Won't drivers just ignore 20 mph limits?

No. The evidence from places which have introduced 20 mph limits is that average speeds do reduce.

In Portsmouth, in places where average speeds were 24 mph or more before the scheme was introduced, the average speed reduction was 6.3 mph. The overall average reduction was lower, but this was because many of the roads which were included already had average speeds which were around 20 mph.

As a rule of thumb, every one mile per hour reduction in average speed results in a five per cent reduction in collisions. For urban roads this figure is six per cent, so a 6 mph reduction might be expected to result in a 36 % reduction in collisions.

The evidence is clear that 20 mph is the right limit for residential streets but in the same way as attitudes to drink-driving, it may take time for some drivers to change their behaviour. The 30 mph limit has been in place since 1935, and changing social norms can take time.

The evidence

Department for Transport: Interim evaluation of the implementation of 20 mph speed limits in Portsmouth (see executive summary on page 1)

Transport Research Laboratory: The effects of drivers’ speed on the frequency of road accidents