Why 20mph is plenty for Edinburgh (and other places)

Why 20mph is plenty for Edinburgh (and other places)

For 6,000 years the street was a place where people met and talked, they traded and did business, above all they could walk where they liked. The street was a democratic space which belonged to no one group or form of transport. This all ended in the 20th Century with the arrival of the motorists who demanded that they had a greater “right” to use the road than anyone else. Pedestrians were forced to the sides and restricted in where they could cross, cyclists were barely tolerated and expected to keep out of the way. How did all this come about?

When the car first arrived, they were few in number and tightly controlled as it was recognised that motor vehicles were a danger to all. However, as the people who owned cars were wealthy and powerful these measures were soon being watered down. First off, the 1865 “Red Flag Act” was replaced by the Highways Act 1896 which set a limit of speed limit to 14mph (23km/h). This change is still commemorated each year by the London to Brighton Veteran Car Run. Then in turn this speed limit was replaced by the Motor Car Act 1903, which raised the limit again, this time to 20mph (32km/h), which is where the speed limit in built up areas should have stayed.

However, after a great deal of pressure from the motoring lobby, all speed limits for cars and motorbikes were removed by the Road Traffic Act 1930. It is very telling that Lord Buckmaster’s opinion at the time was that the speed limit was removed because “the existing speed limit was so universally disobeyed that its maintenance brought the law into contempt”. It is also worth noting that the AA was founded in 1905 to help motorists avoid police speed traps, and the RAC also has a long history of lobbying against speed restrictions.

At this time there were relatively few cars on the roads but the death rate was considerable. Data on road deaths in Great Britain were first collected in 1926, in that year there were 4,886 recorded deaths. The result of the removal of all speed limits for cars meant that in 1931 the death rate hit a new high of 7,343 deaths and 231,603 serious injuries. As a consequence, the new Minister of Transport, Leslie Hore-Belisha, described it as “mass murder” and reintroduced a speed limit for cars at 30 mph in built-up areas (defined in Scotland as areas where the lighting columns are spaced at 185m or less).

But is 30mph an appropriate speed for motor vehicles in a built up area? Well no, as this infographic from Pedal on Parliament clearly shows:

Speed & Fatalities

As speed increases so the survivability of a collision declines. Why would this be?

It’s a simple matter of physics, if streets have large amounts of kinetic (movement) energy moving along them they are more hazardous places to be. Kinetic energy can be shown by the equation Ek = 1/2mv2, where Ek is the amount of kinetic energy (usually given in Joules), m is mass or weight of the vehicle (usually given in Kg), and v2 is velocity or speed which is squared (usually given in meters per second). But what does this mean in reality? Let’s look at the kinetic energy of a small car, these weigh about one tonne (1,000Kg) travelling at 20mph (8.9m/s), so Ek = 0.5 x 1,000 x 8.92 = 39,969J (39.97kJ).

This also tells us that as speed increases, because velocity is squared, the amount of energy increases exponentially. If the vehicle speed doubles, then its kinetic energy quadruples, so at 40mph a one tonne car will now have a kinetic energy of 159.88 kJ. It also tells us that heavier cars are more lethal than lighter ones, so a big 4×4 which can weigh about three tonnes, will have a kinetic energy of 119.91kJ at 20mph and 479.63kJ at 40mph. Just to put all this into context, the kinetic/muzzle energy of a 12 bore shotgun is 4.45kJ. OK so there are some differences is the area over which the kinetic energy would be transferred, but it shows just how lethal cars can be.

Before I move on, just a quick note on the data source for the infographic above. When Donald was drawing it up, we at PoP had a discussion about which paper to take the data from and why. There are a couple of newer papers which appear to give higher survival rates at 30 mph. However, when you read the original source material, you find that the most vulnerable groups, children under 15 years and adults over 70 years, have been excluded from the analysis. No reason is given for this exclusion of data and the reader is left to draw their own conclusions, so we chose to use Aston & McKay 1979. We have received a certain amount of criticism for using “old” data, but as I have shown above, the laws of physics haven’t changed. People say that cars have changed, and yes they have, they have gotten bigger and heavier. A small car in 1979 would have weighed 500-750Kg, now 1,000 Kg, and there were very few 4×4 SUVs. The levels of kinetic energy on our roads for the same speeds are greater than they were before.

