Tuesday, 15 April 2014

Twenty is Plenty fast enough

I had some responses to my Twenty is Plenty entry suggesting that such a move would significantly slow down journey times.

Not so! I shall try to demonstrate.

The limit applies to residential roads, not A and B roads.

There are very few residential roads which are more than half a mile long.

One cannot enter or exit a residential road at speed, certainly not at 20 or 30 mile per hour. It is reasonable to assume that the car might stop or be slowed to a few miles per hour at the start and end of each road.

With that in mind, I'm going to construct a worst-case scenario for a 5 mile urban journey. This assumes:

Only residential roads are used.
Each such road is half a mile long - therefore 10 roads are used in total.
The car never stops, just drops to 5 mph to leave one road and enter the other.

(I am indebted to my son, a maths 'A' level student, for helping with the calculations).

At 20 mph, after allowing 17 feet needed for acceleration and braking between 5  and 20 mph, the car will travel at its maximum of 20 mph for 863 feet in approximately 29.5 seconds.
Over the whole journey, distance travelled at 20 mph is 29.5 x 10 = 295 seconds.

At 30 mph, after allowing 25 feet needed for acceleration and braking between 5  and 30 mph, the car will travel at its maximum of 30 mph for 855 feet in approximately 19.5 seconds.
Over the whole journey, distance travelled at 30 mph is 19.5 x 10 = 195 seconds.

The 20 mph journey will therefore take just an extra 1 minute and 40 seconds for 5 miles.

This, however, assumes absolutely ideal conditions. In practice, the car will come to a halt several times, the roads won't be a nice half a mile long and maximum speed won't be achievable because of other road users.

Because of this, the 20 mph limit probably costs the driver less than a minute over 5 miles in what, in urban terms, is quite a long journey.

And perhaps saves the live of a child.


  1. The same holds true on all but the fastest urban "A" and "B" roads (in Croydon basically everything bar the A22, A23, A232), because of traffic lights, pedestrian crossings, buses and suchlike - although there's some suggestion that leaving the main roads at 30 while making residential roads 20 may discourage people from rat-running on the latter.

    While we're talking maths, it's also worth pointing out that a main road can carry more traffic at 20 than at 30 - although an individual car takes 50% longer to pass through a particular bit of road at 20, you can fit nearly twice as many cars on the same bit of tarmac because of the shorter stopping distances.

  2. Interesting point. I don't object to 30 on the A/B roads but wonder if traffic might not actually flow better with a 20 limit. Good point about rat running.


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