Motorway Speed Limits

h1. Motorway Speed Limits

p(meta). 3 October 2011

The Tories have announced a policy of increasing the speed limit on motorways and dual carriageways by 10mph, making the ‘default’ national speed limit 80mph. They are trumpeting this as a way to increase productivity and boost economic efficiency but, of course, it’s nothing to do with that somewhat bogus claim.

The increase in speed limits is really to serve one main purpose - to garner votes from angry ‘Clarkson-man’ voters who like nothing better than to claim that motoring laws are something it is funny to ignore. This is mostly because they impede their ability to drive as fast or as moronically as they would like to. To these people it is perfectly acceptable to tail-gate other drivers who are going ‘too slowly’ by sticking to the speed limit, flashing their lights and swearing as they go. They claim that speed cameras are some sort of evil ‘revenue generation’ by the government and laugh when they are vandalised (at great cost to the taxpayer). “Speed doesn’t kill” they scream, based on nothing other than their own nausiating ignorance.

Let’s think about that for a moment. Of course speed kills. This is such an obvious truth that I say this to people who say it doesn’t - choose to sit in a stationery car and have another one drive at you head-on at either 30mph or 40mph. Which would you choose? I think we can all guess which one they would go for. The 10mph difference between 70 and 80 might seem insignificant but it isn’t.

At 80mph not only are impact energies significantly higher (thus making damage and injury much more likely on impact) but also thinking distances, stopping distances, chance of mechanical failures, and many other factors that make accidents more likely in the first place. Stopping distances in the dry go from 315 feet at 70 mph to 400 feet at 80. In the wet this changes to 560 and 720 feet (figures from Drive and Stay Alive, inc). These distances are for the best expected ‘thinking times’ too (about 2/3 of a second) - most people will be slower to react. The extra space required between cars grows enormously with the change in speed, as you can see. Do we really think that the sort of drivers pushing for increased speed limits are likely to leave a 2 second gap between themselves and the car in front?

We must also consider the impact on the environment, of course. Cars are typically designed to be most efficient (with minimal fuel consumption) at around 56mph. Going over this sees a drop in efficiency which is not linear with speed - the loss of efficiency at 80mph is more significant than at 70mph than you might think. The DfT uses fuel consumption figures based on WebTAG for its comparisons, but this is a bit underhand as WebTAG is not meant to calculate consumption at steady speeds. Going by the much more standard steady-state model, a car that typically achieves 41 mpg at 70 mph will achieve approximately 32mpg at 80mph - an increased consumption, and hence fuel bill, of around 25%.

According to Jillian Anable of the University of Aberdeenm the 80mph limit will release an additional 1.3 million tonnes of CO2 into the atmosphere. Obviously there is a moral dimension to this in that we should all be doing our bit to combat climate change, but even for the scientifically deficient or just plain seflish who don’t care about such things, there’s another clear reason why this policy is bizarre - economics.

The government claims that an extra 10mph will generate money in the econonomy thanks to people getting where they are going more quickly. Hmm. Let’s think about that briefly, and we’ll start with the most generously government-favourable position. We should come-up with an example, typical journey, so let’s say my old enforced commute of Bath to Newport as this is a nicely convenient trip of 50 miles. I used to make this trip between 7 and 8am as after 8am the traffic was horrendous. Within my journey there were 37 motorway miles where, on a good day, I might have been able to reach the maximum allowed speed (in fact the two last juntions at the Newport end have a 50mph limit, but we will ignore this for now). If I could travelled an extra 10mph faster on these days I might have completed the journey in, at best, four fewer minutes. What would I have done with these minutes? Well, obviously I would have got up later, or spent longer over breakfast. Or just not really noticed what is, really, a very short amount of time. Net gain to the economy? Nothing, and I’d wager that this is a fairly typical bit of ‘business travel’.

Now let’s think of the downsides. By travelling 37 miles at an extra 10mph I would have increased my fuel consumption by about 25% and generated correspondingly higher emissions. I would also have been more likely to be involved in an accident, and should I have been in an accident, obviously I’d be that much more likely to be injured or killed.

By the government’s own statement they claim that the speed increase might kill an extra ten people a year and injure hundreds more. The Parliamentary Transport Committee estimates that an increase in the speed limit from 70mph to 80mph would result in a 10% rise in casualties on those roads. Research shows that reducing the average speed on a road by just 1mph is likely to reduce the frequency of crashes by 5%. Germany, which is famous for lack of speed limits, has a motorway death rate that is 75% higher than in the UK. It’s hard to pin-down the cost to the economy of a death, and then there are the costs to the state for the response of the emergency services, treatment of the injured on the NHS, the insurance industry, and so on. It should also be remembered that most accidents will delay or even close motorways for a period, and what would the cumulative wasted time of the drivers in that queue be?

Of course there are trips made by people on ‘company time’ where their driving is part of their working day. It could be argued that longer trips here do cost money. However, the government could gain far more productive time from these people if more of them caught the train. You can work on trains. Working in this time is clearly not a priority for these people. They may have to make some trips that are not easily served by the rail network. In the long term we obviously need to expand the rail network (which we should be doing now of course, and on an ongoing basis), but in the short term there are some simple ways we can improve productivity from these people.

Why are they travelling at all? A small percentage will have to physically travel, but a lot of them will be people doing the sort of job I do, where they could easily work remotely or video conference. Believe me, there are plenty of companies that are too old-fashioned to allow this sort of arragement, and they impose a much greater cost on the economy than the hundreds of pointless motorway miles that employees over the country are forced to make.</p>

Ultimately this policy makes no sense. It is a shallow and obvious ploy for votes from the wannabe-Clarksons in society. That’s fine in theory, but they should be more honest. Don’t wrap this up in ‘economic productivity’ - it’s a change that will cost the country far more than it may generate. And people will die as a direct result of it.