Of course the level of kinetic energy is not the only factor that makes lower speed limits safer in built up areas. There are other things to think about too, such as stopping distances, here again the same laws of physics come into play. There is the relationship between driving speed and braking distance. The kinetic energy of a vehicle is proportional to the square of its increased speed, this means that as the driving speed is doubled the braking distance quadruples. Of course there will be those who say the brakes on modern vehicles are better than they used to be. But they are not (as some drivers imagine them to be) magic, they can not overcome the laws of physics. Anti-lock brakes do not significantly reduce braking distance on a dry road, they merely reduce the risk of skidding out of control. Of course braking distance is only one component of Stopping Distance, the other part is thinking distance.

The speed with which the human brain can think hasn’t changed in well over 3 million years, no matter how much some people might want to think otherwise. (Insert your own joke here about Neanderthal taxi drivers). For an alert driver the average time between seeing a hazard and applying the brakes is 1.5 seconds. This means that at 20mph (32Km/h) the driver travels 13m while still thinking, this compares with 20m at 30mph (48Km/h) and 27m at 40mph (64Km/h). Remember this is for an alert driver, increasingly drivers are increasingly distracted by things like mobile phones and satnav systems. This can be the difference between life and death for a pedestrian or a cyclist using the same road.

As speed rises so the amount of time the driver has available to look around for hazards reduces and so does their peripheral vision.

peripheral vision image

In a busy environment there is a need for the driver to be more aware of what is going on around them. In less busy environments, i.e. motorways where the traffic is all moving in the same direction and there are fewer hazards coming from the sides, allowing the driver to focus more on what is happening in the distance. This is why motorways are generally considered to be safer than urban roads, even though motorways have higher speeds. As any trained advanced driver can tell you, driving at any speed requires constant attention and observation.

Aside from the clear safety benefits of 20mph speed limits for all road users, there are also other benefits to keeping the speed of motor vehicles below 20mph in built up areas. These include reductions in air pollution and noise pollution, both of which have an impact of human health.

There is increasing evidence that air pollution is shortening all our lives. Reducing motor vehicle speeds reduces levels of air pollution at source, so this has to be a good thing. There are some who claim that there are modelling studies suggesting that 20mph speed limits would increase pollution levels. However, there are no measurement studies I can find that bear this out. Models are only as good as the data and assumptions on which they are based, if they can’t be validated by real world data, they are worthless.

By reducing speeds by 10mph, traffic noise is reduced by about 3dB (depending on road surface). High levels of traffic noise cause stress and discourage active travel – who wants to walk along a noisy urban street? This not only bad for the health of people living in urban areas (and the majority of people do now live in urban areas), it is also bad for the local economy as it makes local shopping less attractive. “High street” businesses rely on footfall, not drive past. A study in the UK showed that people who walked to town centres spent an average of £91 per week on shopping, while motorists would spent £64 per week. Edinburgh’s bold move to bring in an almost blanket 20mph speed limit (excluding major roads), will help to make the whole city more vibrant.

The introduction of the 20mph speed limit in Edinburgh has been a long process, first we had the Southside trial. Then there was a two year consultation which showed there is significant support for 20mph speed limits. There was also cross-party support within the council with only the Tories objecting (and one of them called for more roads at 20mph in his ward, after he had voted against the city wide limit). The local chip wrapper did try to play up the “mass protest march” against 20mph, however this turned out to be 47 people organised by a taxi business. The police didn’t need to estimate the numbers which the organiser would then claim to be double. They just did a head count, as did an independent observer who said there were only 43.

Despite this evidence the politicians keep saying that reducing the national speed limit in built up areas to 20mph is unpopular with the electorate, so the question is, is 20mph so unpopular? In a recent poll commissioned by the Royal College of Paediatrics and Child Health, found that 58% of Britons “support reducing the national speed limit in built up areas to 20mph to attempt to reduce deaths from road traffic accidents”. The RCPCH recommend that we should: “Encourage physical activity for all children and young people – with and without disabilities – by creating more cycle lanes and promoting 20mph speed limits” and “Reduce the national speed limit in built up areas to 20mph to reduce the number of deaths by road traffic accidents”.

