London bus next to a 20 mph speed sign

Major UK study examines impact of sign-only 20 mph zones

  • November 23, 2018

The British Department for Transport has published a major research study on the impact of 20mph zones.

It assesses the outcomes of introducing 20 mph speed limit schemes (ie reducing speed limits from 30mph to 20mph) in residential areas and town centres. The researchers looked at “signs only” schemes, i.e. without the traffic calming measures of earlier 20mph zones.

The study found:

  • Public support for 20mph (signed only) limits but concern about non-compliance;
  • Minor changes in driven speeds: median speed fell 0.7mph in residential areas and 0.9mph in city centre areas;
  • Faster drivers reduced speed more: 1.1mph and 1.6mph respectively (85th percentile);
  • Road characteristics have a much larger impact on driven speeds than whether the road has a 30mph or 20mph limit;
  • No significant change in the short term in collisions and casualties in the majority of case studies;
  • The majority of people have not noticed a reduction in the speed of vehicles, and do not perceive there to be fewer vehicles driving at excessive speeds;
  • Small increase in use of active travel modes; mode shift cannot be determined from data.

Commenting on the study, David Davies, the Executive Director of PACTS, ETSC’s UK member said:

“The changes resulting from 20mph limits are disappointing but not surprising. The study finds that signed-only 20mph limits have very small effects on speed and, surprisingly, no statistically significant effect on casualties in the majority of locations. Local people do not perceive changes and behaviour changes are small.”

“An important finding is that “the speed at which people drive is influenced more by the look and feel of the road, than whether a 20mph or 30mph limit is in place. The DfT will need to take account of this and review its guidance to local authorities on setting local speed limits. They must look again for practical, effective measures to reduce speeds and casualties.”

“In 2007, on the basis of 20mph zones but before signed only 20mph limits had been introduced, PACTS recommended that “a default speed limit of 20mph in all built up areas is implemented in ways that achieve high levels of compliance.” The emphasis on compliance was important. Broadly speaking, we still support this recommendation.”