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University study questions impact of 20mph limits in Belfast city centre

Deniz Huseyin
30 November 2022

Restricting speed limits to 20mph in town and city centres does not seem to reduce road traffic collisions, casualties, or driver speed, a study by Queen’s University Belfast has concluded.

Over a three-year period, researchers drew on data for road traffic collisions, casualties, driver speed and traffic volume in Belfast, before, and one and three years after 20mph speed limits were introduced in 2016 in 76 streets in the city centre.

“Analysis of all the data showed that when compared with the sites that had retained their speed limits, a 20mph speed limit was associated with little change in short- or long-term outcomes for road traffic collisions, casualties, or driver speed,” the researchers said.

The data for roads with 20mph limits was compared with city centre streets where the restrictions did not apply, as well as streets in the surrounding metropolitan area and similar streets elsewhere in Northern Ireland that had all retained speed limits of 30-40 mph.

The study, led by Professor Ruth Hunter from the Centre for Public Health at Queen's University Belfast, noted there were small reductions in road traffic collisions of 3% and 15%, at one and three years respectively after 20mph limits were introduced. “But there was no statistically significant difference over time,” the study found.

Casualty rates fell by 16% and 22%, in years one and three after implementation, but these reductions “were not statistically significant”, said the authors. They also found that average traffic speed fell by 0.2 mph in year one, and by 0.8 mph in year three.

The authors acknowledged that their study was “observational” and “relatively small scale” when compared with similar studies.

Previous research has suggested that 20mph speed limits should “be supplemented with other interventions such as driver training, social marketing, community engagement, CCTV, in-car interventions, community interventions (eg, speed watch), and police communications”, the authors note.

“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 20mph 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.”

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.”

Adrian Davis, senior fellow, Business School at Uwe Bristol and professor of transport & health, Transport Research Institute, Edinburgh Napier University, points out that the Belfast city centre 20mph study was part of a larger study involving evaluation of the city-wide 20mph scheme in Edinburgh. 

He said the evidence from 20mph speed limit interventions involving much larger numbers of roads have revealed significant vehicle speed and casualty reductions. 

He told LTT: “Lazy journalism in the local press has grabbed the headline of no casualty reductions and ignored the fact that only a small number of city centre roads were included where, because of congestion and limited road space, the likelihood of significant changes in casualties was always low. 

“Meanwhile, empirical evidence from UK locations such as Edinburgh, Bristol, and Portsmouth has reported clear evidence of casualty reductions from city-wide roll-out. 20mph speed limits is best viewed as a public health population-wide intervention whereby small reductions in driver speeds lead to bigger reductions in death and injury reduction.”

Rod King MBE of campaign group 20’s Plenty for Us said the report illustrates that implementing 20mph “over a small area in Belfast where speeds are already below 20mph is far inferior to the established experience from city-wide schemes that reduce speed and casualties significantly”. 

King said the report shows how not to implement a 20mph scheme, which was “on just 76 streets in the city centre on which the average vehicle speed was well below 20mph”. 

He added: “Of these 76 streets, 27 were already fully or partially pedestrianised. At the same time other travel initiatives included the introduction of a rapid transit system and extension of the city centre bus lane provision. There was also little community engagement or marketing of the 20mph scheme.”

A scheme that kept 30mph as a norm on all the other streets in the centre “was always going to result in little change, and this report confirms this”, said King. “In fact, other reports which compared Belfast with Edinburgh and its city-wide 20mph scheme show just how more effective 20mph schemes are when implemented city-wide.

“The contrast couldn't be starker. Belfast implements a small and isolated 20mph scheme in its city centre and gets little change on congested streets where speeds are already low. Whilst Edinburgh goes city-wide delivering it to city-centre, suburbs, residential areas, shopping streets with engagement and education and gains a significant reduction in speeds and casualties.”

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