Data doesn’t support Bristol’s 20mph active travel claims

It is no surprise that Rod King (Letters LTT 24 Jan) tried to refute many of the points I raised in my Viewpoint article on 20mph speed limits. I will not go through all our points of disagreement, as readers who wish to compare our arguments can decide for themselves, but there are a couple of statements he made that cannot be allowed to go unchallenged.

Mr King rather contemptuously dismisses the use of the 85th percentile principle for setting speed limits on the basis that it is “a relic from the 20th Century when the 15% fastest drivers (usually law breakers) were used as the basis for setting limits”. In fact, it is the 85% slowest drivers who set the limits, and decades of research both in the US and the UK has shown that limits set that way produce not only a high level of compliance but also a lower accident frequency than when limits are set too low. A summary of the evidence supporting the use of the 85th percentile is on the ABD website at http://www.abd.org.uk/speed_limits_85th.htm.

It is not just the ABD that wants a return to the 85th percentile principle in speed limit setting.  The Association of Chief Police Officers also expressed its preference for this method, instead of the mean (average) speed, in response to the Government’s 2012 consultation on revising the 2006 speed limit circular.

Setting speed limits at the average measured traffic speed means that half of all drivers are criminalised if they continue to drive at what is, in most cases, a safe speed. With the DfT now saying it is acceptable to set 20mph speed limits if the existing average speed on a road is no more than 24mph, this means that probably 75% or more of drivers will be exceeding the limit in such cases. It is madness to set a law that a large majority of people to whom it applies are expected to break.  It simply brings all speed limits, and the law itself, into disrepute. Whether Mr King likes it or not, speed limits only benefit road safety when set at a level that the majority of safe, responsible drivers find acceptable.

When a speed limit is set at the 85th percentile, most drivers will be within the law, which makes it easier for the police to prosecute the minority who break it. As summarised by the Arizona Department of Transportation: “The normally careful and competent actions of a reasonable individual should be considered legal.”

Mr King also criticised my challenge to Bristol City Council’s claims for increases in walking and cycling in its 20mph pilot areas. Professor Danny Dorling has also seized on Bristol’s claims to advocate 20mph limits as a way to “reduce health inequalities” (‘Blanket 20mph limits a priority, says academic’ LTT 24 Jan).

After discovering the unacceptable way in which Bristol had produced its average figures for walking and cycling increases, as I reported in my Viewpoint article (LTT 10 Jan), I submitted a Freedom of Information request to the council for the actual survey data on which its claims were made. From this data it is apparent that walking and cycling levels in both pilot areas increased more at weekends (from a lower base) than on weekdays. In one of the pilot areas, some of the before counts were affected by rain, so comparison with dry after counts produced unrealistically high growth figures. These are the eye-catching upper levels of growth claimed by the council, even though it also calculated alternative figures corrected for weather.

In the first pilot area to be implemented, combining the weekday and weekend increases using the correct 5:2 weighting ratio, shows that pedestrian numbers increased overall by 3% and cyclists by 5%. I am awaiting clarification from the council about additional corrections that appear to have been made to the results from the second pilot area, but which have not been documented in its reports. When I have been able to complete my analysis I will publish the results.

It seems there have been slightly larger increases in the second pilot area, but this could be due partly to the different time periods between the before and after surveys. The before surveys were all conducted in 2009, but the after surveys in the first area were undertaken in 2010, with those in the second area taking place in 2011. It is possible, therefore, that there could be an overall upward trend in walking and cycling in Bristol.

Unfortunately, the council did not undertake comparable surveys in other parts of the city that retained the 30mph speed limit to act as a control, so it will not be possible to differentiate the effect of trend from that of the 20mph speed limits. Even if the increases seen were due entirely to the lower speed limit, however, the true figures of 3% and 5% for walking and cycling in the first pilot area are a long way below the 23% and 20.5% claimed. Bristol cannot be held up as proof that 20mph limits have a significant impact on walking and cycling.

Rod King says that setting a 20mph speed limit “is about setting a better, healthier, safer and more equitable public realm”. Professor Dorling likens the impact of cars on health to that of cigarettes, completely ignoring the immense benefits that motor vehicles have made to the economy and to individuals in terms of mobility and choice. It is clear that the push for 20mph speed limits is more about politics than road safety.

Rod King presents the Time for 20mph Conference on the 18 February 2014 in association with LTT.