City-centre 20mph limits have ‘little impact’ on collisions, casualties and speed, new study suggests

Cutting speed limits to 20mph in busy town and city centres ‘has little impact on road deaths or crashes’, according to a new study which examined data relating to streets with 20mph limits in Belfast.

The study by researchers from Queen’s University Belfast, found that while 20mph limits lead to quieter streets with fewer cars, they ‘don’t even cause drivers to slow down’.

The researchers say that while schemes to reduce speeds to 20mph have become increasingly popular in Britain and across Europe, there is ‘little evidence on their effectiveness and earlier studies have produced mixed results’. They add that ‘few such studies have covered long-term outcomes’.

The Queen’s University study looked at data on road traffic collisions, casualties, driver volume and traffic speed in Belfast before, one year after and three years after 20mph speed limits were introduced in 76 streets in the city centre in 2016.

They compared the data with city centre streets where the 20mph restrictions didn’t apply, as well as streets in the surrounding metropolitan area and similar streets elsewhere in Northern Ireland that had retained 30-40 mph speed limits.

The researchers found 20mph speed limits were associated with ‘little change in short or long-term outcomes for road traffic collisions, casualties or vehicle speed’.

Reductions of 3% and 15% in the number of crashes were found after one and three years, but the researchers say there was ‘no statistically significant difference over time’.

Casualty rates also fell by 16% and 22%, one and three years after implementation, but these reductions were also described as ‘not statistically significant’.

Average traffic speed fell by only 0.2 mph one year and by 0.8 mph three years after roll-out.

Weekly traffic volume fell by 57 vehicles one year after roll-out and by 71 vehicles three years after roll-out. The largest reductions were observed during the morning rush hour (between 8am and 9am) when there were 166 fewer vehicles a week, compared with ‘similarly matched streets’ where the 20mph speed hadn’t been applied.

A statistically significant decrease in traffic volume of 185 fewer vehicles a week was also found when comparing all sites before and three years after roll-out.

The report’s authors said: “Previous research has suggested that 20 mph speed limit interventions should be supplemented with other interventions such as driver training, social marketing, community engagement, closed-circuit television, in-car interventions, community interventions (eg, speed watch), and police communications.

“Such success may then have the capacity to facilitate an ambitious culture change that shifts populations away from the car-dominant paradigm and help us recognise that 20 mph speed limits are not simply a road safety intervention, but instead part of the fundamental reset of the way we choose our life priorities – people before cars.

“Our findings showed that a city centre 20 mph intervention had little impact on long-term outcomes including road traffic collisions, casualties and speed, except for a reduction in traffic volume.

“Future 20 mph speed limit interventions should consider the fidelity [enforcement], context and scale of implementation.”

The findings were published in the Journal of Epidemiology and Community Health.

The RAC described the findings as ‘surprising’ and ‘at odds with other reports.

Simon Williams, RAC road safety spokesman, said: “The findings of this study are surprising as they appear to suggest that drivers on 20mph roads in Belfast hardly slowed down at all, despite the lower speed limit, which is at odds with other reports.

“The study may demonstrate a need for councils to find other ways to get drivers to slow down, whether that’s through enforcement or modifying road design with traffic islands, well-designed speed humps or chicanes.

“It’s also important that 20mph limits are used in places where they stand to make the biggest positive impact, such as in built-up areas and in locations where there are large volumes of motorised traffic, cyclists and pedestrians.

“Our research shows drivers are less likely to comply with a lower limit if they don’t believe it’s appropriate for the type of road.”

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