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Bromley Borough Roads Action Group

The following note was produced in response to calls for a wide area 20 mph speed limit in Bromley

20 Mph Speed Limits on All Roads in Bromley?         

Road casualty figures are still a major public concern, despite the enormous expenditure on road measures in recent years. But before you consider supporting the idea of the wider implementation of 20 mph speed limits in Bromley, you should consider the following.

20 mph zones have been shown to be successful in substantially reducing accident figures in London. However, such zones are usually implemented with a combination of other traffic engineering measures such as road closures, speed humps, road narrowing, entry treatments, improved signage, improved lighting and other measures. It is therefore difficult to separate out the effect of the speed limit change from other changes. Indeed Department for Transport guidelines require that such zones are generally “self enforcing” – in other words physical measures in addition to signs must be used to ensure compliance with the speed limit.

But the recent suggestions from Mr Harvey of the Bromley Road Safety Consultative Panel is for a 20 mph speed limit on all roads in the borough except “priority roads” – the latter apparently includes roads such as Mottingham Road, Whitehorse Hill, Red Hill, High Street Chislehurst, Centre Common Road, Orpington Road, Chislehurst Road, Sundridge Avenue, Yester Road (part), Willow Grove, Elmstead Lane, Prince Imperial Road, Summer Hill, Bickley Park Road, Bickley Road, Southborough Road, Blackbrook Lane, Southborough Lane, Petts Wood Road, Southborough Lane and Franks Wood Avenue (some of these are “A” roads of course and part of the “TfL network” but others are not). To introduce a 20 mph speed limit on all other roads would not be realistic in my view as it would be unlikely to be complied with, and would in reality be difficult to enforce. In addition it would be very costly and would not likely be cost effective as a road safety measure.

There is no public evidence that introducing a lower speed limit by itself, or reducing the speed limit below what most drivers see as a sensible speed, has any beneficial effect on road accidents. To quote from DETR Circular 05/99 on 20 Mph Zones: “Extreme caution should be exercised when considering making 20 mph limits using speed limit signs with no supporting speed reducing features. The weight of evidence points strongly to signed only 20 mph limits have little or no effect on traffic speeds”. But putting up the required signage – and the associated road engineering measures mentioned above, would be enormously expensive. Every side road running off the above mentioned road would require a 20 mph sign at enormous expense, and such signs would add to the existing street clutter significantly. It would be better to spend this money on road safety measures that are known to be cost effective and on roads where accidents are known to happen.

A good example of the ineffectiveness of simply reducing the speed limit on a road as a result of calls for improved improved road safety measures was that of Perry Street, Chislehurst (the A222). This road was reduced from 40 mph to 30 mph some years ago. There were two separate fatal accidents not long after, and compliance with this limit is still very poor – most traffic continues to travel at nearer 40 mph.

There are certainly some streets or zones in Bromley that might benefit from a 20 mph speed limit, in conjunction with road engineering changes, but it is surely best to examine which are the most cost effective sites rather than spread money around without thought.

Incidentally one of the few London boroughs who have introduced proposals for a wide area 20 mph speed limit is Lewisham who have a penchant for road safety “gestures” driven by politicians who believe these are vote winning ideas – just look at the number of speed humps in that borough. But you should be reminded that Lewisham has a worse record than Bromley in reducing traffic accidents. Bromley has done much better because money has been spent more wisely and on more specific measures with a concentration of effort on known accident locations.  Lewisham’s “wide area 20mph” policy is yet another example of the introduction of road safety policies by people who have little real knowledge of road safety issues. 

Mr Harvey claims that “experience shows that motorists who drive safely within 30 mph areas do not reduce their speed much when 20 mph zones are introduced”. Clearly in that case many of them would be breaking the law as the typical average speed of traffic on 30 mph roads is much nearer 30 than 20. I agree that motorists would be unlikely to change their behaviour much, but the result would be that many would be unconsciously breaking the law and be subject to fines of £60, which can be imposed by the use of the hand held laser cameras that Bromley police now have.  

The best way to set speed limits is to use the 85th percentile of traffic speeds. This has also been shown to produce the most cost effective and indeed safest speed limits. Reducing speed limits alone, without reference to existing traffic speeds on roads, and without any associated measures is not sensible and will not be beneficial. 

Mr Harvey has also said “However the top-end speeders, the dangerous ones who are the cause of the complaints and accidents, are dramatically reduced in number and residential areas become safer and calmer”. An interesting statement but one which I believe is untrue. In essence, I believe it to be nonsense. 

He has also produced a leaflet that shows suggested road signs and it includes one which is a standard “30 mph speed limit” sign but with the wording “But 20 on all non-priority roads” underneath. I do not believe that this is an “approved sign”, ie. authorised under the Traffic Regulations, and hence would be illegal unless specifically approved by the Secretary of State, which would be unlikely. 

He also suggests that “main and feeder roads” would have unchanged speed limits whereas “residential and non-transit” roads would have 20 mph limits, which are unfortunately not terms that road traffic engineers use and it might be very difficult to differentiate between the different suggested classes of roads in practice.  

This leaflet makes other claims about the success of such a scheme including the claim that in Graz serious injury accidents were reduced by 24% in the first year. Was this statistically significant in the sense that it could not have happened by chance variation? I doubt it. In any case, any traffic engineer could have advised Mr Harvey that the normal period used for determining the success of any traffic scheme is three years after versus three years before, because it is known that almost any changes to roads reduce the accident figures temporarily. A longer time is required to ensure that there is no distorting effect.

It is also pointed out that Graz police only do “occasional visits” to perform checks. But perhaps they do not have the same financial incentives that the UK police now have to generate revenue from fines imposed on motorists who break posted speed limits. 

Interestingly Mr Harvey also gives some figures for Graz and Bromley (of similar populations) and compares the accident figures in 2006. Although the Graz 20 mph scheme was introduced 15 years ago, the accidents figures for “KSIs” are almost identical and Bromley has one third of the “slight” accidents. There is clearly no evidence that Graz has a better accident record and whereas Bromley is part of the major metropolitan area of London, Graz is an independent city with very different traffic and social problems.  

Inappropriate speed limits, i.e. those that drivers see as unrealistic or unnecessary, simply cause traffic laws to be brought into disrepute. There is much more compliance with speed limits when drivers see them as suitable in all driving conditions for the road concerned.

Regrettably there has been an excessive focus on speed as the main factor in road accidents when recent figures from the Department for Transport show that it is simply not true. If you waste money on trying to reduce traffic speeds, when there are better ways to spend the cash to improve road safety, then you are doing a gross disservice to the community. 

No road safety measures should be proposed without a proper cost/benefit analysis. At present, we do not even know what the cost of these proposed measures might be, and what complementary measures if any are to be used to enforce the speed limit reduction, but in my view these measures are likely to be very expensive, and not produce any significant benefits. 

In summary, the proposals from Mr Harvey are of an amateur nature and are not well founded on established traffic engineering practice and road safety knowledge.

Roger Lawson

Bromley Borough Roads Action Group, www.bromleytransport.org.uk

August 2008

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