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 other the long term, can include new of longer vehicle trips. Recent research indicated that the traffic generated from typical roadway improvements can be significant. Generated traffic has three implications for roadway investment analysis. First, generated traffic provides relatively small user benefits, because it constitutes the trips that vehicle users are most willing to change due to congestion delays. Second, generated traffic reduces predicted congestion reduction benefits, particularly over the long term. Third, generated traffic increases external costs (costs imposed on society) of automobile use. It is important that all three of these impacts be considered when evaluating roadway improvements. Current transportation planning often fails to do this, which tends to overstate the benefits of roadway improvements and undervalue improvements to alternative transportation options. This paper summarizes recent research on generated traffic, describes different types of generated traffic and their impacts, and provides recommendations for incorporating generated traffic into transportation decision making.
Abstract