Motorists are generally in favour of 20mph zones in residential areas but they detest the humps that are used to enforce them.

Humps cause noise and vibration, increase emissions, damage cars and result in longer response times for the emergency services. They also cost up to £5,000 each to build.

Reluctance to introduce “traffic calming”, which also includes chicanes and rumble strips, is the main reason why local authorities have tended to ignore the ample evidence that reducing the limit from 30mph to 20mph saves lives. More than 600 pedestrians and 100 cyclists are killed in built-up areas each year.

Since 1999 the authorities have had the power to change the limit without requiring the approval of the Secretary of State. Many have introduced a few 20mph zones, but only Portsmouth has introduced a blanket 20mph limit on residential roads.

The city has done so by installing signs rather than humps and using peer pressure to make motorists comply. It is heavily promoting the idea that speeding on residential roads is as unacceptable as drink-driving. Guidance issued by the Department for Transport in 2006 encourages 20mph limits on residential roads “where there is a particular risk”. It states that simply changing the limit without introducing physical measures will only reduce average speed by about 2mph. For this reason, it recommends that authorities that do not want humps or chicanes should only consider 20mph limits on roads where the average speed is already 24mph or lower.

However, a growing number of authorities are considering making 20mph the norm regardless of the average speed. Among the advantages of this approach is that it is less confusing for drivers because there are fewer changes in the limit. Last month Hackney Council in East London passed a motion stating that “the standard speed limit on streets within the borough should be 20mph with only very limited exceptions”.

In areas plagued by rat-runners authorities will soon have the option of installing average-speed cameras covering networks of streets. The system, which is due to gain Home Office approval by the end of the year, reads each numberplate as it enters a zone and again as it leaves and calculates the average speed. Similar systems used on motorways during roadworks have very high levels of compliance because drivers know that they cannot simply slow down briefly as they pass a camera.

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