7 traffic-introduction ( Transportation and Traffic Engineering Dr. Sheriff El-Badawy )

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المنصورة-قسم الهندسة المدنية –معهد مصر العالي للهندسة والتكنولوجيا

Introduction to Traffic

Engineering


Traffic Engineering

Definition: Branch of civil engineering that uses

engineering techniques to achieve safe, efficient,

and economic movement of people and goods.

Traffic Engineers: manage, organize, and control

the actual movement of vehicles and pedestrians

on the facility.

Basic Components in Traffic Engineering:

• Road users (driver, pedestrian, cyclists)

• Vehicles

• Roads and highways

• Control Devices

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Traffic Flow

Characteristics


The theory of traffic flow can be defined as:

A mathematical study of the movement of vehicles

over road network.

Definition of Traffic Flow

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1. SPEED (V)

2. VOLUME (q) = FLOW

3. DENSITY (K)

1. HEADWAY (h)

2. SPACING (S)

3. OCCUPANCY ( R )

4. CLEARANCE (C )

5. GAP (g)

Traffic Flow Parameters

Primary Parameters


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1. Speed

• In a moving traffic flow, each vehicle travels at a different speed.

• Thus, the traffic stream does not have a single characteristic speed but rather a distribution of individual vehicle speeds.

• From the distribution of vehicle speeds, a number of “average” or “typical” values may be used to characterize the traffic stream as a whole.

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Speed (contd..)

• Average or mean speeds can be computed in two different ways:

1. Space Mean Speed (SMS) is defined as the average speed of all vehicles occupying a given section of a highway over some specified time period.

2. Time Mean Speed (TMS) is defined as the average speed of all vehicles passing a point on a highway over some specified time period.

• Time mean speed is a point measure, while space mean speed is a measure relating to a length of highway or lane.

it

nd

nit

dSMS

nitd

TMS


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Speed (contd..)

• where, d is the distance traversed, n is the number of travel times observed and ti is the travel time for i-th vehicle.

it

nd

nit

dSMSandn

itd

TMS

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المنصورة-قسم الهندسة المدنية –معهد مصر العالي للهندسة والتكنولوجيا 9

Traffic Stream Parameters

Speed (contd..)



Speed (contd..)

it

nd

nit

dSMS

nitd

TMS

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SPEED ( V )

Rate of motion ( Distance / Unit time)

There are 3 more types of speeds:

3- Spot Speed

يمكن مالحظتها على عداد السيارةوالتىالسرعة اللحظية

4- Running Speed

السرعة مقاسة على أساس زمن الحركة فقط

5- Overall Speed

السرعة مقاسة على أساس الزمن الكلى للرحلة متضمنا التأخيرات


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Volume and Flow

• Volume is defined as the number of vehicles that passa point on a highway, or a given lane or direction of ahighway, during a specified time interval.

• Usually expressed as vehicles per unit time, forexample, vehicles per hour or vph.

• Rate of Flow is the equivalent hourly rate at whichvehicles pass a point on a highway lane during a timeperiod less than 1 hour.

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• Volume and Rate of Flow are two different measures.

• Volume is the actual number of vehicles observed or predicted to be passing a point during a given time interval.

• Rate of flow represents the number of vehicles passing a point during a time interval less than 1 hour, but expressed as an equivalent hourly rate.


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• A volume of 200 vehicles observed in a 10-minute

period implies a rate of flow of:

(200 x 60)/10 = 1200 veh/hr

• Note that 1200 vehicles do not pass the point of

observation during the study hour, but they do pass

the point at that rate for 10 minutes.

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Density or Concentration

• It is defined as the number of vehicles occupying

a given length of a roadway, averaged over time,

usually expressed as vehicles per mile (veh./mi)

or vehicles per mile per lane (v/mi/ln).

q = v * k

where:

q = rate of flow (veh/hr)

v = average travel speed (mph)

k = average density (veh/mi)


Density

No of vehicles/Distance

Density

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Spacing (s)

• Front bumper to front bumper distance

between successive vehicles

S1-2S2-3


Spacing

Spacing

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T=0 sec

Headway (h)

• Time between successive vehicles

passing a fixed point

T=3 sec

h1-2=3sec


Clearance and GapClearance: is defined as the distance

between successive vehicles in a traffic

stream as measured from front bumper of the

behind vehicle to back bumper of the front

vehicle

Gap: is the corresponding time between

end of the front vehicle and front of the behind

vehicle

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Clearance (c) and Gap (g)

• Front bumper to back bumper distance

and time

Spacing (ft) or headway (sec)

Clearance (ft) or Gap (sec)

Clearance (ft) = (spacing) – (average vehicle length)

Gap (sec) = (headway) – (time equivalence of the average vehicle length)

Gap: is the corresponding time between end of

the front vehicle and front of the behind

vehicle


Clearance

Clearance

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Lane Occupancy

• Ratio of roadway occupied by vehicles

L1L2L3

D

D

L

LO i

i


• Speed (v) – ft/sec or mph

• Flow (q) – veh/sec or vph

• Density (k) – veh/ft or vpm

• Spacing (s) – ft/veh

• Headway (h) – sec/veh

• Clearance (c) – ft/veh

• Gap (g) – sec/veh

Remember, units are critical!

Units

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Fundamental Relationships

• q = k v

(veh/hr) = (veh/mi) (mi/hr)

• h = 1 / q

(sec/veh) = 1 / (veh/hr) (3600)

• s = 1 / k

(ft/veh) = 1 / (veh/mi) (5280)


Types of Facilities

• Uninterrupted flow

• Freeways

• Multilane highways

• Two-lane highways

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Types of Facilities

• Interrupted flow

• Signalized streets

• Un-signalized

streets with stop

signs

• Transit lanes

• Pedestrian

walkways


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Speed, Flow and Density Relationship

• Assume

• We have

BkAv

2BkAkkvq

B

vv

B

A

B

vAvq

2

Jam Density

A/2

q=kv

Jam Density

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General properties for any

traffic flow model

• Need to satisfy four boundary conditions

• Flow is zero at zero density

• Flow is zero at maximum density

• Mean Free-Flow Speed occurs at zero density

• Flow-density curves are convex

(i.e. there is a point of max flow)


Connections between speed,

density and flow• A: almost zero density, free-flow speed, very low

volume

• B: increased density, reduced speed, increased

volume

• C: increased density, reduced speed, max volume

• D: jam density, min speed (crawling), very low

volume

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Since a given flow may occur under two completely

different operating conditions (stable and unstable),

volume or rate of flow cannot be used as a measure

describing the operational quality of the traffic

stream.

Speed and density, however, are good measures of

the quality of operations, as both uniquely describe

the state of the traffic stream.


Greenshield’s Model

• Assume a linear relationship between v and k:

High Density = Low

Speed

Low Density =

High Speedvf

kj

kk

vvv

j

f

f

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K

Q

Max flow

qmax

K0 Kj

vf v0

2kk

vkvq

j

f

f

kk

vvv

j

f

f



Q

V

Max flow

Qmax

Vf

V0

1/k0

Density=inverse of slope

7 traffic-introduction ( Transportation and Traffic Engineering Dr. Sheriff El-Badawy )

Engineering

traffic flow parameters

rate of flow

traffic flow parametersvolume

moving traffic flow

theory of traffic flow

traffic flow parameters1

traffic flow parameters

flow volume