20mph speed zones do little to cut collisions and casualties

‘Some drivers were unaware of the 20mph speed limit intervention and enforcement was unlikely’

SPEED limits of 20mph in town and city centres has little impact on road traffic collisions and casualties, partly because motorists are ignorant of the restrictions, research has found.

Limits have been lowered from 30mph in hundreds of urban areas in the past 20 years, and London is due to bring in blanket 20mph restrictions for all inner-city roads by 2024.

However, experts from Queen’s University

Belfast have found that although the limits reduce traffic volume where applied they do little to stop accidents – largely because they are often ignored.

The team gathered traffic and accident data taken in Belfast in 2013 and 2014 and compared them to 2017, a year after 20mph restrictions were imposed, and in 2019, three years later.

The data were also compared with city centre streets where the restrictions didn’t apply, as well as streets in the surrounding metropolitan area and similar streets elsewhere in Northern Ireland that had all retained their speed limits of between 30 and 40mph.

Analysis of all the data showed that when compared with the sites that had retained their old speed limits, a 20mph limit was associated with little change in outcomes for road traffic collisions, casualties, or driver speed.

Average traffic speed fell by only 0.2mph in the first year and by 0.8mph after three years.

Ruth Hunter, professor of public health and planetary health at Queen’s University Belfast, said: “We did find reductions in casualties and collisions. However, these were generally small, and we could not be certain that they were not beyond a chance finding, hence not statistically significant. We did observe significant reductions in road traffic volume.

“The 20mph limit may have reduced traffic volume due to changes to alternative routes or change in travel mode.

“Our evaluation highlights that some drivers were unaware of the 20mph speed limit intervention and that enforcement was unlikely,” she said.

Many towns and cities have brought in reduced speed limits to prevent deaths. At speeds of 30 to 40mph, the risk of pedestrian fatalities was 3.5 to 5.5 times greater than at with collisions at speeds of 20 to 30 mph.

Some cities, such as Edinburgh, have shown significant reductions in road traffic speed, collisions and casualties by bringing in the lower speed limits.

Road deaths fell by nearly a quarter and serious injuries by a third when Edinburgh cut its speed limit to 20mph.

The research was published in the