INTRODUCTION TO TRAFFIC ENGINEERING

of roads, streets and highways, their networks andterminals, abutting lands, and relationships with other modes of transportation

1.1 Definition, scope and goalDefinition of Traffic

Engineering --- It is the phase of transportation engineering that deals with:

• planning• geometric design and• traffic operations

Transportation Engineering is defined as a discipline applying technology and scientific principles to the planning, functional design, operation, and management of facilities for all modes of transportation

Scope of Traffic Engineering --- surface (land) transportation; relationships and connection with other modes of transportation

Major modes of surface transportation --- automobile, bus, truck and bike

Goal of Traffic Engineering --- explore how to provide for the safe, rapid, comfortable, convenient, economical, and environmentally compatible movement of people and goods.

Safe --- public safety Rapid --- time value and customer service Comfortable/convenient --- level of service Economical --- social cost Environmental --- clean air and sustainability Movement = mobility

Basic Elements of Highway Traffic Analysis In order to measure its level of

effectiveness, certain parameters associated with the highway must be measured and analysed. These properties include:

The quantity of traffic The type of vehicles within the traffic

stream The distribution of flow over a period of

time (usually 24 hours) The average speed of the traffic stream The density of the traffic flow. Analysis of these parameters will directly

influence the scale and layout of the proposed highway, together with the type and quantity of materials used in its construction

Speed, flow and density of a stream of traffic

In general terms, q is expressed in vehicles per unit time.where the traffic density, k, is a measure of the number of vehicles, n, occupying a length of roadway, l.

Where, q= traffic flow n=no. of vehicle l=a given length of

roadwayk= density or

concentration of traffic

thus, the three parameters u, k and q are directly related under stable traffic conditions:

It can be seen that if the expression for q is divided by the expression for k, the expression for u is obtained:

This constitutes the basic relationship between traffic flow, space mean speed and density.

Speed-density relationship In a situation where only one car is travelling

along a stretch of highway, densities (in vehicles per kilo-metre) will by definition be near to zero and the speed at which the car can be driven is determined solely by the geometric design and layout of the road; such a speed is termed free-flow speed as it is in no way hindered by the presence of other vehicles on the highway. As more vehicles use the section of highway, the density of the flow will increase and their speed will decrease from their maximum free-flow value (uf) as they are increasingly more inhibited by the driving manoeuvres of others. If traffic volumes continue to increase, a point is reached where traffic will be brought to a stop, thus speeds will equal zero (u=0), with the density at its maximum point as cars are jammed bumper to bumper (termed jam density, kj)

Greenshields (1934) proposed the simplest representation between the two variables, assuming a linear relationship between the two (see Fig. Below)Thus, the limiting values of the relationship between speed and density are as follows:When k=0,u=uf and, When u =0,k=kjIn mathematical terms, this linear relationship gives rise to the following equation:

This assumption of linearity allows a direct mathematical linkage to be formedbetween the speed, flow and density of a stream of traffic.

Flow-density relationshipFrom the equation

derived above, we have:

This is a parabolic relationship and is illustrated in the Figure

In order to establish the density at which maximum flow occurs, is differentiated and set equal to zero as follows:

the term within the brackets must equal zero, therefore:

km, the density at maximum flow, is thus equal to half the jam density, kj. Its location is shown (see the slide above)

Speed-flow relationship In order to derive this

relationship, rearrange the equation to and

Combining it with

You will get

This relationship is again parabolic in nature.

In order to find the speed at maximum flow, is differentiated and put equal to zero:

the term within the brackets must equal zero, therefore:

• um, the speed at maximum flow, is thus equal to half the free-flow speed, uf. Its location is shown in Fig.

(see the above slide)

Combining Equations:

the following expression for maximum flow is derived:

Capacity and level of service analyses

The no. of vehicles on our highways increases every year ,and transportation engineers are often faced with the challenge of designing modifications to existing facilities that will service the increased demand.

Capacity: is the maximum no. of vehicles that a given highway can accommodate.

LOS: is an operating condition under capacity.

Traffic engineers use capacity & LOS analysis to:oDetermine the no. and width of lanes

neededoAssess service levels and operational

characteristics of existing facilities.oIdentify traffic and roadway changes

needed for new dev’t.

Levels of Service

LOS A Free-flow operation

LOS B Reasonably free flow Ability to maneuver is only

slightly restricted Effects of minor incidents

still easily absorbed

From H

ighway C

apacity Manual, 2000

LOS conceptsHighway capacity manual defines the LOS categories for freeways and multilane highways as follows:

Levels of Service LOS C

Speeds at or near FFS Freedom to maneuver is

noticeably restricted Queues may form behind any

significant blockage.

LOS D Speeds decline slightly with

increasing flows Density increases more

quickly Freedom to maneuver is more

noticeably limited Minor incidents create

queuing

From H

ighway C

apacity Manual, 2000

Levels of Service

LOS E Operation near or at capacity No usable gaps in the traffic

stream Operations extremely volatile Any disruption causes

queuing

LOS F Breakdown in flow Queues form behind

breakdown points Demand > capacity

From H

ighway C

apacity Manual, 2000

LOS determinationBase condition: * lane width * terrain * lateral clearance * traffic condition

stream * access frequency * driver population

characteristicsSteps:1) Measure Free-flow speed from the field-is the

mean speed of traffic as measured when flow rates are low to moderate

2) Analysis flow rate –the highest volume in a 24-hr (the peak-hr volume) is used for V in traffic analysis computation.

3) Determine LOS

For further knowledge read Highway Engineering by Rogers M.

THANK YOU

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