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Here are some answers to questions that I am frequently asked about 20 mph as a default for residential streets.

Frequently Asked Questions

  1. When I drive at 20 mph it seems slow?
  2. What's wrong with 30 mph?
  3. What are the benefits of 20 mph?
  4. What are the disadvantages?
  5. Why hasn't the speed been lowered before?
  6. Won't this lead to people watching their speedos instead of the road?
  7. Won't people just ignore the speed limits?
  8. Won't this increase journey times by 50%?

When I drive at 20 mph it seems slow?

Well that's because you have a lot more time to take in all the information that is surrounding you. What you are registering is that you have an extra ability to cope with those events whilst driving. Because we have come to accept 30 mph as our "normal" speed in built-up areas then this has become our reference point.  That's why a default 20 mph limit for residential streets is so important, because it re-sets that reference point. 30 mph is then seen for what it is - 60% faster!

Throughout Northern Europe towns, cities and villages have set a maximum speed even lower at 30 kph (18.5 mph) or even 10 kph (6 mph)

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What's wrong with 30 mph?

Well the 30 mph limit was actually brought in as the national speed limit for built-up areas in 1934. Prior to that the 1903 Motor Car Act designated a specific category for the Motor Car. It also raised the speed limit to 20 mph. The Road Traffic Act of 1930 abolished the 20 mph limit for cars of less than 7 people. This led to such an increase in road deaths that just 4 years later the 1934 Road Traffic Act introduced the 30 mph speed limit in built-up areas. Whilst in 1934 this may have been an acceptable limit, the huge increase in the number of motor vehicles on the roads has created a huge imbalance in vulnerability between pedestrians or cyclists and motor car users.
When driving at 30 mph you have far less time to react to any incident. In addition the kinetic energy in your motor vehicle is proportional to the square of the speed. The stopping distance is also proportional to the speed squared so that means that a car braking from 30 mph will still be travelling at 22mph when one braking from 20mph will have stopped.
Towns that have implemented widespread 20 mph limits in residential areas found that child pedestrian casualties dropped by 74% (Hull City Council).

With the volume of traffic on our roads then the effect on child pedestrians walking on pavements is huge when 30 mph speeds are maintained. If you want to understand the effect of this, then try crouching down next to a busy 30 mph road. That's just what its like for a 6 year old trying to walk to school.

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What are the benefits of 20 mph?

Whilst the safety benefits may justify 20's Plenty on their own, there are additional real benefits for lower speeds. Traffic noise drops considerably, as does pollution. Your street becomes a far more pleasant place to be and this encourages people to walk or cycle instead of using the car. This "modal shift" to walking or cycling enables us all to reduce our dependency on the car, for the good of the environment and our pockets.

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What are the disadvantages?

Well, there are very few. Some journeys will take a few seconds longer.

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Why hasn't the speed been lowered before?

Well, until recently it required the use of physical traffic calming where implementing a 20 mph limit. This is expensive and usually results in a very "patchy" implementation. This therefore misses the opportunity to set 20 mph as the "norm" for residential roads. It has also resulted in lack of enforcement due to technical requirements on the length of road over which a vehicle must be measured.

Recent DfT Guidelines are far less prescriptive and allow Local Authorities to set 20 mph limits over a larger area and without the necessity for physical calming.

The public is also changing its attitude to speeding and a recent report from the Audit Commission showed that 75% of drivers think 20 mph is the appropriate speed for residential roads. This "culture change" has both increased the pressure on Local Authorities to lower speeds and also the public's willingness to abide by them.

Already Portsmouth has used these guidelines to implement a city wide 20 mph speed limit except on arterial roads. Transport for London recently announced plans to encourage all London Boroughs to set 20 mph for residential roads. Lewisham is expected to be the first London Borough to use this opportunity.

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Won't this lead to people watching their speedos instead of the road?

Well once you observe speed limits then drivers become very adept at recognising their speed without even looking at the speedo. Generally its drivers who never observe speed limits who are worst at recognising what speed they are travelling at. This is not really a problem.

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Won't people just ignore the speed limits?

Well its is reckoned that even if you simply change the speed limit from 30 mph to 20 mph there will be a reduction in speed of 2 mph (TRL Mackie 1998). Whilst this in itself is a considerable advantage and links with the fact that such a reduction would decrease accident frequency by 10% (Finch et al., 1993, Taylor et al. 2000).

However, when coupled with the changed public attitudes to speeding since then and when introduced with community involvement and education then far greater reductions in speeds will result.

When a community looks at introducing 20 mph as a default speed then by its nature this starts a debate within the community about the quality of life for residents and the safety on the roads. This discussion is the key to the community then recognising that speed on residential roads does not effect journey times by more than seconds, yet can have a huge difference in safety, noise, emissions, child independence and the ability of people to enjoy our streets as pedestrians or cyclists.

And from these discussions comes the community commitment that becomes the force which enables people to make a change to more responsible driving speed.

Most importantly, setting a blanket speed limit across a whole borough mean that you maximise the number of drivers who gain from a 20 mph speed limit in their street. This therefore encourages a responsible attitude in other people's streets.

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Won't this increase journey times by 50%?

On the face of it, yes. However, when was the last time you were able to drive from your home, to work , school or the shops at a steady 30 mph. Your journey times are invariably effected by the amount of time you are stopped at congestion points, traffic lights, zebra crossings, etc rather than the maximum speedy at which you drive.

When taking into account that Local Authorities may wish to keep arterial roads at 30 mph, then the average urban journey will only take 40 seconds longer, regardless of the size of the town.

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Revised: 12/09/12.


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