Flaws in the DfT’s 20mph limit evaluation

20’s Plenty for Us welcomes the publication of the long-awaited DfT Evaluation of 20mph limits (‘No evidence that 20mph limits cut casualties, says DfT study’, LTT 23 Nov). It confirms the public support and acceptance of 20mph limits, but the report has failed to meet the original DfT study objectives. 

In 2014 the DfT set four objectives for the evaluation, which were confirmed in mid-project presentations by the consultants in 2016, though we note that the final report makes no mention of them. Our commentary on whether the objectives have been met is as follows: 

1. Effectiveness: to evaluate the effectiveness of 20mph speed limits, in a range of settings. The methodology is flawed in the comparisons made; it uses data with inherent bias on speed measurements and the data gathered is insufficient on casualties. Not met 

2. Perceptions: to examine drivers’ and residents’ perceptions of 20mph limits. The wide range of surveys are useful. Met

3. Cost/Benefits: to assess the relative costs/benefits to vulnerable groups e.g. children, cyclists, the elderly.  Cost-effectiveness was not assessed or compared with other interventions such as physically calmed zones. Not met

4. Processes & Factors: to evaluate the processes and factors which contribute to the level of effectiveness of 20mph speed limit schemes. These were discussed, but no evaluation made of the possible contribution in case studies chosen. Not met

Let us now discuss each objective in detail. 

The evaluation of effectiveness has been compromised by:

• Only including two authority-wide case studies, Portsmouth and Middlesbrough. All six others were part-authority or small isolated implementations. No case studies were in London boroughs or capital cities. This is hardly comprehensive or nation-wide.

• There is potential dissimilarity between the case studies and comparators used to assess background trends. 

• The assumption that comparators had “done nothing”, yet each had adopted alternative road safety interventions on the unproven premise that these would provide better value than wide-area 20mph schemes. 

• Casualty level in the chosen case study areas were so low (typically < 20) that any change would have no statistical significance. This was clearly predictable and led to false media headlines of “No proof 20mph speed limits make roads safer, official report says”.

• Using TomTom data as the primary mechanism for speed measurement with an estimated 3 per cent of all traffic is not credible.

• Acknowledging “affluence and behaviour bias” of TomTom data, but failing to quantify how this may have skewed results. Delivery drivers, taxi drivers and others with incentives to reduce journey times will be over-represented in the data.

• Using median rather than the mean speeds which in the words of the authors “dampen the impact of slower moving vehicles”.

We note that generally the reduction in casualties in the 20mph case study residential areas was greater than that on the comparator areas. In one city centre case study (Brighton) the casualty reductions were found to be statistically significant (18 per cent and 29 per cent) based on sample size but would benefit from further time periods. 

As for perceptions, post-implementation surveys showed that the majority of residents (78 per cent) and non-resident drivers (67 per cent) said that 20mph was an appropriate speed limit. It was also good to see some reference to the support from such organisations as WHO, OECD, the Global Network of Road Safety Legislators, Directors of Public Health etc for 20mph/30kmh as the safe speed for where motor vehicles mix with cyclists and pedestrians. A lack of police commitment to enforcement was recognised as the primary factor in low compliance with the limit.

On the relative cost/benefits of 20mph limits, there was no evaluation comparing wide-area 20mph limits with physical traffic calming. Neither was there any evaluation of the cost of even low-level routine and random enforcement, which could have radically changed the outcome and benefits. This objective seems to have been entirely dropped from the evaluation.

As for ‘processes and factors’, we have seen schemes implemented in many ways and we see that compliance levels (and hence effectiveness) would depend on a number of “success predictor” factors:

• Level of cross-party local political support

• Whether police gave supportive or negative messages regarding enforcement and the actual levels of enforcement that are seen to occur

• Whether public health and behaviour change specialists had been used

• Whether a multi-agency team had been used to promote wider community benefits 

Whilst the evaluation report references these factors as being important, it made no attempt to evaluate their relative importance, or levels of costs or benefits. This is a serious omission.

We welcome the section of the report covering lessons and considerations for national decision-makers. Much of this echoes some perspectives of 20’s Plenty for Us. However, it could have been condensed down to the following:

• 70 per cent of drivers in residential and 85 per cent in city centre 20mph limits reduce their speeds to 24mph or less. That is good and can be improved.

• We need lower speeds on our community streets for a wide variety of objectives that go way beyond casualty reduction and include public health and well-being, liveability, air quality, modal shift and active travel. 

• There is widespread evidence that 20mph is the safe speed limit where motor vehicles mix with pedestrians and cyclists. The 30mph limit set in 1934 is no longer fit for purpose. This is a reality that needs to be communicated at national level in an awareness campaign. 

• Lack of commitment to enforcement is a key factor in reducing compliance levels. Smarter and more cost effective enforcement and compliance is possible through automation, delegation to local authorities and rollout of Intelligent Speed Adaptation, first to works vehicles and eventually all vehicles.

From 2014, when the evaluation was commissioned, to 2017, 2,263 people have died on roads with a national 30mph speed limit that was set in 1934. There have been a further 369,000 casualties on those 30mph roads, of which 49,000 were serious injuries. The speed that these roads permit and endorse intimidates pedestrians and cyclists and deters the young, elderly and disabled from using such roads. That is why 30mph has been deemed inappropriate by so many progressive local authorities.

The evaluation report touches on the methods and benefits of making our places better places with 20mph limits. But we have serious questions about the data limitations through quantity or bias. 

Regardless of our concerns with the failings in this report, communities across the country are saying that in their streets 20 is plenty. The Government needs to start getting to grips with how it can deliver those limits effectively and start assisting those communities. It can do so best by setting a national 20mph limit with exceptions decided locally. It can maximise compliance through better enforcement, a national awareness campaign and use of Intelligent Speed Adaptation. It’s time it started building towards a 20mph default limit on community streets for the 2020s.