ug geometric design i

Indian Institute of Technology, Kharagpur

Geometric Design of Highways

Lesson 1Functional Classification, Design

Elements and Controls

Dr. Bhargab MaitraDepartment of Civil Engineering

Indian Institute of Technology KharagpurIndia

Email: [email protected]

mailto:[email protected]


Specific Instructional objectives

• Appreciate the need for Geometric design

• Classify the road system

• Identify broad elements of geometric design

• Identify design control and criteria


General

• Layout and dimensions of visible features of roadways

• Optimum efficiency and maximum safety at reasonable cost cost

• Improvement of design standard at a later date- A difficult task


Functional classification• Volume of traffic• Type of traffic• Purpose of road• Importance / Priority

• Expected Speed• Long/Short distance traffic• Access control • Traffic control at intersections• Parking, loading/unloading• Gradient, curves


Urban Roads• Expressway• Arterial Street• Sub-arterial Streets• Collector Streets• Local Streets

Rural Roads• National Highways• State Highways• District Roads• Village Roads

• Functional classification and roads in reality


Elements of Geometric Design• Cross section elements

• Sight distance consideration

• Horizontal Alignment details

• Vertical Alignment details

• Intersection elements


Design Control and CriteriaDesign Speed• Mixed traffic, different types of vehiclebut single design value is required

• Selected value should satisfy requirements of most of the drivers/ conditions

• Cumulative distribution of speed has a typical “S” shape


0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Speed (kmph)

% o

f veh

icle

at l

ower

spe

edth

an x

-axi

s

85th percentile-Safe speed

95th /98th percentile-Design Speed

15th percentile-Lower speed limit


Design Speed (km/h)

Plain Terrain Rolling Terrain

Mountainous Terrain Steep Terrain

Ruling Min Ruling Min Ruling Min. Ruling Min.

National andState Highways

100 80

Other District Road (ODR)

Village Road (VR) 20

Major District Road (MDR)

Road classification


Topography

IRC• Plain 0 to 10 %, • Rolling 10 to 25 %, • Mountainous 25 to 60 % • Steep terrain >60%

AASTHO• Level • Rolling • Mountainous


Traffic factorsTraffic volume• Average Daily Traffic (ADT)

• Annual Average Daily Traffic (AADT)

Traffic composition

• Heterogeneous• Passenger Car Unit / Equivalency (PCU/PCE)• Suggested PCU/PCE values


Design Vehicle Selected motor vehicle- weight, dimensions and operating characteristics

• Single unit truck• Semi-trailer• Truck-trailer combination


Design Hourly Volume • 30th Highest Hourly Volume V• 8-10 % AADT (Typical Indian condition)

No of hours in one year with traffic volume exceeding that shown in Y-axis

Hou

rly tr

affic

vol

ume

- % o

f A

AD

T


Environmental and other factors

• Aesthetics and landscaping

• Air pollution

• Noise pollution

• Local site conditions


SummaryThe need for geometric design• Functional classification of road and its relevance for geometric design• Broad elements of geometric design• Design control and criteria

Design SpeedTopographyDesign VehicleDesign Hourly VolumeEnvironmental and other factors


Geometric Design of HighwaysLesson 2

Cross Section Elements







• Identify different cross section elements,

appreciate their needs and justify variations in

shapes and dimensions


• Side slope• Lateral and vertical

clearances• Kerb• Guard rail• Side drain• Other facilities

Elements• Carriageway• Shoulder• Roadway width• Right of Way• Building line• Control line • Median• Camber


CarriagewayRoadway width

ShoulderShoulder Carriageway width

RO

W

Bui

ldin

g Li

ne

Bui

ldin

g Li

ne

Con

trol L

ine

Con

trol L

ine

RO

W

Width of Carriageway (meters)

Single lane Intermediate lane

Two lanes without raised

kerbs

Two lanes with raised

kerbs

Multi lane road(Width per

lane)

3.75 5.5 7.0 7.5 3.5

Paved traveled width – Carries vehicular traffic


Two Lane Carriageway

7.0m

0.625m

Shoulder

0.625m

2.5m

Shoulder

3.75m

2.5m

2.5m 0.5m0.5m

Maximum width of vehicle as per IRC: 2.44m

Single Lane Carriageway

1.0m


Shoulder• One-half the difference between roadway width and

carriageway width

• Supports carriageway

• Provides space for stopped vehicle

Roadway width

ShoulderShoulder Carriagewaywidth

RO

W

Bui

ldin

g Li

ne

Bui

ldin

g Li

ne

Con

trol L

ine

Con

trol L

ine

RO

W


Roadway• Carriageway (including separator) + shoulders

Plain and Rolling terrain(meter)

