There is clear evidence of the effect of reducing traffic speeds on the reduction of collisions and casualties, as collision frequency is lower at lower speeds; and where collisions do occur, there is a lower risk of fatal injury at lower speeds. Research shows that on urban roads with low average traffic speeds any 1 mph reduction in average speed can reduce the collision frequency by around 6% (Taylor, Lynam and Baruya, 2000). There is also clear evidence confirming the greater chance of survival of pedestrians in collisions at lower speeds.

Yet while 20mph limits have been shown to reduce speed, and in some instances reduce casualties among vulnerable road users, concerns about compliance are valid, not least for the potential perception that the widespread contravention of 20mph limits cheapens the rule of law. The guidance retains and builds upon many of the underlying principles of DfT Circular 01/2006, but provides additional evidence of the safety and wider benefits of setting appropriate speed limits. It builds on the responses received to the consultation held by the Department in 2012 as well as to an earlier consultation held in 2009. Lower maximum speed limits apply on certain roads to certain traffic classes of vehicles. These are set out in Schedule 6 of the RTRA 1984 and in the Highway Code. Drivers of these vehicles are expected to be aware of this and follow these special limitations without having to be reminded by specific speed limit signs for particular vehicles. Traffic authorities are not permitted to erect different speed limit signs relating to different classes of vehicle.Department for Transport Web-based Transport Analysis Guidance SECTION 6: URBAN SPEED LIMITS Key points The tool has been developed to be economical to apply and straightforward to operate, and to provide informative outputs that can be flexibly interpreted in the context of the local highway authority’s requirements. At its basic level, it does not call for specialist skills such as demand modelling and environmental analysis. When the policy is new, there may be little support until people understand what it is hoping to achieve. Public support will then grow. Support then drops when the details emerge and as the policy is implemented. After the policy is implemented and people can experience the benefits personally, support then rises again. This might take some time. It is important that traffic authorities and police forces work closely together in determining, or considering, any changes to speed limits.

Speed limits should be evidence-led and self-explaining and seek to reinforce people’s assessment of what is a safe speed to travel. They should encourage self-compliance. Speed limits should be seen by drivers as the maximum rather than a target speed. The output spreadsheets should be considered as a starting point for developing the appraisal into a case that can be readily understood and appreciated by a range of people, and which reflects wider considerations than the quantitative values that the tool provides. At least one traffic calming feature as defined in direction 16(2) TSRGD must be placed in a 20 mph zone and the features and signing must still be placed at intervals not greater than 100 metres: it is not the intention to remove physical features, but to ensure that the most appropriate measure is used to ensure the continuity of the zone. Only where speeds are already constrained to near the limit should local authorities consider placing the speed limit sign or a roundel marking, in addition to physical features within a zone. The Department has recently made significant changes to facilitate and reduce the cost for providing 20 mph zones in England. Traffic authorities can now place any of the following:On dual carriageways where the road environment and characteristics allow, traffic authorities can also implement 40 mph and, in exceptional circumstances, 50 mph limits. Generally, efforts should be made to promote the use of suitable routes for urban through traffic and to manage the speed of traffic requiring access to residential streets using traffic calming and associated techniques. The overall speed limit framework, including the setting of national limits for different road types, and which exceptions to these general limits can be applied, is the responsibility of the government. Table 2 Speed limits for single carriageway roads [footnote 8] with a predominant motor traffic flow function Speed limit (mph) Full User Guidance is provided with the tool covering instructions on how to run the appraisal tool, and also a practical guide to the assessment of a range of aspects that local authorities should consider when planning to introduce a change in speed limits. The guidance should therefore be read in conjunction with this circular. Roads should be designed so that mistakes made by road users do not result in death or serious injury. Effective speed management is part of creating a safe road environment which is fit for purpose. It involves many components designed to work together to require, encourage and help road users to adopt appropriate and safe speeds below the speed limit. As well as being the legal limit, speed limits are a key source of information to road users, particularly as an indicator of the nature and risks posed by that road both to themselves and to all other road users. Speed limits should, therefore, be evidence-led and self-explaining, and seek to reinforce people’s assessment of what is a safe speed to travel and encourage self-compliance. They should be seen by drivers as the maximum speed rather than as a target speed at which to drive, irrespective of conditions. It is often not appropriate or safe to drive at the maximum speed limit.

During consultation to introduce 20mph limits in Witney, Oxfordshire, a traffic management officer working for Thames Valley Police warned: “compliance with 20mph limits is a challenging issue”, and added: “If a speed limit is set too low and is ignored then this… could bring the system of speed limits into disrepute.” The officer also warned setting limits too low “can also cause a disproportionate number of drivers to criminalise themselves.”

20mph isn’t in the Highway Code. I’ll keep driving according to that

we want to contribute to wider public health and safety outcomes by contributing to a reduction in road casualties Where a 20 mph zone leads into a 20 mph limit, it is important to use the correct signing to indicate this. It is not appropriate to use the sign that indicates the end of a 20 mph zone and the start of a different, higher speed limit. Instead, a standard 20 mph terminal sign (TSRGD 2002, diagram 670) must be used. 20 mph speed limits