20mph speed limits: what’s the data on pros and cons?

SouthLondonUser

20mph speed limits have been introduced in most of London and, AFAIK, in many other cities.

Unsurprisingly, the debate has been very polarised, with supporters claiming it makes roads safer, and detractors saying they’re useless. However, I have not seen any evidence supporting one or the other view. In an ideal world, people would be substantiating their views with hard facts, but in the polarised, fake-news world we live in, facts and data seem to be optional.

It is self-evident that a collision at 20mph is preferable to one at 30mph, but that’s not all, there are other considerations, otherwise the speed limit should be 5mph because 5 is even safer than 20.

I am sceptical because:

I understand the DfT found the results of an initial trial in some English towns (can’t remember which ones, not London) inconclusive, and commissioned a more extensive multi-year trial. Why have so many councils not waited for these results?

There are many cases where a lower speed limit would be irrelevant, e.g. motorists speeding at 50mph in urban areas, motorists under the influence, or any low-speed collision. I have no idea what kind of collisions are more common, and therefore how effective a lower limit can realistically be, but I would have expected (hoped) the proponents of the 20mph limits would (should) have. However, not a word on this.

It was reported in the press that Lambeth council spent more than £700k changing all the traffic signs. Without an answer to the previous point, how can we be sure it was money well spent? How can we be sure that, for example, spending that money to fix the roads or improving their design would not have contributed more to making roads safer?

I do not expect the 20mph limit to make much of a difference to total journey times in the rush hour traffic of a busy city like London. However, even in London, it can make a difference in less busy areas and/or times. Going slower means more vehicles on the road for longer periods of time, ie more pollution, which damages everyone, even non-motorists.

Lower speed limits could, paradoxically, be more dangerous to the extent they provide an even greater incentive for pedestrians to cross the roads where they shouldn’t, rather than being bothered to walk 20 metres to the next crossing and wait 20 seconds for the green man. Yes, I know, jaywalking is illegal in Northern Ireland only, but, still, pedestrians can cross where they want “when it’s safe to do so”, not whenever they like.

The scientific approach would be to compare the accident numbers on two roads/areas which are as similar as possible, one with a 20mph and the other with a 30mph. However, accidents are luckily not enough to make this kind of comparison feasible. A given road may have 2 accidents one year, one another, 4 another. Technically, accidents have decreased by 50% then increased by 400% but, practically, any kind of inference on such few data points is meaningless. To be precise, TFL’s statistics talk about circa 2,000 “killed or seriously injured” every year in the whole of London, which is 32 boroughs. In an inner borough like Lambeth it’s about 100 per year. The accidents which are speed-related will of course be fewer.

Thoughts? Note the question is “what’s the data?”, not “what is your totally unsubstantiated opinion?”

agrinnall

If you want data rather than opinion this really isn't the right place to be asking. Why don't you ask your MP to raise it in Parliament?

nicechap

Does the OP's computer not have google?

https://www.rospa.com/rospaweb/docs/advice-services/road-safety/drivers/20-mph-zone-factsheet.pdf

SouthLondonUser

@agrinnall, since these are local decisions I am not sure contacting an MP makes a lot of sense - maybe a local councilor?

@nicechap, thank you for bringing the existence of google to my attention I had read the document you posted. I tried to keep the post brief and therefore decided not to mention the press releases and other documents I had already read (and found unconvincing).

The experience in other countries is not necessarily representative of what can happen in England; many habits and rules are different, eg the fact that jaywalking is an offence in many countries but not in England. The ROSPA document mentions that, with respect to Bristol:

Given the relatively low numbers of casualties in each of the two areas, it was not possible to draw conclusions
about the effect of 20mph limits on injuries from the data available

which is exactly one of my points. The document also states (my highlighting) that

Although a high proportion of urban roads are suitable for 20mph limits, RoSPA does not believe that 20mph
speed limits are suitable for every road in a local authority area. They should be targeted at roads that are
primarily residential in nature and on town or city streets where pedestrian and cyclist movements are high (or
potentially high), such as around schools, shops, markets, playgrounds and other areas. Roads which are not
suitable for 20mph limits are major through routes.

Lambeth council stated on its website that lower speed limits would reduce traffic congestion; why, they never bothered explaining. I understand that variable speed limits can reduce congestion on a motorway, but how and why does the same apply in a city where you have to stop at a traffic light every 3 minutes?

Also, I’m sure you are all better drivers than me, but, whether I’m riding a (motor)bike or driving a car, I find the chance of going slightly over the 20mph limit all too easy, especially if I have an analogue speedometer. This wouldn’t be a problem if there were some tolerance for speeding fines, but there isn’t, so I can be fined (a % of my income thanks to new rules) even if I am going only at 21 mph. I frankly believe that spending more time looking at the speedometer than looking at the road to make sure I am at 19 and not at 21 makes me not safer but, on the contrary, more dangerous.

Finally, I had read the British Medical Journal study quoted in the report. http://www.bmj.com/content/339/bmj.b4469
It covers a very long period, and divides London in a very large number of different subsets and areas, without clarifying how many data points there are in each small subset. Also, data from the DfT showed that ca. 24% more people were injured in 20mph zones in 2011 vs 2010. The researchers defended their results (http://www.bmj.com/content/345/bmj.e5580 ) claiming, correctly, that the number of accidents alone doesn’t mean much; the concept itself is correct, but it’s not clear at all if/how they took it into account in their own study, e.g. did they exclude those accidents which were not speed-related (eg driving under the influence)?

