CE 436 Traffic Engineering

Topic 2: Components of the Traffic System

Prof. Saad AlGadhi

CE 436 –Traffic Engineering

King Saud University

email: salgadhi@ksu.edu.sa

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Traffic Stream Components and

Characteristics

Five Elements of Traffic System:

Users (drivers, pedestrians, etc.): Ch.2

Vehicles (private, commercial): Ch. 2

Roadways: Ch. 3

Controls: Ch. 4

General environment (weather, lighting, enforcement policies, etc.)

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Road Users

Human as active component of traffic system,

distinguishes it from virtually all other engineering

fields.

Component highly variable and unpredictable in

capabilities and characteristics.

Physiological –Measurable and Usually Quantifiable

Psychological –Much more difficult to measure and

quantify

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Examples of Psychological Factors

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Road User Diversity

Most human characteristics follow Normal distribution.

Traffic Engineering standard on 85th percentile or 15th percentile. Example on reaction time and speed.

Addressing diversity through uniformity, avoiding surprise (e.g Uniform Traffic Control Devices).

Two important human factors in traffic engineering: Visual Acuity and reaction process.

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Fields of Vision

1- Acute Vision Cone (30_ 100): can see legend

2- Fairly Clear (100_ 120 ): color and shape

3- Peripheral (1200_ 1800 ): 900 right/left, 600 up/700

below – moving objects

Traffic Signs: Location and Height

Shapes

Colors

Visual Deficit Issues

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Perception and Reaction Time:

P.I.E.V. (PIJR/PRT)

Perception of cue or stimulusInterpretationEvaluation of appropriate response (i.e.,

decision)Volition or physical response (i.e., reaction)Factors Affecting PIEV/PIJR (or PRT):

Expectancy ( = 0.5 s) Age Fatigue Complexity of Cues Presence of Drugs or Alcohol (enforcement)

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Expectancy

People react quicker to situations they expect to encounter as opposed to those they do not expect to encounter.

The PRT under “expected” situation is

0.5 seconds faster than the “unexpected” situation

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Reaction Times

AASHTO Recommendations: For braking reactions on Highways: Perception

and Reaction Time: 2.5 seconds (90th percentile) For reaction time to traffic signal: Perception and

Reaction Time: 1.0 Second (85th percentile)

Travel Distance During PRTdr= 1.47 * v * t

dr in feet , v in mph, and t in secondsdr= 0.278 * v * t

dr in m , v in km/h, and t in seconds

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Pedestrian Characteristics

Walk Speed: (used for signal timing)

4.0 fps (1.22 m/s)

Safe or 15th (accommodates 85th) percentile

Gap Acceptance: (crossing un-signalized intx.)

Median 84 ft (25.6 m)

125 ft (38.1 m) for 85th [between ped. & veh.]

Pedestrian’s comprehension of controls

e.g. Flashing “DON’T WALK” not understood by 50% of

road users !

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Vehicle Characteristics

Vehicle Characteristics Vary Acceleration/Deceleration

Turning characteristics

AASHTO Classification: 4 main categories (PC, B, T, RV)

20 “Design Vehicles”: accommodate 95th percentile.

AASHTO Standards for: Height

Width

Length

Minimum and Maximum Turning Radii

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Vehicle Performance

Impact of vehicle performance on

Road Design

Traffic operations

Truck Performance on Grades

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Vehicle Braking (or Stopping) Distance

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Stopping Sight Distance (1)

2 ft

3.5 ft

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Stopping Sight Distance (2)

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Change Interval at Traffic Signals

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Calculation

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Example

HW # 1

Problems #:

2-2

2-3

2-4

2-5

2-7

Due: Wednesday 22/4/1436 (4/2/2015)

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