Many streets in the UK have roads which restrict drivers to a 20mph limit.

The 20mph Restriction

The main road and feeder roads around these 20mph zones usually have a 30mph speed limit so this usually results in areas with variable speed limits.

The 20mph speed signs are an important indicator to drivers that they are in a zone where there are schools and other institutions and they have to drive with more attention and care. These 20mph zones also tend have an associated speed hump in order to reinforce the lower speeds.

Even with these restrictions in place, do 20mph zones help prevent crashes and fatalities? Do 20mph zones really work?

The Research

Several studies have shown that minor accidents actually increased in 20 mph zones by nearly 17%. Studies by the Institute of Advanced Motorists, based on data collected by the government, has shown that minor casualties in 20 mph zones increased by 19% while major casualties in these zones saw a 29% increase.

The Theory

There have been several theories put across as to why 20mph zones are not working as effectively as they should.

One theory is that the 20mph sign does not alter the behaviour of drivers because the driver still feels that the road is a 30mph zone. This could be because both the roads feel and look the same, especially as a lot of feeder roads are 30mph. This variable change can go unnoticed by drivers.

Another theory of 20mph zones not working is distractions. When the driver keeps checking his/her speedometer to ensure compliance with the speed limit of 20 mph less attention is paid to the road, potentially causing accidents.

In addition to these theories, it has been reported that traffic congestion and pollution increase when the speeds are lower. This conflict will be experienced more by cyclists in a 20mph zone when they find that cars are going at the same speed as them, instead of going by them. This could potentially lead to tailgating accidents between cyclists and vehicles in a 20mph zone.

How to change

Overall, these theories suggest that roads themselves should be self-regulating, i.e., have obvious limitations that suggest to the driver that he/she needs to drive slower. This could be more effective than putting up a sign that indicates the speed at which to drive. When roads are designed to appeal to the intuitive speed limits of drivers, there could be a much lesser chance of fatalities and casualties. So one way to make sure drivers comply with the speed limit in 20 mph zones would be to change the structure or character of the roads per se.

The zones which are 20mph could potentially have further self-regulating traffic controlling methods that are not just restricted to speed humps to address the issues discussed. There could be narrower roads, chicanes, planting of trees and shrubs, etc.

Several studies have shown that when 20mph zones are introduced with associated traffic-controlling methods mentioned above, there is a significant reduction in traffic accidents.

So the way to address these issues could be to introduce these traffic-controlling measures even on roads which are wider, leading to a potentially significant reduction in road traffic accidents.