice the normal safe stopping distance. IRC 73-1980 28
Sight Distance Headlight Sight Distance o In valley curves roadway ahead is illuminated by vehicle headlights to a sufficient length enabling the vehicle to break stop (equal to SSD) 29
Horizontal Alignment o Directional transition of the roadway in a horizontal plane o Relationship between design speed and curvature and on their joint relationships with superelevation (roadway banking) and side friction 30
Horizontal Alignment Superelevation V2 e + f = 127R 31
Horizontal Alignment Superelevation o Plain and rolling terrain 7% o In snow bound areas 7% o In hilly areas not bound by snow 10% IRC 73-1980 32
Horizontal Alignment o Radius of Horizontal Curve R = V 2 127(e + f) IRC 73-1980 33
Horizontal Alignment Widening of Pavements on Horizontal Curve o When curves are not of large radius 34
Horizontal Alignment Widening of Pavements on Horizontal Curve o Extra Widening = Mechanical Widening + Psychological Widening W e = nl2 2R + V 9.5 R IRC 73-1980 35
Horizontal Alignment Horizontal Transition Curve A transition curve has a radius which decreases from infinity at the tangent point to that of the circular curve. o Objectives: o To introduce the centrifugal force gradually o To steer the vehicle gradually and comfortably o To allow for gradual introduction of superelevation and extra widening o To improve aesthetic appearance 36
Horizontal Alignment Horizontal Transition Curve o Spiral curve: o Ideal transition o Calculation and implementation are easy 37
Horizontal Alignment o Length of Transition Curve i. Rate of change of centrifugal acceleration L s = 0.0215V2 ; C = 80 CR 75 + V ii. Rate of change of Superelevation o For plain and rolling terrain L s = 2.7V2 R o For mountainous and steep terrain L s = V2 R 38
Horizontal Alignment IRC 73-1980 39
Horizontal Alignment Setback distance o Distance from the road center line within which the obstructions should be cleared to ensure the needed visibility m = R (R n)cosθ 40
Vertical Alignment o The vertical alignment is the elevation or profile of the center line of the road to accommodate changes in grades o Consists of grades and vertical curves vehicle speed, acceleration, deceleration, SSD and comfort 41
Vertical Alignment Gradient the rate of rise or fall along the length of the road with respect to the horizontal (1 in x; n in 100) o Gradient types o Ruling gradient o Limiting gradient o Exceptional gradient o Minimum gradient 42
Vertical Alignment o Ruling Gradient The maximum gradient within which the designer attempts to design the vertical profile of the road (design gradient) o Limiting gradient where topography compels adopting steeper gradients than ruling gradients o Exceptional gradient steeper than limiting; not exceeding100 m at a stretch o Minimum gradient from drainage point of view; 1 in 500 in concrete drains 43
Vertical Alignment IRC 73-1980 44
Vertical Alignment Grade Compensation o At horizontal curves, the gradients should be eased by an amount known as the grade compensation (reduction in gradient) o Grade compensation % = 30+R R ; max. = 75 R o Not necessary for grades flatter than 4% 45
Vertical Alignment Vertical Curve o At intersections of different grades to smoothen out the vertical profile Summit curves/crest curves Convexity upwards Valley curves/sag curves Concavity upwards 46
Vertical Alignment o Summit Curve o Governing design factor Sight Distance o Circular / Parabolic curves 47
Vertical Alignment o Length of Summit Curve for SSD i. When L>SSD L = NS 2 ( 2H + 2h) = NS2 2 4.4 ii. When L<SSD L = 2S 4.4 N 48
Vertical Alignment o Length of Summit Curve for OSD or ISD i. When L>OSD/ISD L = NS2 8H = NS2 9.6 ii. When L<OSD/ISD L = 2S 8H N = 2S 9.6 N 49
Vertical Alignment IRC 73-1980 50
Vertical Alignment Valley Curve o Governing design factor comfort of passengers& availability of stopping sight distance under headlights of vehicle o Allowable rate of centrifugal acceleration influences the design (transition curves) 51
Vertical Alignment o Length of Valley Curve i. For comfort condition L = 0.38(NV 3 ) 1/2 ii. L > Headlight Sight Distance L = NS 2 (1.5 + 0.035S) iii. L < Headlight Sight Distance (1.5 + 0.035S) L = 2S N 52
References IRC codes 1. IRC: 73-1980, geometric design standards for rural (non-urban) highways 2. IRC: 86-1983, Geometric Design Standards for Urban Roads and Plains 3. IRC: 66-1976, Recommended Practice for Sight Distance on Rural Highways 53
Thank You 54