TRL Report 363 Urban speed management methods
Possible changes in legislation, which might relax the consent requirement for making 20mph speed limit orders, would allow Local Highway Authorities greater freedom to introduce 20mph speed limits. 20mph zones with engineering measures to make them self-enforcing have been very effective at reducing traffic speeds and casualties on residential and other urban roads (Webster and Mackie, 1996). However, treatment of all residential roads in the manner of existing 20mph zones would be expensive, and they are therefore likely to continue to be selectively applied according to priority and budget. Whilst any legislation changes would be intended to encourage Local Highway Authorities to continue with the successful formula for 20mph zones, there could be local political pressure on them to increase the number of schemes and therefore reduce the cost of implementing 20mph zones, and possibly to reduce them to signs-only zones without self-enforcing physical measures. The Department of the Environment, Transport and the Regions therefore wished to assess the effectiveness or otherwise of 20mph speed limits which are not self- enforced, so as to be in a position to offer well informed guidance to Highway Authorities. The Transport Research Laboratory was commissioned to carry out a study to assist in ensuring that policies for reducing urban speed limits to benefit more road users result in the most effective use of resources and implementation of measures which secure an overall reduction in the number of casualties. The work comprised: l a review of studies of the effectiveness of attempts to manage speeds in urban areas, with particular reference to those roads which have not or could not be treated by the application of engineering measures in, for example, 20mph zones; l collection of information on low speed limit zones in other countries which use signs-only and do not make use of physical traffic calming enforcement measures; l a review of the effectiveness of existing 20mph zones in the UK where physical measures have not been used; l measurement of speeds at sites in new 20mph zones where the local authority agreed to install signs before implementation of the physical measures so that the effects of using signs only could be assessed. The main conclusions from the study are: 1 The most effective measures for controlling speed in urban areas are physical traffic calming measures, particularly speed humps. 20mph zones using such measures have generally achieved mean and 85th percentile speed reductions of around 10mph and mean speeds after installation of less than 20mph. 2 Speed cameras have reduced mean and 85th percentile vehicle speeds by about 5mph on average, but the effect has been very localised to the installation. Speed camera signs, informing drivers of the possible use of speed cameras, have not been effective in reducing speed.3 Various forms of ‘flashing’ signing (usually made using fibre-optics and often vehicle-activated) have achieved mean and 85th percentile speed reductions of around 4mph on average. 4 The use of static signs only has had a small effect on mean and 85th percentile speed. On average there have been reductions of about 2mph for a range of speed limits for which data are available, but for 20mph zones the reductions were about only about 1mph on average. 5 20mph zones (and 30kph zones) which used signs only did not show any reduction in injury accidents, apart from in the city of Graz (Austria) where there was a 13% accident reduction. However, the signs-only zone installations in Graz were accompanied by a comprehensive publicity and enforcement campaign. 6 From the studies where there were associated public awareness campaigns and/or enforcement, it appears that further reductive effects on speeds of up to 3mph have been achieved, over that achieved by signs only. To sum up, where speeds of around 20mph are desired in urban areas, traffic calming remains the best option to achieve this. Where funding or other reasons preclude its use, the use of only static signs appears insufficiently effective to reduce speeds to 20mph or to achieve accident reductions. Where signs-only schemes are used, small speed reductions and accident savings can be achieved if associated publicity and enforcement campaigns are also used. However, speeds are still likely to remain well above 20mph. The use of ‘flashing’ signing and speed cameras have a substantial effect on speed, generally reducing mean and 85th percentile speeds by around 5mph, and appear likely to have safety benefits in specific locations. However, at current costs, their comprehensive use to control speed over a whole network of urban streets could prove more expensive and less effective in accident reduction terms, than traffic calming. In-vehicle technology to automatically control speeds may eventually be a further option to manage vehicle speeds, but full implementation of such technology is still many years away as a practical solution to the danger of speed on urban roads.