“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).