Suppressed Demand
1.Many would-be drivers are put off by congestion and the difficulty in finding places to park.
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2.Building a new road (or widening an existing one) creates more space to drive in.
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3.The space fills with more would-be drivers and congestion quickly reaches the same level as before(1).
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This effect has confounded many anti-congestion efforts, particularly 'soft' measures such as travel plans for company employees or increasing public transport provision. There are simply too many would-be drivers. Road pricing, on the other hand, is a form of demand management - it deters people from wanting to drive. But even so, it is not enough on its own.
The mirror image of this effect also occurs. When roads are removed or become too congested, many drivers simply decide not to drive. This effect has been called 'Traffic Evaporation'(2), and it was famously observed during the closure of Lendal Bridge for a month in the late seventies, which utterly failed to bring the city to the predicted standstill.
Notes and References
1.
The effect, also known as 'Traffic Induction' has been written about widely. See, for example "New roads generate new traffic" (R.H.H.Pfleiderer and M Dieterich, in World Transport Policy and Practice, Vol.1 No.1, pgs. 29-31). Perhaps the most definitive study is Trunk roads and the generation of traffic (Standing Advisory Committee on Trunk Road Assessment, 1994), available from The Stationery Office.
2.
This effect was most famously supported and promoted in the study Traffic Impact of Highway Capacity Reductions; Assessment of the Evidence (Cairns, Hass-Klau and Goodwin, 1998) - available from Landor Publishing for £35. You can read a synopsis or a shorter summary report on-line. The article Disappearing traffic? The story so far (Cairns, Atkins and Goodwin, in Municipal Engineer 151 Issue 1) is also worth reading.
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