Thursday, September 29, 2011

80MPH Limit

Philip Hammond's crowd-pleaser for the Tory conference is a proposal to raise the motorway limit to 80MPH.

"the motorway speed limit nearly 50 years old, and out of date thanks to huge advances in safety and motoring technology"

Unfortunately drivers abilities or reaction times haven't advanced much, and the laws of physics haven't changed. If you raise the limit to 80MPH, those drivers who are doing 80 today will be doing 90 instead. That's 30MPH more than that HGV that just pulled out into the middle lane...oops! Hammond had previously indicated that the 80MPH limit would be rigorously enforced, but somehow I can't see that lasting long, especially with the Government's police cuts and war on speed cameras.

"Increasing the motorway speed limit to 80mph would generate economic benefits of hundreds of millions of pounds through shorter journey times. So we will consult later this year on raising the limit to get Britain moving."

Let's see if we can tease out the flaws in that argument. Businesses value predictability of journeys rather than duration. HGVs will still presumably be doing 60MPH, so they won't benefit from the higher limit. But a lot of congestion is due to collisions. A higher limit will likely mean more collisions, and collisions between faster-moving vehicles will tend to be more severe. So any economic benefits from some faster journeys may be neutralised by increased congestion, less predictable journeys and the costs of more casualties.

Even ignoring collision-related congestion, faster journeys won't necessarily result from a higher limit. It may simply mean that drivers get to the next bottleneck quicker and spend more time queueing. That after all is why the M25 variable speed limit works - if people drive slower, they actually travel quicker. Traffic flow may suffer from the increased speed differential between vehicles - drivers wishing to pull out into another lane will have to allow a bigger gap, and drivers will need to keep a greater distance behind the car in front to be able to stop safely (something not all drivers bother much about). If the prospect of faster journeys attracts more drivers, that will mean more congestion.

Then of course there are the environmental considerations. Higher speeds mean considerably higher emissions. Philip Hammond pointed out on BBC TV news that transport emissions must be reduced but there's more than one way to do that - and suggested that rolling out ultra-low-emission cars would solve the problem. Maybe he's not been reading this blog, which has pointed out that the public are for good reasons about as enthusiastic about electric cars as the Pope is about gay marriage. And an 80MPH limit would actually make electric cars less attractive, as 80MPH in an electric vehicle will drain the battery faster than you can say 'lithium ion technology'. It's those outdated laws of physics again, you see. Maybe Philip Hammond should change them, which should be easy enough as he seems to spend a lot of time in fantasy-land.


  1. The Government often makes incoherent announcements:
    Sustainable growth [in a finite world, there can be no such thing].
    Carbon emissions must be reduced.
    Speed-limits will be increased.

    Either they are confused, or it's me. I believe it's them.

  2. On the Today programme this morning, they asked where the "new money" to pay for weekly bin collections was coming from - from Cycling England is my guess.

    Looks like the Daily Mail has taken over the asylum.