Question of the week: Are 20mph limits having any effect?

David Ross, Editor
Fri, 31 Jan 2025

Dear Honest John,

"I've had to put up with 20mph limits for a year now, are they really worth the trouble?"

- MN

Dear MN,

Speed limits have always been a contentious issue and probably will remain that way forever. But 20mph limits that were introduced in Wales and in selected urban areas in England do appear to have caused significant debate.

12 months of data on road casualties has recently been published for Wales and there appears to be positive benefits in this respect.

Between July and September 2024, the number of casualties on roads with a 20mph or 30mph limit were the lowest for that time of year since data was first collected in 1979. The data also showed that 100 fewer people were killed or seriously injured in the 12 months following the introduction of the 20mph limit than in the previous year.

However, this does not necessarily mean that all the reductions in accidents are solely down to the reduction in speed limits. Only more data and further analysis will provide any substantial conclusions.

Data from other areas where 20mph limits or 20mph zones had been introduced does appear to support their effectiveness - in 1994 Hull introduced 20mph zones and over a six-year period accident rates fell by 56% with fatal or serious injuries by 90%.

Speed limits are a matter of risk mitigation versus practicality - a blanket 10mph speed limit may well reduce casualties still further, but would be wholly impractical.

How 20mph limits are implemented is also a key factor, as a blanket limit avoids confusion for drivers but potentially introduces a limit lower than the road conditions can comfortably support. More targeted 20mph limits are arguably preferable, but require sufficient signage and enforcement.

What is clear is that a reduction in speed both reduces the likelihood of accidents and the severity of any accident that occurs. According to RoSPA, the risk of a fatality for a pedestrian struck by a car at 20mph is 1.5%, and the risk rises to 8% at 30mph. 

Ask HJ

Are speed warnings now mandatory?

We have just taken delivery of a 24 plate Toyota Aygo Cross. The car is all good accept every time you go above the speed limit the car pings quite loudly and keeps pinging. Unfortunately the digital Speedo is 2 miles an hour fast. So it pings at 29 and not 31mph. You can turn it off until the next time you drive it starts pinging again. Yes we should not speed but is very annoying constantly being pinged for speeding at 29mph. Goodness knows what it will be like in a 20mph zone. Is the speed warning system now mandatory in all new cars?

EU legislation introduced in 2022 mandated that all new cars sold in the region must have a speed limiting device fitted, but the operation of these will depend on the specific vehicle. The reason the warning appears at a true 29mph is that vehicle speedometers must not under read by law, so we would expect that the vehicle's display is indicating 31mph before it is activated.

Answered by David Ross

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