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Is 20 Plenty?

Is 20 Plenty?

How can we increase safety and reduce speed?

Tags: Increased Safety Innovative Solutions Speed Feedback Unipart Dorman

Many UK local authorities have implemented 20mph limits in residential areas as best practice for protecting the vulnerable road user and to deliver safer, healthier, people-friendly streets.  Key to the effectiveness of these zones is getting drivers to firstly recognise their speed and then take steps to reduce it.

This can take a number of forms, from simply providing the statutory 670 signage and repeaters with ‘low level’ physical measures such as diagram 1075 and ‘dragon teeth’ road markings, through to the more expensive vertical measures such as speed humps all the way to a complete re-engineering of the road system to incorporate things like safety cameras, narrow lanes and chicanes or even total road closure.

Money is tight though and it is all about balancing the needs of all road users against the safety consideration. It could be argued that the most effective traffic calming measure ever, was the 1865 Red Flag Act which restricted vehicle speeds to roughly 4mph by requiring a man with a flag to walk in front of any moving motor vehicle, but that is simply not a realistic measure. Nor is it cost effective when you consider things like 24/7/365 manning levels.

So what can we do?

Education and information to influence driver behaviour is the most effective way to get drivers to slow down. The education piece is often driven centrally whether by public or charity based agencies such as the excellent work done by

So that leaves us with information. 

The Unipart Dorman DF11 delivers a bright LED display of the approaching vehicle’s speed which in itself will usually cause drivers to unconsciously reduce the pressure on the accelerator pedal.  

The highly effective campaign run by Lincolnshire Road Safety Partnership combined fixed safety signage with a DF11 that was moved around the parish to prevent complacency setting in and this allows drivers to receive clear visual cues of what the speed limit in force is and also their speed compared to the limit.

Reducing speeds to 20mph is vital in school zones.  When you consider that the difference between the typical stopping distance quoted in the Highway Code for 20mph is 12 metres which nearly doubles to 23metres at 30mph and trebles to effectively 9 car lengths at 40mph; any reduction in speed and subsequent increase in reaction time will have a significant effect on the chances of a child surviving being hit by a car.  If you throw in some good old British adverse weather conditions and hard pressed parents dashing to get to work after dropping their children off at the school breakfast club, it is suddenly a very dangerous place for children, who can often have more interest in finding the latest Pokemon or having fun with friends than the danger of traffic.

Cost effectiveness is sometimes the most significant consideration when planning for traffic calming.  Much as every Highway Department would like to have the budget to meet the aspirations of residents by narrowing the road and putting in other visual clues such as planting trees and installing chicanes etc. to make it ‘appear more like’ a 20mph zone - this simply isn’t possible given the funding restraints.

Speed feedback devices such as the Unipart Dorman DF11 have proved time and again to reduce the road speed and when used in conjunction with 20 zones, local residents are more engaged and very supportive of the benefits not only in speed reduction but making their local area more friendly places to live in as the amount of air and noise pollution are reduced.  The data generated by the DF11 can be used for a large number of applications, from a simple notice on a busy road saying ‘X number of people exceeded the speed limit on this road in the last month’ to presentation at community engagement meetings as a method of informing local residents and developing the data to make a compelling case for more enforcement action.

Unipart Dorman can support you in your quest to make your roads safer with trial installations of a DF11 and access to case studies and full UK based technical support.  

Unipart Dorman are proud to manufacture DF11 in the UK and support the Made in Britain Campaign.



About this author
Graham Scott

Technical Support Manager

I am the Technical Support Manager at Unipart Dorman providing technical and sales support across the Rail, Traffic and New Product Development areas of the business. In my spare time I write Flash Fiction and I also volunteer at a local community creative writing group and for various transport heritage groups.


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