Car_54

SouthLondonUser wrote: »

Also, I’m sure you are all better drivers than me, but, whether I’m riding a (motor)bike or driving a car, I find the chance of going slightly over the 20mph limit all too easy, especially if I have an analogue speedometer. This wouldn’t be a problem if there were some tolerance for speeding fines, but there isn’t, so I can be fined (a % of my income thanks to new rules) even if I am going only at 21 mph. I frankly believe that spending more time looking at the speedometer than looking at the road to make sure I am at 19 and not at 21 makes me not safer but, on the contrary, more dangerous.

1. What has the type of speedo to do with it?

2. It is vanishingly unlikely that any action would be taken at 21 mph.

3. Even if it were, the question of an income-related fine would only arise if you refused the inevitable offer of a course or fixed penalty.

takman

SouthLondonUser wrote: »

Also, I’m sure you are all better drivers than me, but, whether I’m riding a (motor)bike or driving a car, I find the chance of going slightly over the 20mph limit all too easy, especially if I have an analogue speedometer. This wouldn’t be a problem if there were some tolerance for speeding fines, but there isn’t, so I can be fined (a % of my income thanks to new rules) even if I am going only at 21 mph. I frankly believe that spending more time looking at the speedometer than looking at the road to make sure I am at 19 and not at 21 makes me not safer but, on the contrary, more dangerous.

A car speedo will read atleast 2 mph an hour more than the speed you are going so you would need to be going an indicated 23 mph to have an actual speed of 21. But like the poster above said the chance of action being taken for 21 mph is very slim.

But why would it be more difficult to keep at 20 mpg mph than 30 mph?

Johno100

SouthLondonUser wrote: »

@agrinnall, since these are local decisions I am not sure contacting an MP makes a lot of sense - maybe a local councilor?

But it was central government that gave councils free reign to set speed limits. Unless in exceptional circumstances local councils used to have to set speed limits based on Department of Transport guidelines, in particular the 85th percentile rule.

The 85th percentile
Much road engineering and speed limit design refers to the "85th percentile of vehicle speed". What does it mean and why is it important? Refer to figure 1 below.



For any road situation there will be a spread of vehicle speeds. This spread of speeds is indicated by the green line. There's an average speed of vehicles and some travel faster and some slower. The 50th percentile speed is the speed which 50% of vehicles are not exceeding. (equally it's the speed which 50% of vehicles ARE exceeding).

The 85th percentile speed is the speed which 85% of the vehicles are not exceeding. The blue "accumulated frequency" curve shows how at each higher speed point the number of vehicles not exceeding that speed point increases.

http://www.safespeed.org.uk/speedlimits.html

takman wrote: »

But why would it be more difficult to keep at 20 mpg than 30 mph?

20mpg is about what my car does at 20mph - really good for the environment.

SouthLondonUser

@Car54, with a digital speedometer a quick glance tells me whether I'm at 19, 21 or whatever. With an analogue one, a quick glance at the needle tells me that I am very close to 20, but I cannot with absolute certainty tell whether I am doing 19 or 21 or else. And, before you ask, I regularly have my eyesight checked and I do wear the correct type of glasses when driving.
Also, even if the speedometer is digital, it doesn't take a Ferrari or an MV F4 Serie Oro to go from 20 to 21mph without actually realising...

It is unlikely that any action would be taken at 21 mph. Unlikely but not impossible. The law has no leeway, no tolerance whatsoever.

Similarly, the offer of a course is very probable, but is neither a requirement not certain. The fact remains it is illegal to drive at 21mph in a 20mph zone. I think it's wrong and stupid, not because speed limits must be breached, but because spending more time looking at the speedometer than at the road makes driving more dangerous, not less.

@Talkman, AFAIK UK law is similar, if not identical, to EU law, and dictates that, for real speeds between 25 and 75 mph, the difference between real speed and speedometer reading must be up to 10% +6.25 mph. Look at point 19 of this link: http://www.legislation.gov.uk/uksi/2001/25/schedule/3/made
In other words, the law dictates a maximum difference, not a minimum one, and only for speeds between 25 and 75mph. In fact, the few times I have compared GPS vs speedometer when riding at 20mph I didn't notice any difference, while I do notice differences at motorway speeds.

Also, keeping at 30 mph is easier because it's easier to keep a few mph below without slowing down so much that you become an annoyance to the rest of the traffic and even grannies going uphill on a pushbike overtake you.

SouthLondonUser

Johno100 wrote: »

But it was central government that gave councils free reign to set speed limits. Unless in exceptional circumstances local councils used to have to set speed limits based on Department of Transport guidelines, in particular the 85th percentile rule.

Yet this does not stop different councils in London from setting different speed limits (eg 30 in Westminster and 20 in Lambeth, Southwark and most others).

Nor did it compel these councils to wait for the DfT to complete it multi-year study into the effects of 20mph speed limits.

SouthLondonUser

Johno100 wrote: »

20mpg is about what my car does at 20mph - really good for the environment.

I have no idea if emissions are higher at 20mph or at 30mph - AFAIK opinions are very mixed on this point. A study commissioned by the City of London seems to say: "we have no clue" https://www.cityoflondon.gov.uk/business/environmental-health/environmental-protection/air-quality/Documents/speed-restriction-air-quality-report-2013-for-web.pdf

However, if emissions are really lower at 20mph, the fact remains that, when driving for long periods of time outside of rush hour (eg from North to South London, or from East to West London), with 20mph limits your journey time increases, so you'll be on the road for longer. Is having less emissions per minute but for a longer period of time the same, better or worse? I genuinely have no idea.

mark1959

I think you just want to drive faster, i.e. 30-35.