“You get the traffic you build for” is an axiom that all of Oxfordshire’s localities should live by. Years of research conclude that building more roads generates additional traffic, and reducing the amount of road-space evaporates some traffic.

This post is about evaporation. As the image implies, some of the traffic switches to other modes; some chooses completely different routes. It’s fallacious to think of traffic as water, which has to find a way through. Some of it actually goes away.

In terms of the research, Rachel Aldred in Motor traffic on urban minor and major roads: impacts on pedestrian and cyclist injuries (page 2) cites Cairns S, Hass-Klau C and Goodwin P (1998) Traffic Impact of Highway Capacity Reductions: Assessment of the Evidence. Landor Publishing, London, UK).

Todd Litman in Generated Traffic and Induced Travel: Implications for Transport Planning (page 5) cites that same Cairns et al (1998) paper, plus five others:

Such [induced demand] impacts can also occur in reverse: reducing urban roadway capacity often reduces total vehicle travel (Cairns, Hass-Klau and Goodwin 1998; Cervero 2006; CNU 2011; ITDP 2012; Miller 2006) which is sometimes called traffic evaporation (EC 2004).

1 reply
  1. Barb Chamberlain
    Barb Chamberlain says:

    You said it isn’t like water, and yet it is since water evaporates. Keep that metaphor streaming right along.

    I wish the graphic showed trips diverted to walking/rolling with an illustration of people walking and using wheelchairs. Great illustration of the concept, though. I’d couple it with the funnel illustration I’ve seen that points out that highway volumes of traffic have to enter local streets so adding highway capacity doesn’t actually alleviate congestion in that sense either. In my part of the world (Seattle) the highway backlog can easily result from the volume of drivers at an exit backed up onto the highway lane because the surface streets aren’t designed for highway volumes, nor should they be.


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