The main beneficiaries of this change are the drivers themselves.
20mph improves traffic flows. 20mph limits can actually shorten journey times, rather than making them longer. New research from Prof Dr Niels Benedikter at the University of Milan confirms 30 km/h (18.5 mph) as the optimal speed limit for minimising time wasted due to traffic congestion.
The assumption that 20mph limits increase journey times is wrong. Since most urban speeds are well below 20mph owing to congestion and junctions, traffic can flow more freely at 20mph due to:
- Reduced braking distances enabling drivers to leave a smaller gap to the vehicle in front
- Increased junction capacity, as drivers are able to pull into slower traffic more easily
- More people choose to walk, wheel or cycle as streets become safer, reducing congestion
All road users – on foot, on a bicycle, or in a car – gain safety from a limit reduced from 30mph to 20mph.
“A car travelling at 20mph is able to stop three car lengths sooner than a car travelling at 30mph” – UK Highway Code
UK 20mph zones were first evaluated in 1996. Results were:
- injury collisions reduced by 60%
- child injury collisions reduced by 67%
- traffic decreased by 27%, largely due to through traffic re-routing
The 20mph zones in Hull recorded a decrease in total collisions of 56% and in fatal and serious injuries of 90%. The biggest reductions were pedestrian casualties, which fell by 54%, child casualties, down by 54% and child pedestrian casualties, down by 74%.
Read more about the benefits of lower speed limits in urban areas…
The local environment becomes measurably ‘greener’ at 20mph
Data from existing 20mph speed zones proves that vehicle emissions drop significantly. It encourages more walking and cycling. And by the overall reduction in traffic speed, and by the removal of the ‘stop – start’ constant braking and accelerating by impatient drivers negotiating heavy traffic. A 20mph speed limit encourages a consistent adherence to a lower constant speed, generating a smooth overall traffic flow.
The ‘sweet spot’ in terms of traffic flows
The case for making communities better for walking, wheeling and cycling is well documented, but the latest research from Prof Dr Niels Benedikter at the University of Milan confirms earlier studies that capacity increases when speed limits are lowered. At lower speeds, the safe braking distances between vehicles is lower. Benedikter’s work was aimed at finding the speed ‘sweet spot’ where the maximum number of cars can pass a specific point in a given time. By using mathematical modelling, he determined this speed to be 31 km/h (20mph).
Reduction in injury and death
A 2007 review of 78 new 20mph zones in London found that they reduced injury collisions by 42% and fatal or serious collisions by 53%. A review of road casualties in London between 1986 and 2006 recorded that 20mph zones reduced the number of casualties by over 40%, and also prevent more fatal or serious injuries to children, which were reduced by half. The study results suggest that 20mph zones have a very positive effect on public health outcomes, such as improved air quality, safety for children walking and playing in the street, and better walking and cycling conditions.
Why does it matter?
30mph is a deadly speed. In fact, in an impact with a pedestrian at 30mph, research warns us with utter clarity that 50% will die. This claim is frequently repeated in support of speed camera policy and comes from ‘Ashton and Mackay, 1979’