Anecdotally though, speed cameras may not seem as prevalent on roads with 20mph limits as they do on other roads, and this may be linked to the fact that 20mph limits are supposed to be “self enforcing”. Evidence from successful 20 mph schemes shows that the introduction of 20 mph zones generally reduces mean traffic speed by more than is the case when a signed-only 20 mph limit is introduced. Historically, more zones than limits have been introduced. The Road Safety Foundation has assessed the safety of the trunk road network, assessing the protection levels that the design and engineering features of roadsides, medians and junctions on these roads offer in case of a crash. This assessment uses a star-based European Road Assessment Programme (EuroRAP) Road Protection Score, and has found that two-thirds of single carriageway trunk roads achieve only a 2-star (out of 4) rating. Even though this assessment has only been applied to trunk roads it suggests that engineering measures may often be more appropriate to manage speed and reduce collisions on rural single carriageway roads. On rural roads there is often a difference of opinion as to what constitutes a reasonable balance between the risk of a collision, journey efficiency and environmental impact. Higher speed is often perceived to bring benefits in terms of shorter travel times for people and goods. Many of these limits were introduced in London, where a quarter of all roads were 20mph by 2015, while in 2020 all roads within the central Congestion Charge zone were set to 20. Transport for London intends to roll out 20mph limits across 137 miles of road by 2024.

The objectives of this guidance also fit into the context of some wider transport and cross-government priorities, which those responsible for setting local speed limits should bear in mind: It is on urban roads that the majority of road casualties occur, including 87% of all pedestrian and 83% of all pedal cyclists casualties (DfT, 2011). Collisions typically involve pedestrians and cyclists, including children, and knowledge of the relationship between vehicle speed and injury severity in any collision must inform decisions on speed limits. mph limits covering most streets in Portsmouth have demonstrated that it is possible to introduce large-scale 20 mph limits in some built-up environments. Traffic speeds in most of the streets treated were relatively low (less than 20 mph) to start with. The early evidence suggests that it is likely that some speed and casualty reductions have taken place and this is consistent with previous research that has indicated that 20 mph limits without traffic calming reduce mean speeds by about 1 mph on average. 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. For C and Unclassified roads with important access and recreational function, the following speed limits are deemed appropriate and traffic authorities should use these as guidance when reviewing the speed limits on these roads:The choice of speed limits should take account of whether there is substantial roadside development and whether the road forms part of a recognised route for vulnerable road users, including whether there is a footway. The same study found the introduction of 20mph limits increased journey times by 3% in residential areas, and 5% in city-centre areas. Mr Waters laid out a timeline to look at the success of the policy to the Senedd chamber during his no confidence vote. He said a formal framework will soon be published. Then, in January speed data will be published and by summer a six month review. There will be annual monitoring for the next five years. "We're not digging our heels in, we want to take a common sense approach," he said. Rural dual carriageways with segregated junctions and facilities for vulnerable road users would generally be suitable for 70 mph limits. However, a lower limit may be appropriate if, for example, a collision history indicates that this cannot be achieved safely.

A comprehensive and early consultation of all those who may be affected by the introduction of a 20 mph scheme is an essential part of the implementation process. This needs to include local residents, all tiers of local government, the police and emergency services, public transport providers and any other relevant local groups (including for example, groups representing pedestrians, cyclists, drivers, or equestrians). Further details about consultations are set out in Appendix A. Based on this positive effect on road safety, and a generally favourable reception from local residents, traffic authorities are able to use their power to introduce 20mph speed limits or zones on: Local authorities are invited, though not required, to use the Speed Limit Tool. Its use is free of charge and is not restricted to local authorities.

Why are the Fire Service involved in 20mph enforcement events?

enabling the setting of more appropriate local speed limits, including lower or higher limits where conditions dictate