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Edinburgh imposing 20mph speed limits by 2017; air pollution may worsen

Most of Edinburgh’s city streets could see a 20mph speed limit implemented as early as 2017 if councillors on the Transport Committee in Edinburgh get their way. They are backing a consultation, but have been very quiet on the potential benefits of such a change. They’ve been (unsurprisingly) even quieter on the downsides.

Presumably the goal is safety, which is admirable, but there are better ways such as segregating cyclists and cars, making it easier for people to get public transport, and so on. Neil Greig of the Institute of Advanced Motorists’ Motoring Trust spoke to BBC Scotland and said that the IAM has reservations about the plans. “Our main reservation is about the blanket approach, but it’s also about the fact that we need to do a lot more in Edinburgh.” He went on to say that it needs investment in infrastructure. Tell me any initiative that doesn’t!

While proponents are very vocal about their one goal of reducing speed limits (usually safety), very little is said about the negative consequences, or how technology is gradually improving to render some of these changes redundant.

Worse fuel economy

Modern cars are designed to be most efficient at around 50mph. If you’ve got a hybrid that will work on electricity only, and you can plug it into the mains in your garage at night, then that’s best, but trucks, motorbikes and all non-hybrid cars (i.e. most of them), have the best operating efficiency at 50mph, and it gets worse and worse as you drop the speed. 20mph uses more fuel per mile than 30mph. Above 50-60mph, air resistance plays a huge part, but air resistance is much less at lower speeds.

This paper concerns fuel consumption and emissions at different speeds. Look for table 5 on page 7. For a vehicle without a catalytic converter driven at a constant 50kph (30mph) or 30kph (20mph), the slower car uses 36% more fuel. For a vehicle with a catalytic converter it is more even, but it still uses 22% more fuel to travel at 20mph rather than 30mph.

This will cost more money not only for consumer motorists, but couriers, delivery drivers and tradespeople. That will increase their costs, which they will pass onto the end user.

Worse air pollution

Because we’ll be burning more fuel and spending more time on the roads, this will increase air pollution which (ironically) could lead to more deaths.¬†Again, in the same paper I linked to above, table 5 indicates for a non-catalytic converter vehicle that the slower car emits 44% more CO2 (carbon dioxide) and 36% more NOx (nitrous oxides), but 27% less CO (carbon monoxide). If the vehicle has a catalytic converter it’s much better for CO2 (-4%), slightly less bad for NOx (+22%), and much worse for CO (+7% – that’s a whopping 34% increase).

That is not going to encourage people to walk or cycle to work.

It also has the effect of shifting the deaths to be caused by air pollution, which is not a ‘visible’ form of death to the public like a road accident is.

You can also check out this diagram from The Department of Transportation in the USA.

Wasted time

Time is money, and it takes 50% longer to travel any distance at 20mph than it does at 30mph. That could lead to a 50% increase in call-out fees for people like plumbers, and it will increase freight and delivery costs. It will also degrade the quality of the average driver’s life by causing them to spend more time in the car. Already, the IAM reports the 30% of drivers don’t like driving.

The extension of rush hour

In general (and it is general, because the fluid dynamics equations used to model traffic flow are quite complex), if you slow people down from getting somewhere, it means they are on the roads for longer. As roads have a certain capacity per hour, which is generally governed by speed, road capacity decreases when speed decreases because cars are not cleared from the roads. That could increase your rush hour commute by 50%.

The dumbing down of our driving abilities

I’m going to write a more in-depth article on this as soon as I’ve found the right brain expert to give me an expert opinion. We acquire a set of skills that allow us to drive. Our brains become used to processing information that comes at us at a certain speed, and are able to react effectively and accurately. By reducing the overall speed we are reducing the ability of our brains to cope with higher speeds. The speed differential between 20mph and 70mph is significant, as it is between 30mph and 70mph, but it is less between 30-70mph. If we get used to driving at 20mph, we experience a gradual (albeit initially imperceptible) reduction in our skills and abilities to control a car.

Taking this to the extreme, how many Formula 1 drivers have had a sabbatical and been able to come back as sharp as before? Not many – they tend to languish in the middle or back of the pack, and it’s only Kimi who has had any real success recently. Something like F1 requires a driver’s reaction to be superb, and you can see the impact a year off has on those reactions. For us, driving around a city also requires reactions, and we shouldn’t be doing anything to dumb them down.

New technology

Many new cars now come with automatic braking technology that almost eliminates low-speed crashes. Volvo has City Safety, Subaru has EyeSight, Ford has City Safe, Honda has Collision Mitigation Braking System (CMBS) and so on. We might be reducing the limits now, but over time, it will become safe to drive fast because we won’t be the ones in control: we will have autonomous vehicles (read about them here).

False sense of security for pedestrians and cyclists

Just like you should all be extremely cautious if you are climbing a mountain because conditions can change rapidly for the worse, cyclists and pedestrians should also be extremely cautious when using the roads. Yes, it would be ideal for them not to have to be (and that’s what completely segregated cycling lanes and paths help with), but on busy roads they are sharing the space with large blocks of fast-moving steel that are using that road to transport all manner of goods and perform all manner of services that cyclists will benefit from. That’s just the way it is.

The worst thing that could happen is that cyclists become complacent because certain areas of roads because less dangerous. It is not practical to make all roads like this. However, it should be mandatory that cyclists follow the road rules like other road users (that means no more running the red lights), and it should also be strongly encouraged that lorries have cameras or sensors to warn when a cyclist is on their inside.

The conclusions

As I said, I’ve made a few gross generalisations and used some fairly basic science to make a point: reducing the speed limit might make a difference to safety, but then again, it might not. We do need to improve safety and try to reduce fatalities where possible. The proposal would exclude some main thoroughfares, so it might not be all bad. There are times where reducing the speed limit is sensible – around schools during the time that children will be travelling to and from school, for example – and then there are times when we should go with a pragmatic solution, otherwise we will end up driving with a man walking in front of us carrying a red flag.

Of course, it’s not very politically correct for a councillor to hold those opinions, so it shall be left for us to debate, and I welcome a healthy one in the comments (particularly if you have hard evidence to refute my claims).

Darren has owned several companies in the automotive, advertising and education industries. He has run driving theory educational websites since 2010.

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  1. […] January we discussed Edinburgh’s proposed 20mph limit and why it might be flawed thinking. The public is only ever given the side of the story that suits […]

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