Roadway improvements that reduce traffic congestion tend to increase total vehicle travel, due to latent demand. This is called "generated" or induced traffic. Generated traffic consists of trips that are shifted in time, route and mode, and new or longer vehicle trips. Recent research indicates that a typical roadway improvements can generate significant amounts of traffic. Generated traffic has three implications for transport planning. First, generated traffic reduces predicted congestion reduction benefits of increasing highway capacity, particularly over the long term. Second, generated traffic increases external costs (costs imposed on others) of automobile use. Third, generated traffic provides relatively small user benefits, because it consists of vehicle travel that consumers are most willing to do without if congestion increases. It is important that all three of these impacts be considered when evaluating roadway capacity expansion projects. Failing to do so tends to overstate the benefits of roadway capacity expansion, and undervalue other transportation improvements. Alternative congestion reduction strategies that make more efficient use of existing capacity may provide greater social benefit in many situations. This paper summarises recent research on generated traffic, describes different types and their impacts, provides recommendations for incorporating generated traffic into transportation decision making and describes ways to encourage more efficient use of existing roadway capacity. (Author/publisher)
Abstract