Edinburgh has now committed to doing something which Graz in Austria managed to do 23 years ago. Now if we could just get just Edinburgh to pick up a few other ideas from cities across Europe, maybe Zurich’s policy of one car out, one car in? But then that would need the City of Edinburgh Council to have the ambition to make Edinburgh a world class city.

Possibly Related Posts: (automatically generated)

5 thoughts on “Why 20mph is plenty for Edinburgh (and other places)

  1. “For 6,000 years the street was a place where people met and talked, they traded and did business, above all they could walk where they liked. The street was a democratic space which belonged to no one group or form of transport. This all ended in the 20th Century with the arrival of the motorists who demanded that they had a greater “right” to use the road than anyone else”

    Historically this is not true. According to British Road engineer J J Leeming’s famous 1960s analysis of historical data for British transport fatality rates there were 470 deaths on the road per million population per year in 1863-1870. This breaks down as 76 rail, 143 road and 251 water deaths per million population. The first proper motor cars came in in 1886. That’s far more dangerous than today. There was always road death. If you accelerate the human body over about 10mph and then decelerate it rapidly you expose it to forces it is not designed to withstand. A lot of people died falling off horses … because it is not just speed or energy that is important in causing death but forces which are a function of the momentum divided by the dTtime of the collision. Furthermore not all energy in a collision is absorbed as injury. Collisions may be elastic or inelastic or partially ineleastic. Therefore when people are hit they absorb some of the energy in injury, the car absorbed some of the energy in damage and they are usually projected which often means they are injured a second time when they land. So where and how they land are important. If you land on your head it’s extremely dangerous… which is why there were so many equine death … the roads were not magically safe before the car came along. Loads of people drowned on water transport despite it going deadly slow. After all there was no HSE in 1888.

    On the plus side I award a bonus point for an Ashton and MacKay graphic that does actually look like their data.

    1. Your comment fails to grasp the point, the roads today are dominated by one form of transport, which has annexed public space. The only reason that our roads are supposedly so “safe” for non-motorised road users, is because they are largely excluded from using the roads in the first place. What stats on drowning has to do with 20mph speed limits it is anyone’s guess, evidently a red herring.

  2. A very comprehensive and thoughtful post: many thanks.

    I must admit I get more and more assertive when I see cars driving fast down our local ‘shared space’ streets (apeed limit 7Km/h or 4 MPH), especially when they are driving fast enough to make their suspension bounce.

    These were built with no pavement, and thus no refuge for pedestrians, with the goal of making them places for everyone, as you said. To me that makes driving fast down them to me a form of theft: it is taking something that belongs to us all, and assuming the exclusive right to use it, in fact, threatening anyone else in the street (who isn’t in a car) with injury if they don’t relinquish the part of the road they want to use.

    I’ve had cars drive behind me and try to push me out of the way, and my usual response ids to find a narrow section, stop in the middle of the road, and ask them what the problem is…

  3. If there was a blanket ban in Edinburgh it would mean significant changes to things like the bus system would be needed – you can imagine how every single bus stop would need it’s times changed. Maybe that’s not a as big a deal as it would have been in the past, LRT have their apps and everything now a days.

    If they could slow the traffic over the bridges that would be good, there are so many people crossing the road and things also the noise levels are tremendous.

    I thought the pic showing the peripheral vision is pretty alarming but also make a lot of sense. There is a trick to driving slowly, it’s called being patient and starting your journey five minutes earlier than you otherwise would have.

    I like that we are a forward thinking city and making city centre areas 20mph would be of benefit to everyone.

    1. Thank you for your comment. I would just like to come back on the issue of buses. There have been a number of claims that a blanket 20 mph speed limit would significantly change the bus service in Edinburgh. However, these don’t actually stand up to scrutiny as Dave McCraw has shown, because of the distances between stops the buses seldom reach 20mph along their routes anyway, so a 20mph speed limit would have no real affect on timetables. Then there is the small matter that the timetables are constantly being revised anyway.

      If anything the bus companies stand to gain from the reduction in the speed limit as it takes away the perceived speed benefit of using private cars and taxies over buses, which is why the taxi drivers are so against it. This is despite the fact actual journey time will change relatively little as most taxi journeys within the city are less than 5 miles, and the average speeds less than 30mph, so the reduction in journey time will be less than a 3 minutes, hardly a deal breaker.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Do NOT follow this link or you will be banned from the site!
%d bloggers like this: