Highway Engineering

150 Highway Engineering

Example 5.9 Contd

Let us assume a speed of 30 km/h which means that it takes a vehicle just

under 48 seconds to progress from one junction to the next. Knowing this,

together with the minimum and maximum actual green times (41 s and 43 s),

the time/distance diagram illustrating the ability of vehicles to progress in

both directions through the network of intersections can be compiled.

An illustration of the time-distance diagram for the above example is

shown in Fig. 5.34. Note that the vehicles travel between junctions during

approximately half the cycle time.

104 s

N/bound

N/bound

Distance (m)

A3

A2

A1

S/bound

S/bound

100

200

0

Cycle time

Time (s)

Figure 5.34Illustration of time-distance diagram for Example 5.7.

The computer programs TRANSYT (Robertson, 1969) and SCOOT (Hunt

et al., 1981) can be used for signal optimisation and traffic control system analy-

sis. TRANSYT automatically finds the optimum timing that will co-ordinate

the operation of a signalised network of intersections. It produces what is

termed a performance index for the network, assessing it on the basis of a

weighted combination of delays and stops. As in the above worked example,

TRANSYT assumes that all signals have a common cycle time and that the

minimum green and red periods are known. SCOOT, unlike TRANSYT, is an

on-line traffic control system, using measurements from road detectors in order