Updated: Is 20 mph speed limit practical?

A Bill was presented to the Scottish Parliament recently by Green MSP Mark Ruskell, seeking to introduce a law making 20 mph the default speed limit in built-up areas across Scotland.

20 mph speed limit as default in Scotland

The bill was introduced with the support of 12 members from the ruling SNP, seven from Scottish Labour, five from the Scottish Green Party and one from the Scottish Liberal Democrats.

There already are many 20 mph zones across Scotland’s towns and cities, but the bill is aimed at reducing many current 30 mph speed limits to 20 mph. Under the proposed bill, Scottish local authorities would still be able to designate 30 mph as the limit on certain roads.

But, the default urban limit would be 20 mph and they would have to follow a procedure – similar to that currently required to lower 30 limits to 20 mph – to raise the limit on any road to 30 mph.

TRL research across England, Scotland and Wales found that 20 mph zones reduced traffic flow by 27%.

Road safety charity Brake supports the speed limit reduction.

“At slower speeds, drivers have more time to identify potential hazards and take action to avoid them,” say the charity, adding that at 20 mph the average stopping distance is 12 metres, rising to 23 metres at 30 mph according to the Highway Code.

It is also an undoubted fact that, if the worst happens, accident injuries are reduced dramatically at lower speeds.

Cutting the speed limit from 30 mph to 20 mph can increase fuel consumption by 5.8 mpg and push up emissions by more than 10% according to research by the AA•.

How practical are 20mph limits?
They make obvious sense in residential streets and busy city centres, but this bill seems to be aimed at extending 20 mph to the majority of roads in our towns and cities.

What would be the impact on fuel usage?
Research by the AA shows fuel consumption increases when you drop the speed from 30 mph to 20 mph. By an average of 5.8 mpg according to their research.That should come as no surprise, At 30 mph a car may be travelling at very low revs in fourth, or even fifth gear. At 20 mph, it is likely that drivers will have to change down a gear, resulting in the engine running at higher revs and burning more fuel.

What would the impact be on emissions?
If a car is burning more fuel, it will produce more emissions. It’s been shown that speed bumps result in significantly higher localised pollution. So it should be no surprise that the AA research found that emissions would increase by more than 10% if speeds were dropped from 30mph to 20 mph.

What would happen to traffic flow?
It is ironic that when trying to clear traffic jams police officers are often to be seen exhorting drivers to speed up. The reason is, quite simply, that faster traffic clears congestion quicker than slow movement. The highly-respected and formerly Government operated TRL say that 20 mph limits reduce traffic flow by a highly-significant 27%.

What about frustration?
Frustration causes accidents. We know that. It is emblazoned on warning signs on our roads and motorways. Would extensive 20 mph speed limits increase frustration and, thereby, increase the number of accidents? We really need to know, before we blindly introduce more extensive 20 mph limits.

What about driver concentration?
It seems more likely we will let our minds wander when driving at low speeds. Some may even be tempted to fiddle with mobile phones, radios and the like.

What about the economic impact?
What studies have been done into the economic issues of a 33% reduction in traffic speeds? What will be the impact on delivery deadlines and lost work time? Again, these need to be fully understood.

How enforceable is a 20 mph speed limit?
Police Scotland’s roads chief has already said that catching 20 mph speeders “will not be a priority”.

Is the cost justified?
Make no mistake about it, introducing a 20 mph default speed limit would cost millions of pounds for all the administration and planning, not to mention the cost of changing speed limit signs and road markings.

What about road safety?
The argument of road safety is an easy win for those who want us to reduce our speeds. It is impossible to argue that slower speeds will reduce the severity of any accidents that happen – especially the most horrifying accidents that involve pedestrians.

But, we need to stop and think about the repercussions of a rush to reduce speeds.

After all, Britain’s roads are currently some of the safest in the world. We should, therefore, be especially careful that we don’t do something that has unintended consequences.

The AA research found that at a steady 30 mph small to medium-size petrol cars would return 58.15 mpg, but at 20 mph that dropped to 52.3 mpg.

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