Mountainous and Steep terrain (meter)

Single lane Two lane Single lane Two laneNH and SH 12.0 12.0MDRODRVR

Road classification

Roadway width

ShoulderShoulder Carriagewaywidth

RO

W

Bui

ldin

g Li

ne

Con

trol L

ine

Con

trol L

ine

RO

W

Bui

ldin

g Li

ne


Culverts (up to 6.0m span)-normal roadway width (measured from outside to outside of the parapet walls)

Bridges (greater than 6.0m span)- clearway between kerbs

- Single lane bridge- Two lane bridge - Multi lane bridge bridge


Right of Way (ROW)/ Land Width

• Land secured and preserved for road purpose

• Should be adequate to accommodate all the cross section elements

• Should provide space for future upgradation

Roadway width


RO

W

Bui

ldin

g Li

ne

Bui

ldin

g Li

ne

Con

trol L

ine

Con

trol L

ine

RO

W


Plain and Rolling terrain(meter)

Mountainous and steep terrain

(meter)Open areas Built up areas Open Built-up

Normal Range Normal Range Normal Range

NH and SH 45 30-60

MDR

ODR

VR

Road classification


Building lineControl line

Plain and Rolling terrain Mountainous and steep terrain

Open areas Built up areas Open Built-up

Overall Width betwn. building lines

Overall Width betwn. control lines

Set back distance betwn. building line and road boundary

Set back distance betwn. building line and road boundary

NH and SH 80 150

MDR

ODR

VR

Road classification

Roadway width


RO

W

Bui

ldin

g Li

ne

Con

trol L

ine

RO

W

Bui

ldin

g Li

ne

Con

trol L

ine

• Control construction/ developmental activities


Median• Longitudinal space separating dual carriageways• Separates directional traffic streams• Should be as wide as possible• Width is restricted by economic consideration• Uniform width is preferable• Width depends on type of road/cross drainage

structure and availability of land

• Transition length for change in width


Camber• To drain off rain water from road surface • Depends on type of road surface and amount of rainfall

Surface Type Camber (per cent)

Earth road

WBM and gravel road

Thin bituminous pavement

High type bituminous surfacing or rigid pavement 1.7-2.0


Shape of camber• Parabolic, straight line or combination

Providing camber in the field• Templates or camber boards -To check the lateral

profile of finished pavement during construction

Straight line Shape

Combination of Straight and parabolic Shape

Parabolic Shape


Side slope• Type of soil

• Height of embankment or depth of cutting

• A flatter slope - conducive for erosion control but is costlyCondition Slope (H:V)Embankment in silty/sand/gravelly soil 2:1

Embankment in clay or clayey silt or inundated condition 2 ½ : 1

Cutting in silty/sandy/gravelly soil 1:1

Cutting in disintegrated rock or conglomerate ½:1


Lateral and vertical clearances

• Generally required – ROB, Underpass locations

• Lateral clearance- Distance between the extreme edge of the carriageway to the nearest face of the structure

• Vertical clearance- Height above the highest point of traveled way to the lowest point of the overhead structure.


Kerb

Vertical or sloping member along the edge of a pavement or paved shoulder

• Desirable for urban roads • Facilitates and controls drainage• Strengthens and protects pavement edge• Delineates pavement edge• Presents more finished appearance• Encourages orderly roadside development


Barrier Semi barrier Mountable


Guard rail• Prevents vehicle from running off

• Vertical W-beam or box-beam along the edge of shoulder

• Painted guard stone

• High embankment• Outer side of sharp horizontal curve• Approach of bridge

Guard StoneW-Beam Guard Rail


Side drain• Proper drainage to enhance the life of pavement• Surface drainage- Efficiently remove surface water

and lead them to natural water channels• Along the toe of embankment

V-Shape Trapezoidal shape

• Sub-surface drainage- Drainage of underground water


Other facilitiesParking lane• Urban roads• On street parking

Truck Lay-bye• Road side amenities• Repair / rest

Bus-bay• Widening • Avoids conflict

Foot-path• Urban roads• Safety of pedestrians



Stopping Sight Distance and Decision Sight Distance







• Appreciate the need for providing Stopping Sight

Distance and Decision Sight Distance

• Identify influencing factors and understand the

basis for suggesting design values


GeneralSight Distance- Length of road visible to a driver

Stopping Sight Distance- Length of road required for safe stopping of a moving vehicle

