A blog about cars in Aberdeen.

This is a blog about cars in Aberdeen because most people aspire to the convenience of personal motor transport, pay dearly for the privilege, provide much employment, contribute greatly in taxes, and then people expect them to ‘leave the car at home’, while their money is spent creating cycle lanes and the like for freeloading cyclists.

Friday, 30 September 2011

Speed Limits

Everyone will have noticed the Government's announcement that they aspire to raise the motorway speed limit to 80 mph. Initially we were very excited by this, for when Aberdeen's new bypass Motorway, the Aberdeen Western Peripheral Route (AWPR) is built, when we roar along the "Fastlink" at 80mph, we will not risk getting any more points on our licence.

But then we thought about this policy in more detail, And, frankly we were disgusted. Firstly, where is the corollary for other types of road? If the government are going to decriminalise going at 80 on the motorway, will they also please legalise going 40 and even 50 or 60 in town? Our endorsement points are mounting up, despite the fact that we pay dearly through road taxes for our right to use Her Majesty's pot-holed highway.

But secondly, and this is the subtle bit, we began to notice something rather sinister in this motorway speed policy. Now, pay attention:

Increasing speeds on a road leads to a proportionate decrease in capacity (i.e. number of vehicles passing a point, per lane, per hour). Braking distance is proportional to the square of the speed. So if drivers are doing 80 mph instead of 70 mph (a speed increase of 14%), the safe braking distance is increased by a factor of (80/70)^2, or about 30%. The lane capacity (ignoring vehicle length for simplicity) is the vehicle speed divided by the separation between vehicles. The throughput of vehicles at 80 mph as a fraction of the throughput at 70 mph is given by:
 (80 / 70) / ((80 / 70)^2)
 = 1 / (80 / 70)
 = 70 / 80
= 7/8 or 87.5%

So that's an immediate decrease in road capacity of 12.5% as a result of raising the speed limit. Shocking!

Seems that, while pretending to be the "motorists' friend" and with Phil "Hoverboard" Hammond declaring that "Labour's War on Motorists is OVER", our Tory government is actually intent on reallocating roadspace AWAY from we hard-pressed motorists. Horrible!

Also, the tragedy of the commons applies - a shared resource which is free to exploit at the point of use will be over-subscribed. All motorists rightly and naturally like to go really really fast; Oh the thrill of speeding along the open road; yes those deserted switchback mountain roads; night-time urban highways; laser-straight racetrack autobahn, like in the car adverts - and the new government policy speaks to this dream of unhindered open-road high-speed freedom. This will, quite naturally, attract more motorists to our newly high-speed motorways where these extra motorists will actually find constricted capacity because of that self-same increase in speeds. The prospect of driving faster will attract more car journeys onto already oversubscribed roads and will actually mean more congestion. This will, in the longer run, detract from motoring and push motorists onto alternate modes of travel. Disgusting!

So, while we naturally and rightly support faster driving, we can only support this policy if:

  1. it is rolled out across the entirety of the UKs roadspace, including town centres, school zones and residential areas, and
  2. it is supported by a massive increase (at least 12.5%, but better to be 15%, no 20%) in the amount of roadspace in the UK. This means a massive new roadbuilding programme which will bring great benefits of economic growth.

We trust that, during the consultation process, Transport Minister Phil Hammond will take our criticisms and recommendations into account. After all, here in Aberdeen, we know a few things about cars and economic growth!


  1. You seriously think it is safe to drive at 60mph in town? Someone should take your license away. 30mph is safe, and entirely sensible in the majority of instances.

    Although 80mph on a Motorway is a sensible idea, surely it shouldn't be applied indiscriminately? As you've pointed out, there's limited capacity on roads, and the thought of doing 80 on many stretches of motorway (especially in Scotland) is laughable at best. Particularly on two-lane motorways where the difference in speed between 80mph cars and lorries restricted to 56mph would be potentially dangerous.

    Unless I've missed something, I suspect that you won't see 80 on the Aberdeen bypass, which isn't going to be a Motorway,