Chapter III Geometric design of Highways

Tewodros N. www.tnigatu.wordpress.com

tedynihe@gmail.com

• Introduction • Appropriate Geometric Standards • Design Controls and Criteria

• Design Class • Sight Distance • Design Vehicle • Traffic Volume and • Design Speed

• Geometric Design Elements – Horizontal Alignment

• Straights (Tangents) • Circular Curves • Super elevation • Transition Curves • Widening of Curves

– Vertical Alignment • Vertical Curves • Length Of Vertical Curves • Sight Distances At Underpass Structures: • Grades and Grade Control

– Cross-Section

Lecture Overview

Sight Distance • Sight Distance is the distance visible to the driver of a

vehicle ahead of him • Stopping sight distance • Passing sight distance • Meeting sight distance

• For highway safety, the designer must provide sight distances of sufficient length so that drivers can control the operation of their vehicles. They must be able to avoid striking an unexpected object on the traveled way.

• Two-lane highways should also have sufficient sight distance to enable drivers to occupy the opposing traffic lane for passing maneuvers, without risk of accident.

Stopping sight distance • Stopping sight distance is the total distance traveled by a given vehicle

before stopping during three time intervals • The time to perceive the hazard • The time to react • The time to stop the vehicle

• During the first two intervals, the vehicle travels at full speed, during the third interval, its speed is reduced to zero, and must happen before hitting an object or vehicle ahead.

• As speed increases the reaction time increases ????

speed (km/h) Perception- reaction

time (sec) 16 3.5 32 3.25 48 3.0 64 2.75 80 2.50 96 2.0

Stopping sight distance

• Stopping Reaction distance Dr = 0.278 Vt

• Braking distance

( )Gf

VDb±

=254

2

Db = braking distance V = initial velocity when brakes are applied f = coefficient of friction G = grade (decimal)

Stopping sight distance

( )GfVVtSSD

±+=

254))()(278.0(

2

SSD = Stopping Sight Distance (meter)

= Dist. traveled during perception/reaction time + Braking Dist.

t = Driver reaction time, generally taken to be 2.5 seconds

V = Initial speed (km/h)

f = Coefficient of friction between tires and roadway Note: 1. Safe SSD on upgrades is shorter than on downgrades 2. Min. SD should be adjusted where steep grades and high speed occur in

combination

Stopping sight distance Cont…

Pavement condition Maximum Slide

Good, dry 1.00 0.80

Good, wet 0.90 0.60

Poor, dry 0.80 0.55

Poor, wet 0.60 0.30

Packed snow and Ice 0.25 0.10

Stopping sight distance Cont…

Practical Stopping Distance

0100200300400500600700

0 20 40 60 80

Speed in MPH

Stop

ping

Dis

tanc

e

Series1

Grade impacts on stopping

Varying Grade

0

100

200

300

400

500

600

0 20 40 60 80

Miles per Hour

Stop

ping

Dis

tanc

e in

Fee

t

2 percent grade0 percent grade-2 percent grade

Friction impact on stopping

Varying Friction

0200400600800

10001200140016001800

0 20 40 60 80

Miles per Hour

Sto

ppin

g D

ista

nce

in F

eet

Friction = 0.5Friction = 0.25Friction = 0.1

Impact of reaction time

Varying Reaction Time

0200400600800

0 10 20 30 40 50 60 70 80

Miles per Hour

Stop

ping

D

ista

nce

2.5 secondreaction time1.5 secondreaction time0.6 secondreaction time

Example

A driver of a car applied the brakes and barely avoided hitting an obstacle on a roadway section which has a 5 percent gradient. The vehicle left skid marks 26 meters. Assuming that the coefficient of friction is 0.6 and the driver was travelling down the grade; determine whether the problem was the speed limit of 70kph on the section or driver violation of the speed limit.