Minimum required sight distance for safe operation of traffic – Absolute Minimum Sight Distance


Criteria for measurement• Height of driver’s eye above road surface (H)• Height of object above road surface (h)

hH

AASHTO• H = 1.08m• h = 0.6m

IRC• H = 1.2m• h = 0.15m


The value of ‘h’ (as per AASHTO) is based on

• Rationalization of the size of object that may be encountered

• Impact on construction cost: A value lesser than 0.6m (height of passenger car taillights) may increase construction cost substantially

• Drivers ability to perceive and react: At high speeds most drivers are limited in their ability to detect small objects


Factors Influencing Stopping Sight Distance• Perception / Reaction time of driver• Speed of Vehicle• Efficiency of brakes• Frictional resistance between road and tires (IRC)

or Deceleration rate (AASHTO)• Longitudinal gradient of road


Perception/Reaction timeTime taken from the instant an object is visible to the instant brakes are applied effectively

Perception/Reaction time depends on

• Characteristics of driver

• Characteristics of obstacle

• Speed of vehicle

• Distance between vehicle and object

• Characteristics of vehicle


Basis for Design Value

• Design value should satisfy requirements of nearly all drivers under most operating conditions

• Required time is generally higher under normal conditions than under alerted conditions

• A time of 2.5 sec exceeds the 90th percentile of reaction time for all drivers

• For extremely complex conditions, 2.5 sec may not be adequate


Speed of VehicleHigher the speed, higher will be the required SSD

Efficiency of brakes100% braking efficiency is not desired

Frictional resistance between road and tiresCoefficient of friction depends on• Speed of vehicle• Type and condition of pavement• Type and condition of tyres• Brake efficiency

Design value: 0.35 to 0.40 depending on speed


Lag Distance• Distance covered during the reaction time• Lag Distance = vt (if v in m/sec)

or 0.278 Vt (if V in km/h)Braking Distance• Distance covered by a vehicle to come to stop

after application of brakes

Stopping Sight Distance = Lag Distance + Braking Distance

Calculation of Stopping Sight Distance


F = fWW

Work done = Kinetic energyKinetic energy = ½ mv2 = Wv2/2gWork done = Frictional force x braking distance

= F x L = fWLEquating work done in stopping and kinetic energy

L = v2/2gf

IRC

L = Braking distance, mv = Speed of vehicle, m/secf = Coefficient of friction (longitudinal)g = Acceleration due to gravity, 9.8m/Sec2


If the speed of vehicle is expressed in km/h then

L = V2/254fStopping Sight Distance = Lag Distance +

Braking Distance

= vt + v2/2gf or

0.278 Vt + V2/254f


AASHTO • Approximately 90% drivers decelerate at rates

greater than 3.4 m/sec2

• Such decelerations are within drivers’ capability to

maintain steering control during braking on wet

surfaces

• Most vehicle braking systems and tyre-pavement

friction levels are capable of providing a

deceleration rate of at least 3.4 m/sec2


• A constant deceleration rate of 3.4 m/sec2 is

suggested for the calculation of braking distance

• Suggested deceleration rate corresponds to a

value of coefficient of friction of about 0.35• Braking Distance = 0.039 V2/a (by replacing ‘f’ with a/9.81 in V2/254f)

Stopping Sight Distance = 0.278 Vt + 0.039 V2/a

V in Km/h, t in sec and a in m/sec2


F = fW Cos αα

W W Cos αW Sin α

Work done = (W sinα + fW Cosα)L or (WN +fW)LKinetic Energy = ½ mv2 = Wv2/2g

L = v2/2g(f+N)

Effect of Grade

IRC

L = Braking distance, m v = Speed of vehicle, m/secf = Coefficient of friction (longitudinal)g = Acceleration due to gravity, 9.8m/Sec2

N = Percentage of Grade divided by 100



L = V2/ (254 (f+N))Stopping Sight Distance = Lag Distance +

Braking Distance

= vt + v2/(2g(f+N)) or

0.278 Vt + V2/(254 (f+N))


F = Wa/gα

W W Cos αW Sin α

AASHTO

Work done = (W sinα + Wa/g)L or (WN + Wa/g)LKinetic Energy = ½ mv2 = Wv2/2g

L = v2/(2g(a/g + N))

