Why not just enforce the 30mph limit?

There are three main reasons why 30 mph is too fast for residential streets:
1. Chance of death – 2. Stopping distances – 3. Children’s speed perception

1. Chance of death

You are seven times more likely to kill someone if you hit them at 30 mph rather than 20 mph.
If they are over 60, they have just a 50/50 chance of surviving.
Around half of all pedestrian deaths are from impacts of 30 mph or less. At 20 mph, most people have a 99% chance of surviving.

2. Stopping distances

You are much less able to avoid a collision if you drive at 30 mph rather than 20 mph.
Your stopping distance is almost double. If a child ran out 40 feet in front of you:

at 20 mph you would just stop in time
at 30 mph you would hit them still doing 27 mph (the same impact speed as falling to the ground from a third storey window)
at 40 mph you wouldn’t even have time to brake – and you will most likely kill them.

Research shows that for every one mile per hour reduction in average speed on urban roads, there is a six per cent reduction in the number of collisions.

3. Children’s speed perception

New research has shown that children cannot accurately judge the speed of vehicles above 20 mph.
Being hit by a car is still one of the leading causes of preventable death for our children. If a child makes a mistake and misjudges your speed, do they deserve to die? And even if it was the child’s mistake, would you want to live with the consequences for the rest of your life? How would you feel if you knew that at 20 mph you’d have been able to stop in time?

The evidence:

Department for Transport: Relationship between Speed and Risk of Fatal Injury: Pedestrians and Car Occupants

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

Government information: Stopping distances

RoSPA: Stopping distances simulator (Note – in this example the pedestrian is about 80 feet / 25 metres away)

Royal Holloway University of London: Traffic at 30 mph is too fast for children’s visual abilities