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Stop Sprawl
Generated Traffic Implications

Generated Traffic Implications for Transport Planning

By Todd Litman
Victoria Transport Policy Institute

Abstract

Traffic congestion tends to maintain an equilibrium. Congestion reaches a point at which it constrains further growth in peak-period trips. If road capacity increases, the number of peak-period trips also increases until congestion again limits further traffic growth. The additional travel is called "generated traffic." Generated traffic consists of diverted traffic (trips shifted in time, route and destination), and induced vehicle travel (shifts from other modes, longer trips and new vehicle trips). Research indicates that generated traffic often fills a significant portion of capacity added to congested urban road.

Generated traffic has three implications for transport planning. First, it reduces predicted congestion reduction benefits of road capacity expansion, particularly over the long term. Second, it increases many external costs, particularly over the longer term due to changes in transportation choices and land use patterns. Third, it provides relatively small user benefits because it consists of vehicle trips that consumers most willing forego.

It is important that transport planning incorporate generated traffic. Failing to do so tends to overstate the benefits of roadway capacity expansion and undervalues alternatives. This paper defines various types of generated traffic, discusses the impacts of generated traffic, recommends ways to incorporate generated traffic into transport planning, and describes ways to use existing roadway capacity more efficiently.


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