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Chapter 2 | Prevailing strategies to manage congestion

How significant is the induced travel effect

A decline in congestion and the consequent reduction in travel will invariably attract

more traffic. Exactly how much more traffic depends on a number of factors, including

time savings involved. In general, the magnitude of the induced travel effect will

depend on:

• the behavioural response to the capacity expansion; and

• how broadly induced travel is interpreted.

The following section reviews the relative importance of the factors that determine

the magnitude of the induced travel effect.

Behavioural response to the capacity expansion

The response of users to a reduction in congestion due to capacity expansion will

vary between locations and, for the same location, at different times. People can

make immediate and simple adjustments to travel patterns, such as leaving for work

a little later in the morning, while in the longer term they can change their work or

home location.

Because the level of induced travel is so much a function of location, it is not

surprising that there is wide variance in the short- and long-term responses to

capacity expansion. Estimates of the traffic growth due to network expansion vary

widely and in the following table range between 10 per cent (in the short term) to up

to 100 per cent (in the long-term), as illustrated in the following summary compiled

by the Sierra Club (Table 2.1).

Table 2.1

Share of new capacity filled with induced travel

Short‐term Long‐term (3 years)

(per cent)

SACTRA 50–100

Goodwin 28 57

Johnson and Ceerla 60–90

Hansen and Huang 90

Fulton, et al 10–40 50–80

Marshall 76–85

Noland 20–50 70–100

Source:

Sierra Club (undated).

Economics has a term for the responsiveness of the level of consumption of a good

or service to a change in price: the elasticity of demand. The more elastic the demand

for road use, the greater the increase in use following a fall in price. In this case, the

‘price’ is the generalised cost of travel.

Essentially, demand is regarded as elastic if a small reduction in the generalised

cost results in a large increase in demand. Such a high degree of responsiveness

is suggested by the studies where traffic congestion ‘recovers’ to its previous level

within 3 years of capacity expansion. For instance, dramatic traffic growth followed

the completion of the M25, the 188 kilometre orbital motorway that encircles London

(see Figure 2.8). Within one year of opening, traffic levels had reached the levels

forecast for the 15th year after opening:

29

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