Road Safety News

20mph limits improve traffic flow, campaign claims

Wednesday 13th June 2012

The ‘20’s Plenty for Us’ campaign suggests that traffic in cities flows more freely with a 20mph limit than when the limit is 30mph.

The campaign argues that while many people wrongly assume that 20mph limits delay journey times, in fact average city speeds are generally well below 20mph because of congestion and queues. The campaign for lower speed also claims that in 20mph zones drivers make better use of road space and junctions work more efficiently and at a higher capacity, as it is easier to merge.

20s Plenty for us says that “optimal speeds for maximum urban traffic flow have been mathematically modelled and 20mph is more efficient than 30mph”. The campaign also claims that walking and cycling levels rose by up to 12% after 20mph limits were introduced in Bristol.

For more information contact Anna Semlyen, 20’s Plenty for Us campaign manager, on 07572 120439.


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Of course it works that if you reduce speed by 1mph collisions reduce by 5% - so what happens if you drive at more than 20mph below the limit? Negative crashes?
Andy , Warwick

Agree (1) | Disagree (1)

20 mph limits are simple common sense really. I'm sure that everybody here doesn't hit the 30 mph limit on urban roads? You're too busy looking for children, pedestrians, stray dogs whilst negotiating the crazily parked cars.

When driving in a 30 mph zone, are you constantly checking your speedo and therefore crashing because of your lack of attention on the road?

There is lots of evidence to show that 20 mph zones give a feeling of safety and encourage more people to get out of their cars and use their feet and bicycles. They give more sociable feeling neighbourhoods.

All these arguments against 20 mph zones were used before, when the 70 mph limit was first introduced. If you remember this then resulted in a 20% decrease in crashes.
Steve, Merseyside

Agree (5) | Disagree (9)

As is the case with pretty much every road safety intervetion, what works in one situation cannot automatically be applied to others elsewhere. Implying an approach is 'one size fits all' is misleading and irresponsible.

Let's see the promotion of an evidence led, common sense approach to reducing urban casualties where the most appropriate solutions for the local communities are sought, rather than expectations being falsely raised.
Matt, Cambs

Agree (7) | Disagree (1)

I'm all for safe drivers and not just the ever quoted speed by those who have most likely not had a very long time behind the wheel. I am not the best driver in the world but I consider myself to be considerate and safe driving at the speeds which should be acceptable to others. Through this I achieved 62.5 MPG from a 2.0 ltr vehicle so cannot be considered a speed demon although I observe many which the lack of road policing fails to do. Let's achieve a zero RTC figure by taking everything off the roads, then all the 20s plenty brigade will be satisfied when they have to walk everywhere.
reg oliver derbyshire

Agree (7) | Disagree (6)

The comment that vehicle speeds at 20mph at road junctions would reduce accidents must be questioned. At what junctions does traffic move at higher than 20mph already? Precious few.

There is, however, a strong case for removing many traffic signals from junctions, as a green light is seen by many to mean 'GO', and not: 'You may proceed with caution' which is the true meaning of a green light. Such junctions where lights have failed temporarily show a far greater tolerance between road users than when give a 'green', or held up unnecessarily on red.

An overall enforcement to drive at speeds below that which are sensible and safe, where drivers are not conditioned to observe their instruments but observe instead the road ahead and the environment in general, is a far safer environment to live in.

Most accidents/incidents/collisions occur due to lack of proper attention to the road, hazards, and surface conditions thereon. This includes all those users who are unable or unwilling to reach or exceed 10mph, let alone 20mph.
Derek Reynolds, Salop.

Agree (7) | Disagree (3)

The speedometer in my car emphasises 10, 30, 50 and 70 in bold numbers. Will I have to retrofit a new one with 20 also in bold? Also the M25 was "mathmematically modelled" as are our banking system and government taxes. Just slow down.
Peter Wilson, Westminster

Agree (8) | Disagree (4)

I would wish to think that 20 mph limits on our roads would indeed reduce the number of accidents or at least reduce the degree of injury should the number of accidents happen to increase.

I would like to see fewer twv accidents at junctions and roundabouts involving smidsy's but I doubt it will happen.

I would however, conceivably think that more accidents would occur as drivers etc constantly observe their speedo and take their eyes of traffic whilst doing that. Also trying to avoid the traffic cushions or road humps that will inevitably be put down by local authorities as passive road policing.

Talk about making matters worse.
bob craven Lancs

Agree (10) | Disagree (11)

What I am "getting at" is that casualties increase in 20mph areas (when allowance is made for reduced traffic volume). The reason is that pedestrians and cyclists are encouraged to feel safer and take less care. This behaviour can be witnessed when driving through any 20mph area. I made this point at the 20mph Places conference in May, and Anna Semlyen and the rest of the 20's Plenty contingent were unable to counter my evidence and data. You say "There’s no doubt that 20 mph on a lot of urban roads is about the right speed to prevent road collisions happening" - sorry, but there is a lot of doubt about such a spurious claim. No speed is magically safer than any other in all circumstances.
Eric Bridgstock, Independent Road Safety Research, St Albans

Agree (8) | Disagree (12)

Speed limits are inevitably a compromise and sometimes it is not so much the actual limit that matters, but the individual motorist’s compliance with the prevailing limit - whether it seems ‘right’ or not. It indicates good discipline, awareness and an inclination to be safety conscious.

There’s no doubt that 20 mph on a lot of urban roads is about the right speed to prevent road collisions happening – the problem is persuading a significant proportion of the motorists to stick to this upper limit, which they will not do unless forced to do so through enforcement (which is unlikely without average speed cameras or continual Police presence which is obviously not practical) or through physical traffic calming which is not an ideal solution either.

Also, the physical characteristics of the roads themselves will have a bearing on whether a 20 limit will be accepted by the average motorist. Older, narrower, residential roads where parked vehicles may proliferate practically cry out for a 20 limit but a more modern wider road with better visibility for all road users may make a 20 limit seem unreasonably slow.

Not quite sure what Mr Bridgstock is getting at though.
Hugh Jones Cheshire

Agree (7) | Disagree (6)

Anna - it is not a "fact", it is a claim for a model which, as I explained, cannot have been validated and based on a number of assumptions which equally may not be valid. I note that you have ducked the issue of increased casualties in 20mph areas.
Eric Bridgstock, Independent Road Safety Research, St Albans

Agree (7) | Disagree (11)

The fact that a maximum traffic speed of 20mph is more efficient than 30mph is in the Dept for Transport COBA manual. For mote see the site briefings pages.
Anna Semlyen, York

Agree (8) | Disagree (7)

I would find this story more credible if it came from an organisation who were not overtly anti-car. The speed management world have been quoting the use of mathematical models for years to prove all manner of unlikely or unbelievable claims (eg reducing speed by 1mph reduces collisions by 5%). They are based on false assumptions and have never been validated. 20's Plenty know that 20mph increases casualties, so they are desperately looking for other "benefits".
Eric Bridgstock, Independent Road Safety Research, St Albans

Agree (8) | Disagree (12)