Passing Sight Distance • Minimum distance required to safely complete passing

maneuver on 2-lane two-way highway • Allows time for driver to avoid collision with approaching

vehicle and not cut off passed vehicle when upon return to lane

• Assumes: 1. Vehicle that is passed travels at uniform speed 2. Speed of passing vehicle is reduced behind passed vehicle as it

reaches passing section 3. Time elapses as driver reaches decision to pass 4. Passing vehicle accelerates during the passing maneuver and

velocity of the passing vehicle is about 16km/hr greater than that of the passed vehicle

5. Enough distance is allowed between passing and oncoming vehicle when the passing vehicle returns to its lane

Passing Sight Distance

d1

(1/3)d2

d3 d4

(2/3)d2

Passing Sight Distance

PSD = d1 + d2 + d3 + d4

d1 = distance traveled during perception/reaction time and distance traveled while accelerating to passing speed and when vehicle just enters the left lane

Where t = perception/reaction time and the time for

acceleration, for example,70-85km/h, t=4 sec, for 100-110 km/h, t=4.5sec

V= design speed km/h a = acceleration (km/hr/sec), for 70-110 km/h, a =2.4

)2

(278.0 111

atmVtd +−=

Passing Sight Distance PSD = d1 + d2 + d3 + d4 d2 = distance traveled during overtaking time d3 = clearance distance between the passing vehicle and the opposing vehicle at the moment the passing vehicle returns to the right lane. Usually d3 varies b/n 30 and 90m.

d4 = distance traveled by opposing vehicle during 2/3 of the time the passing vehicle is in the left lane. (d4 usually taken as 2/3 d2 )

Design speed (Km/h) 50 60 70 85 100 120

Passing sight distance(m) 140 180 240 320 430 590

Meeting Sight Distance

• Is the distance required to enable the drivers of two vehicles traveling in opposite directions to bring their vehicles to a safe stop after becoming visible to each other.

• Meeting sight distance is normally calculated as twice the minimum stopping sight distance.

DESIGN VEHICLE

• Design Vehicle are vehicles with representative dimensions and operating characteristics used to establish highway design controls for accommodating vehicles of a designated class.

• Design vehicles are used to define several geometric features. Some examples are:

• Turning radii • Vehicle height • Driver Eye Height

For design purpose each design vehicles has larger physical dimensions and a larger minimum turning

radius than in the class.

DESIGN VEHICLE • As per ERA Design Manual there are four general classes of design vehicles has

been established: • 4*4 utility vehicle • Single unit truck • Single unit Bus • Semi-Trailer Combination

The design vehicles are therefore, hypothetical vehicles, selected to represent a particular vehicle class.

typically the 85th percentile or 15th percentile value of any given dimension.

DESIGN VEHICLE

Dimensions and Turning Radius for a Single Unit Truck (DV2)

TRAFFIC CHARACTERISTICS

• Traffic volume directly affects the geometric features such as no. of lanes, widths, alignment, and grade.

• Average Annual Daily Traffic (AADT) is the average of 24-hr counts collected every day in the year.

• Average Daily Traffic (ADT) is the average of 24-hour counts collected over a number of days greater than 1 but less than a year.

• Peak-Hour Volume: traffic volumes for an interval of time shorter than a day more appropriately reflect the operating conditions that should be used for design.

TRAFFIC CHARACTERISTICS

• Directional distribution: is an indication of the tidal flow during the day. – Directional distribution is relatively stable and

does not change materially from year to year. • Traffic composition: Vehicles of different sizes

and mass have different operating characteristics. – The percentage of truck traffic during the peak hours

has to be estimated.

TRAFFIC CHARACTERISTICS

TRAFFIC CHARACTERISTICS

• Design Speed: the highest continuous speed at which individual vehicles can travel with safety on the road when weather conditions are favorable, traffic volumes are low and the design features of the road are the governing condition for safety.

Horizontal and vertical alignment, superelevation and sight distance. Other elements such as lane width, shoulder width and clearance from obstacles are

indirectly related to design speed.

• The facility should accommodate nearly all reasonable demands (speed) with appropriate adequacy (safety and capacity) but should not fail completely under severe load, i.e. the extremely high speeds maintained by a small percentage of drivers.

Thank You!