Table 3

Societal benefits and net present value (NPV) over 5- and 10-year time horizons of 20 mph zones

Time after implementation (years)Low casualty roadsHigh casualty roads
Benefit per km of road (£) (SD)NPV (90% credible limits)Percent of zones with positive NPV*Benefit per km of road (£) (SD)NPV (90% credible limits)Percent of zones with positive NPV*
523344 (3668)−36117 (−128225 to 20862)4178940 (14660)18947 (−75252 to 82021)74
1037278 (5905)−22183 (−114294 to 35289)53127299 (24232)67306 (−29157 to 137890)85
  • *Societal benefits outweigh costs.