L = Braking distance, m v = Speed of vehicle, m/seca = Deceleration rate, m/sec2

g = Acceleration due to gravity, 9.8m/Sec2

N = Percentage of Grade divided by 100



L = V2/ (254 (a/9.8 + N))Stopping Sight Distance = Lag Distance +

Braking Distance

= vt + v2/(2g(a/9.8 + N)) or

0.278 Vt + V2/(254 (a/9.8 + N))


Variation for Trucks• Recommended SSDs do not explicitly consider

design for truck operations

• Trucks (larger and heavier units) need longer SSD than Passenger Cars

• Balancing Factor: Higher position of seat in the vehicle (truck)

• Separate SSDs for Trucks are generally not used

• Provide more SSDs where horizontal sight restrictions occur at the end of long downgrades


Example• Speeds are : 90 km/h and 50 km/h• Vehicles from opposite direction • Single lane road• Coefficient of friction: 0.76• Brake efficiency : 50 %Minimum SD required to avoid a head-on collision?

f = 0.5*0.76 = 0.38

LD1 = 0.278 x 90 x 2.5 = 62.55 m

LD1 = 0.278 x 50 x 2.5 = 34.75 m


BD1 = 902/(254 x 0.38) = 83.92 m

BD2 = 502/(254 x 0.38) = 25.90 m

SSD1 = LD1 + BD1 = 62.55 m + 83.92 m = 146.47 m

SSD2 = LD2 + BD2 = 34.75 m + 25.90 m = 60.65 m

Therefore, minimum SD required

= SSD1 + SSD2 = 146.47 m + 60.65 m

= 207.12


Decision Sight DistanceSSD may be inadequate

• Complex or instantaneous decisions

• Information is difficult to perceive

• unexpected or unusual maneuvers


Error in either information reception, decision making or control actions at critical locations • Interchange and intersection locations

(unexpected or unusual maneuvers)

• Changes in cross section such as toll plazas and lane drops, and areas of concentrated demand (visual noise from competing sources of information)

Provide DSD at critical locations or shift critical decision points to locations where DSD is available – otherwise use suitable traffic control devices


Decision sight distance to

• detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered

• recognize the hazard or its potential threat

• select an appropriate speed and path

• initiate and complete the maneuver safely and efficiently


Different DSD values are recommended

• Avoidance Maneuver A: Stop on rural road

• Avoidance Maneuver B: Stop on urban road

• Avoidance Maneuver C: Speed/path/direction change on rural road

• Avoidance Maneuver D: Speed/path/direction change on suburban road

• Avoidance Maneuver E: Speed/path/direction change on urban road

Indian Institute of Technology, Kharagpur• Avoidance maneuvers A and B: Pre-maneuver time

is increased and the braking distance is added to the pre-maneuver component.

DSD = 0.278 Vt + 0.039 V2/a(t = pre-maneuver time)

• Avoidance maneuvers C, D and E: Pre-maneuver time is increased and the braking distance component is replaced with a maneuver distance based on maneuver times that decrease with increasing speed

DSD = 0.278 Vt(t = total pre-maneuver and maneuver time)



Overtaking Sight Distance, Intermediate Sight Distance and

Headlight Sight Distance







• Appreciate the need for providing Overtaking Sight Distance (OSD), Intermediate Sight Distance (ISD) and Headlight Sight Distance (HSD)

• Identify influencing factors and understand the basis for suggesting design values for OSD


Overtaking Sight Distance (OSD) • Minimum sight distance that should be available

to a driver for overtaking another vehicle safely

• Restrictions for passing opportunities affect the Level of Service for highways (two-lane, two-way) as the MOEs used are percent time spent following and average travel speed


Overtaking Maneuver • Overtaking vehicle follows the slow vehicle for

some time

• Finds a favorable condition for overtaking

• Pulls out, overtakes and returns back to original lane before meeting an oncoming vehicle

Assumptions• A single vehicle overtaking a single vehicle• Overtaken vehicle travels at uniform speed


• Overtaking vehicle follows the vehicle ahead for a short while

• Overtaking vehicle then accelerates rapidly, pulls out and occupies the opposing lane, overtakes slower vehicle and returns to original lane

• Vehicle travels from opposite direction at the same speed as the overtaking vehicle

• Difference between average speed of overtaking and overtaken vehicle is 15 km/h (AASHTO)

• Speed of overtaken vehicle is 16 km/h lesser than Design speed of the road (IRC)


