Highway Engineering

Chapter 4

Basic Elements of Highway Traffic Analysis

4.1 Introduction,

The functional effectiveness of a highway is measured in terms of its ability to

assist and accommodate the flow of vehicles with both safety and efficiency. 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. This process of examination is termed traffic analysis and the

sections below deal with relationships between the parameters which lie at its

basis.

4.2 Speed, flow and density of a stream of traffic,

The traffic flow,q,a measure of the volume of traffic on a highway, is defined

as the number of vehicles,n, passing some given point on the highway in a given

time interval,t, i.e.:

(4.1)

In general terms,qis expressed in vehicles per unit time.

The number of vehicles on a given section of highway can also be computed

in terms of the density or concentration of traffic as follows:

k (4.2)

n

l

=

q

n

t

=