CE 413 Highway and Traffic Engineering Lecture 4...Establishing minimum sight distance onhighway Length of the yellow phase at a signalizedintersection Typical Perception-Reaction

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CE 413Highway and Traffic Engineering

Lecture 4

Elements of Geometric Design

Engr. Amjad Khan

MS Transportation Engineering (NUST)

Important Driver Characteristics

Two most important driver characteristics

Visual and hearing perceptions

Perception-Reaction Process

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Perception

Identification

Emotion

Reaction (volition)

PIEV or PRTUsed for Signal Design and Braking Distance



Perception

Sees or hears situation (sees deer)

Identification

Identify situation (realizes deer is in road)

Emotion

Decides on course of action (stop, change lanes, etc)

Reaction (volition)

Acts (time to start events in motion but not actually do

action)

Foot begins to hit brake, not actual deceleration

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1. Detection/Perception: In this phase, an object or condition of concern enters the driver’s field of vision, and the driver becomes consciously aware that something requiring a response is present.

2. Identification: In this phase, the driver acquires sufficient information concerning the object or condition to allow the consideration of an appropriate response.

3. Decision/Emotion: Once identification of the object or condition is sufficiently completed, the driver must analyze the information and make a decision about how to respond.

4. Response/Reaction: After a decision has been reached, the response is now physically implemented by the driver


PRT is important factor:

Determination of braking distances

Establishing minimum sight distance on highway

Length of the yellow phase at a signalized intersection

Typical Perception-Reaction time range - 0.5 to 7 seconds

For stopping sight distance - AASHTO recommends 2.5 sec

PRT

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Perception-Reaction Time Factors

Environment:

Urban vs. Rural, Night vs. Day, Wet vs. Dry


Older drivers

May perceive something as a hazard but not act quickly enough

More difficulty seeing, hearing, reacting

Drive slower

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Younger drivers

May be able to act quickly but not have enough experience

Drive faster

Are easily distracted

Poorly developed risk perception


Physical Condition:

Fatigue

Increases perception/reaction time

Major factor in 41% of truck accidents in US

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Physical Condition:

Drugs

Slows reaction time

Increases risk taking

Slows decision-making

Peripheral vision difficulties


Distractions

Mobile

Music/ Video

Discussion/ talking with accompanied passengers

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Medical condition

Visual acuity - Ability to see fine details of an object


Complexity of situation (more complex = more time)

Expected versus unexpected situation (traffic light turning red vs. some animal coming on road)

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Perception-Reaction Process –Reaction Distance

where:

dr = reaction distance (feet/meters)

V = velocity (mph/Kmph)

tr = perception-reaction time

The Policy recommends the 2.5-second reaction time.

dr 1.47tr V

Perception-Reaction Distance Example

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Perception-Reaction Distance ExampleHow much longer does it take an impaired driver to perceive/react than an unimpaired one at 65 mph?

(1) Unimpaired has P/R time of 2.5 seconds

(2) Impaired Driver has P/R time of 4 seconds

Unimpaired has P/R time of 2.5 seconds

Dr = 1.47(V)(t) =

1.47(65 mph)(2.5 sec.) ~ 240 feet

Impaired Driver has P/R time of 4 seconds

Dr = 1.47(65 mph)(4 sec) ~ 380 feet

Difference is 380 – 240 = 140 feet

Difference is safety problem!

Sight Distance: Sight distance is the length of the roadway a driver can see ahead at any particular time.

Stopping Sight Distance (SSD)

Passing Sight Distance (PSD)

Sight Distance

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Safe and efficient operation of a vehicle - driver’s ability to see ahead

Constant changing path and speed

The designer should provide sight distance of sufficient length

Two-lane highways - sufficient passing sight distance

Sight distance provision - compatible with the intended function of the highway and desired LOS

Why Sight Distance is Needed

Stopping Sight Distance (SSD) Stopping Sight Distance (SSD) - Length of the roadway ahead that is visible

to the driver or the distance along a roadway throughout which an object ofspecified height is continuously visible to the driver.

