TRL research shows that, on urban streets in the UK, a 1mphreduction in vehicle speeds typically produces a 5% reduction incollisions
ēThe profile of car speeds on roads with a 20mph speed limit and roadswith a 30mph limit are quite different, e.g. 49% of cars using roadswith a 20mph speed limit in free flow conditions travel at 24mph orless, compared with 12% of cars using roads with a 30mph limit.
ēThe imposition of 20mph speed limits on roads previously subject to a30mph limit typically produces a small, but statistically significant,reduction in average (both mean and median) vehicle speeds.
ēCity-wide 20mph speed limits generally reduce road collisioncasualties, although some smaller schemes have not reducedcasualty numbers.
ēThe imposition of 20mph speed limits on roads previously subject to a30mph limit can produce small increases in modal share for walkingand cycling.
ēThe air pollution impacts of 20mph speed limits are mixed, and soshould not be assumed to be conclusively positive or negative.
ē20mph speed limits generally have no significant impact on journeytimes or traffic congestion.