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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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