Minimum sight distance available on the highway at any spot should be ofsufficient length to stop a vehicle traveling at design speed , safely withoutcollision with any other obstruction.

Composed of Two Parts

Distance traveled during perception/reaction time

Distance required to physically brake vehicle

SSD = PRD + BD

PRD = dr = 1.47(Vi)(t)

dr = Distance traveled during PRT(feet)

Vi = velocity (mph),

t = PRT= 2.5s (generally)

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Braking Distance (BD)

The basic physics equation on rectilinear motion.2aX= Vf

2 - Vi2

Vi = Initial Vehicle speed= ft/secVf = Initial Vehicle speed= ft/sec

a = acceleration (negative for deceleration) in ft/sec2

X = deceleration Distance in ft.

X = ��

� – ��

if Vf =0 vehicle comes to complete stop

X = – ��

��

a = – ��

�� AASHTO recommends a deceleration rate of 11.2 ft/sec2


Consider a vehicle traveling downhill with an initial velocity of Vi in mi/h,as shown in Figure

X

Y

W

W Cosθ

W Sinθ

θ

θ

NVi

F = ��

�

X

Db = is the horizontal

component of distance along the incline

X

θ

Db=X Cosθ

WfCosθ

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Derivation on Board


A student trying to test the braking ability of her car determined that she needed 18.5 ft. more to stop her car when driving downhill on a road segment of 5% grade than when driving downhill at the same speed along another segment of 3% grade. Determine the speed at which the student conducted her test and the braking distance on the 5% grade if the student is traveling at the test speed in the uphill direction.

Numerical

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Stopping Sight Distance – Level Terrain

V 2

d 1.47 * (V ) * ( 2 . 5 ) 1 . 0 7 511 .2

Stopping Sight Distance – On Grades

Vi 2 V f

2

d b 1.47 * V * (2.5) 30F 0.01G

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SSD Example

fi

fi

2S 2

2S 2

d 220.5344.83 565 ft

(60)2 (0)2d 1.47(60)(2.5)

300.3480

300.348 0.01G S

d 1.47Sit

If a 11.2 ft / s2 , g 32.2 ft / s2 , t 2.5sec

30F 0.01G S

d 1.47Sit

Use basic assumptions to determine SSD at 60 mph on

a) 0% grade, b) 3% grade

(a) G 0%

ftd

Gb

537

%3)(

Passing Sight Distance (PSD) - The passing sightdistance is the minimum sight distance required on atwo-lane, two way highway that will permit a driverto complete a passing maneuver without collidingwith an opposing vehicle and without cutting off thepassed vehicle

Minimum distance open to the vision of a driver of avehicle intending to overtake slow vehicle ahead withsafety against the traffic of opposite direction

Passing Sight Distance – Two-lane Highways

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Two-lane, Two way Highway

Two-lane, Two way Highway

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d1

d1

1/3 d2

2/3 d2

d2 d3 d4

First Phase

Second Phase

Passing VehicleOpposing Vehicle

Elements of Passing Sight Distance

d(1) = distance traversed during perception and reaction time and during the

initial acceleration to the point of encroachment on the right lane

d(2) = distance traveled while the passing vehicle occupies the right lane

d1

d1

1/3 d2

d2

2/3 d2

d3 d4

First Phase

Second Phase

Passing Vehicle Opposing Vehicle


d(3) = distance between the passing vehicle at the end of its maneuver and the

opposing vehicle

d(4) = distance traversed by the opposing vehicle for two-thirds of the time the

passing vehicle occupies the right lane

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Passing Sight Distance for Design of Two-Lane Highways


d3= clearance distance varies from 100 to 300 feet

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Documents

CE 413 Highway and Traffic Engineering Lecture 4...Establishing minimum sight distance onhighway Length of the yellow phase at a signalizedintersection Typical Perception-Reaction