Criterion for MeasurementIRC AASHTO

Height of driver’s eye above road 1.2m 1.08m

Height of the object above road 1.2m 1.08m


Estimation of OSD: Approach-1

d3

s

d2d1

s bA2

Speed of overtaking / opposing vehicle = v m/secSpeed of overtaken vehicle = vb m/sec

B1 B2 A3

C2

A1 C1

Calculation of d1d1= vbt ; t = reaction time (2.0 sec)


Calculation of d2d2 = b+2s = (vbT + aT2/2)Now, b = vbT Therefore, 2s = aT2/2

or, T = sqrt(4s/a)s = 0.7vb+6 (empirical formula)T = Total time required for overtaking maneuver, seca = Maximum overtaking acceleration, m/sec2

d3

s

d2d1

s bA2 B1 B2 A3

C2

A1 C1


Calculation of d3d3 = vTOSD = d1 + d2 + d3

d3

s

d2d1

s bA2 B1 B2 A3

C2

A1 C1


Example• Design Speed: 80 km/h • Maximum overtaking acceleration a = 0.72 m/sec2

• Reaction time = 2.0 secMinimum safe OSD required?

Vb = 80 – 16 = 64 km/h = 17.79 m/secd1 = 0.278Vb t = 0.278 x 64 x 2 = 35.58 ms = 0.7vb+ 6 = 0.7x17.78+6 = 18.45mT = sqrt ( 4x18.45/0.72) = 10.124secd2 = 0.278Vb T +2s= 0.278 x 64 x 10.124+2x18.45

= 217.02 m


d3 = 0.278V T = 0.278 x 80 x 10.124= 225.16 m

OSD = d1+d2+d3

= 35.58 + 217.02 + 225.16 m= 477.76m


Estimation of OSD: Approach-2 (IRC)• Total of 9 to 14 seconds for the completion of

overtaking maneuver by a vehicle closing at design speed

• Add 2/3rd additional time to account for opposing vehicle

Time (sec)Speed (km/ h) Overtaking

vehicleOpposing

vehicleTotal

Recommended OSD (m)

40 9 6 15 165

100 14 9 23 640


Approach-3 (AASHTO)

d3

d2d1

d2/3

BAC

d4


Calculation of d1

d1= 0.278 t1 (V-m+at1/2)t1 = time of initial maneuver, seca = average acceleration, km/h/secV = Average speed of passing vehicle, km/hm = difference in speed of passing and passed

vehicle, km/h

Speed Range (km/h)50-65 66-80 81-95 96-110

a 2.25 2.37t1 3.6 4.3


Calculation of d2d2= 0.278 Vt2

Speed Range (km/h)50-65 66-80 81-95 96-110

t2 9.3 10.7

d3d2/3

BA

d2d1

C

d4


Calculation of d3

Speed Range (km/h)50-65 66-80 81-95 96-110

d3 (m) 30 75

d3

d2d1

d2/3

BC

A

d4


Calculation of d4 d4= 2/3rd of d2

OSD = d1 + d2 + d3 + d4

Likely and logical relation between the average passing speed and the highway design speed to express the minimum OSD needed for design purpose.

d3

d2d1

d2/3

BC

A

d4


The speed of passed vehicle is taken as average running speed at a traffic volume near capacity

The speed of passing vehicle is assumed to be 15 km/h greater

Assumed Speed (km/h)

OSD (m)

Overtaken Vehicle

Overtaking vehicle

Calculated Rounded

80 65 80 538 54090 73 88 613 615

Design Speed (km/h)


Effect of Grade on OSDDowngrades• overtaking vehicle can accelerate rapidly - less

time of overtaking• overtaken vehicle can also accelerate easily –

racing contest situationUpgrades• More sight distance is required due to reduced

acceleration of the overtaking vehicle and likely speeding up of opposing vehicle

• Compensated by the loss in speed of overtaken vehicle which is frequently a heavy truck

Generally no Grade adjustment on OSD


Intermediate Sight Distance (ISD)• Sections where providing OSD is impractical for

reasons of economics or otherwise ISD is provided

• ISD = 2 x SSD

• ISD provides reasonable opportunities to drivers to overtake with caution


Headlight Sight Distance (HSD)

• In valley curve, during night travel design must ensure that the roadway ahead is illuminated by vehicle headlights for a sufficient length

• Minimum value of HSD provided is equal to SSD from safety consideration

ug geometric design i

Documents