20mph limits: The heavier and faster a vehicle, the more danger it poses

Road safety 20mph

When drivers question the case for 20mph limits, they risk overlooking the laws of physics. After all, it’s indisputable that the faster and heavier a vehicle, the more likely it is to cause injury or death in the event of a crash. It’s the reason the driver of a car who collides with a child is 7 times more likely to kill her at 30mph than 20mph.

When the Vias Institute examined data from all road traffic collisions in France over a five-year period, they found that in a collision between two cars, when one weighs at least twice as much as the other, the occupants of the light vehicle are 3 times more likely to suffer serious injuries than when the two vehicles have a similar mass.

20mph limits are vitally important in the areas where people live, work and play. And given the disproportionate danger posed by heavier vehicles, it’s alarming to witness public service vehicles like buses flouting the law.

We filmed our journey through quiet and narrow residential streets this week and found we were travelling at 25mph – 25 per cent above the local 20mph limit.

speeding bus

As well as representing less of a risk to other road users’ safety, buses travelling within the speed limit have a powerful ‘pace car effect’ on surrounding traffic. It’s the reason a number of buses in London have already been fitted with with intelligent speed adaptation (ISA) – a type of speed restrictor that recognises and enforces the local limit. Results of the trial have been impressive, particularly in 20mph zones, but rollout of the tech is proving to be slow.

Speed limiters have been a legal requirement on all new cars since July 2022. However, the feature needs to be manually selected by the driver and can be overridden at will.

Plus ça change

The noisy minority so vocal in its opposition to safer speeds might be interested to learn the 30mph limit established in 1935 prompted protests of its own. The protests are now little more than a curious historical footnote.

30mph protest 1935

The ethical choice

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