How are speed limits set?

Speed limits are set using a mix of practicality, science and the public’s expectation of being able to travel with certain freedoms. A speed limit is not necessarily the safe speed on a road – there are plenty of narrow lanes in the UK countryside where you would struggle to safely drive parts of them at half the actual speed limit.

There are three national speed limits:

• 30mph – urban areas
• 60mph – open road
• 70mph – motorways and dual carriageways.

These are set based on the characteristics of the road and the environment it runs through. They are legally enforceable without road signs, removing the need for you to be reminded on every city street that the limit is 30mph.

Local authorities have the power to set limits outside this for various reasons permanently or temporarily. For example, some urban roads might be set at 20mph, or some rural roads with sharp bends might be set to 50mph.

Department for Transport issues guidance for how speed limits should be set by local authorities and also covers traffic calming measures and street lighting.

Factors that planners may take into account when setting a speed limit include:

• Whether the road is challenging to drive – sharp corners, blind crests, narrow sections, poor sightlines, etc.
• The presence of junctions and access points
• Frequency of accidents – if it’s an accident blackspot, speeds may be reduced to reduce the number of crashes
• Pedestrians – is it an area where there are a large number of vulnerable road users such as cyclist, the elderly, and so on
• Is it an area where people tend are choosing a speed they feel safe (i.e. should the limit be increased?)
• Is it an area where higher speeds are causing noise issues for local residents
• Is the current speed causing an air quality issue
• Is the speed limit designed to act as a transition between a high speed and low speed environment, e.g. having a stretch of 40mph limit between a 60mph and 30mph limit to give drivers time to slow down.

A speed limit that is not temporary (i.e. due to road works) should generally be more than 600 metres long (but could be as short as 300m in low-speed environments, such as around a school) and should not be used just to attempt to solve the problem of an isolated hazard such as a sharp bend. Other signage or engineering controls should be used.

Councils can use the local speed limit appraisal tool.

What happens when new speed limits are proposed?

When speed limits are proposed on a local level, consultation must happen with the local community, police and local services. Education measures must be put in place. The impact of the lower speed on traffic flow and engineering requirements must be considered, as must the objective of the road, i.e. on a road which has a purpose of transiting a large number of vehicles at higher speeds, it might be a better idea to improve the road’s engineering rather than lower the speed limit.