20mph – Societal Savings?

There are many amazing things you can find out if you look through South Wales Police’s disclosure log. Ever wondered how many bonsai trees have been registered as stolen in the South Wales Police force area? Fret no longer – the answer is revealed here.

But something even more interesting than the fate of miniature topiary grabbed my attention recently. Because someone asked for the injuries and fatalities recorded on the road network of Penarth.

I’ve dipped my toe in this issue before:

I’m no specialist in transport planning, but it turns out that Friends of the Earth  has done a bit of work on 20mph zones that makes the case pretty watertight. And that makes me think that we should go the whole hog, and make the entire communities of Penarth and Sully into 20mph zones (like you, I’m wondering why Llandough has been excluded from this report).

And the combination of injury statistics and various reports mean we can make some suggestions as to the number of injuries and fatalities that could be avoided by a 20mph limit in Penarth.

Firstly it’s as well to look at the factors that contribute to “pedestrian impacts”. The most common factor is “pedestrian entered carriageway without due care”, accounting for 73% of impacts. There are several others, but it’s clear that in not one case would injury or fatality be less likely if the car was moving more slowly. In fact, the reverse. By providing additional thinking and reaction time, injuries would be less severe, and fatalities less likely, if cars were moving more slowly.

Next, let’s see exactly what impact speed has on injuries.

www nrd.nhtsa.dot.gov pdf esv esv20 07 0440 W.pdfwww nrd.nhtsa.dot.gov W.pdf

Unsurprisingly, the faster the impact, the more severe the injuries. At 40-50km/h (that’s 25-31mph), 21% of injuries are minor, 66% are ‘non-minor’, and 13% are fatal. At 20-30km/h (12-19mph), 67% of injuries are minor, 33% non-minor and none are fatal.

If we assume that everyone driving in Penarth is law-abiding, but drives reasonably close to the limit, that means that for every collision with a pedestrian, you currently have a 66% chance of seriously injuring someone and 13% chance of killing them. In a 20mph future, you have just a 33% chance of seriously injuring someone, and no chance of killing them (bear with me, we’re dealing with statistics here!).

Ok, so I’m aware that rather a lot of traffic in Penarth doesn’t reach 30mph, but with a 20mph limit (using the assumption above), none of it would.

Back to the South Wales Police stats. It’s important to note that not all of the statistics will relate to pedestrians. But the same logic applies to any impact involving a vehicle. Some basic maths helps us clarify this. Kinetic energy (energy of motion) is defined as 1/2 (mass)*(velocity squared). So a one tonne vehicle travelling at 20mph has (roughly) 40 kJ of kinetic energy. The same vehicle travelling at 30mph has 89 kJ of kinetic energy – more than twice as much. Incidentally, the reason a motorway crash can be so devastating is that your one tonne vehicle travelling at 70mph has 490 kJ of kinetic energy.

In the 13 calendar years since this brave new century, a grand total of 1,280 people have been slightly injured on Penarth’s roads. Actually, it may be less than that number, because some people may have been slightly injured more than once. But it’s close enough. In a 20mph limit, you’re presumably much less likely to be injured at all in a collision. I’m going to assume you’re less than half as likely, because there’s less than half the kinetic energy to injure someone (45%, to be precise). That means that 704 people – people like these –  were slightly injured solely because a 20mph zone hasn’t been in place in Penarth.

How about the serious injuries? The statistics tell us that you’re half as likely to be seriously injured in a 20mph collision. That leaves 50 people who would have had slight injuries, rather than serious injuries, over the last 13 years. If you think that a 20mph zone is some esoteric idea with no concrete impacts, try talking to someone who’s suffered a broken pelvis as a result of a collision.

The title of this post is about societal savings. There are all manner of savings we make as a society when someone is reprieved from serious injury. Think of the police time, the medical time, the time spent off work or unable to care for relatives by the person injured. If you’re a hard-nosed economist, think of the reduced productivity of the workforce because people are convalescing at home instead of busy being productive at work.

But there are other societal savings, too. In a town where 20mph is the speed limit, we’re likely to see an increase in people walking and cycling. That’s because poll after poll tells us that most people don’t think it’s safe to cycle on the roads. Well, guess what? A 20mph limit just made it safe.

And people who bike to the town centre, rather than hop in their cars, cause considerably less wear and tear to the roads. Let’s take a similar car to the example above, whose weight plus driver is one tonne. Then let’s take a bike, whose weight plus rider is 100kg. The damage caused to a road surface increases to the fourth power of the axle weight. The axle weight of the car is 10 times that of the bike. That means the damage caused is 10 x 10 x 10 x 10 as much, or 10,000 times as much. A ten tonne lorry (on two axles) causes 10,000 times the damage of the car and 100 million times as much as a bike. Of course, you can’t carry a washing machine on a bike! But still, we’re now looking at real road maintenance savings for our hard-pressed, austerity-wracked Vale of Glamorgan.

And the serious footnote to this post?

Given that by and large people don’t die in collisions taking place at 20mph, if the 20mph zone had been in place since 2000, there’s a chance we’d have up to 12 people alive in Penarth, spreading love and happiness to their friends and families, who are lying in their graves right now.

Think about that next time someone canvasses your vote.

Rho sylw

Filed under Democracy, Transport, Vale of Glamorgan Council

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