19 January 2015

10 reasons 20mph's plenty for Edinburgh

A 20mph limit's not going to transform Edinburgh drivers into sweetness and light, nor is it going to get rid of tram-related traffic snarlups, or over-heated busses, but it's still got a lot going for it.  

Here's my top ten reasons why 20's plenty:

  1. Safety: there's no two ways about it, the biggest argument for 20mph is the increased safety for anyone out shopping, walking the dog,  on their way to school, or even just getting out of a car. I dare you to read this and not think, yeah that could so easily have been me, and that kid could have been my kid.  Every day five children and 20 adults are killed or seriously injured while walking or cycling on UK roads (Department for Transport, 2013).  Lower speed limits reduce the severity of accidents. Whatever data you choose to look at, there is study after study showing that survival rates increase dramatically when speeds drop from 30mph.
  2. Reaction time:  Stopping distances at 20mph are half that at 30mph, which means that you're more likely to be able to avoid hitting someone at 20mph, even if a child runs out, or a pedestrian slips and falls while crossing a road. 
  3. Time: We're all busy people. Can we really afford to take it slow? Most city journeys on 20mph roads will only take a few seconds or minutes more. Living Streets research suggests that an urban journey of three miles, which would take 30 minutes in a 30 mph limit,  only increased to 33 minutes in a 20 mph setting. That's a tiny price to pay for safer roads.
  4. Congestion:  Slower speeds actually reduce congestion by smoothing out the bunching that occurs at higher speeds, so we should see some reduction in congestion.
  5. Pollution: 20mph should also reduce most forms of air pollution in our cities  30 km/h zones reduce CO2 emissions by 15%, NOX emissions by 40% and CO emissions by 45%. Only hydrocarbons will increase, by 4%.  And bonus - cars should also be more fuel efficient. 
  6. Noise: Reducing traffic speeds by  10 km/h, a noise reduction of 2-3 dB is achieved.
  7. Good for business: All of those things mean that 20mph zones are also good for business, making shopping, chatting, and socialising more pleasant and more common. 
  8. Fairness: At present there are 20mph zones in lots of residential areas - especially our leafier suburbs - but this would make streets that are densely populated by families, students and the elderly safer.  
  9. Happier mornings: In 20mph zones, more parents are happy to let their kids walk or cycle to school, which means fewer cars on the school run, which has got to make life saner for all of us.
  10. Demand: Surveys (scientifically conducted ones) repeatedly show high levels of support for 20mph.  The  2011 British Attitudes Survey showed that well over two-thirds of us, including motorists, would like a 20mph speed limit in the streets where we live. As I've already reported, 60% of the people surveyed in the Edinburgh People's Survey in 2012 supported further extension of 20mph in the city centre and busy streets, and only 5% opposed it.    
Reading material: 

I've not footnoted everything. I do enough of that at work.  The stats I have cited - and more - are to be found here.  Happy reading: 


Dave Robertson said...

One simple question for you.

How many killed or injured by cars doing between 20-30mph.

If you can't answer that, the rest of your data is pretty irrelevent.

It's fine quoting theory that roads might be safer at 20-30mph. And it's fine quoting stats saying x number of people killed.

I just want one simple question answered. How many people killed/injured in Edinburgh by cars doing between 20-30mph.

Sara Dorman said...

Thanks for this. I think you'd be better off asking the police or someone who collects data.

I'm completely convinced by the wealth of robust data collected in other cities that accidents decrease in number and severity when 20mph zones are introduced.

I have yet to see any reason why that would not also be the case in Edinburgh.

Dave Robertson said...

I'm sure there are plenty of well minded people who believe the same same as you.

As a sceptic, All I'm asking for is the figures to back up the claims.

Yet not one person (including a number of councillors) have been able to provide the stats to back up claims that this will save lives/injuries.

Sara Dorman said...

But we *have* provided data, of the sort that policymakers use all the time. It's just not the dataset that you think is most relevant.

One obvious problem with your requested data set is that often we do not know how fast vehicles are moving at time of impact - especially in hit and runs.

Also, the data sets of actual KSIs within one city are luckily relatively small, so we draw on multi-sited data trends to have more reliable results.

And, as I have suggested on twitter, you are ignoring the other benefits of 20mph. Yes safety is a huge part of it, but by no means all. Data also shows that streets become quieter, less polluted, and people walk more often. These are all benefits.

Dave Robertson said...

The fact that policy makers use wishy washy data to make policy, does not mean it is correct.

If the 20mph folks wish to gain support from others, they need to provide actual stats, not wishy washy "oh it'll make things better, honest, we're sure it will".

Is see the Say No To 20 campaign already has 5000 people signed up. Double the number that did the councils "consultation".

Sara Dorman said...

your wishy-washy is my 'reliable data based on extensive research by professionals'.

similarly, a facebook 'like' is not equivalent to actually attending a meeting, sending a letter or completing a survey.

Unknown said...


The above map (ignore the linkname it does link to a map for Edinburgh shows us the incidents of traffic collisions (recorded by the police which we also know is not all of them) and has them broken down by fatalities etc for the period 2001-2010.
AS these are in the city centre I would suggest that we could suspect that the vehicle was going at between 20-30mph

I can try and find the data set and more recent data this evening.

The point is I think that some of the more serious routes are the ones that will be kept at 30mph... so in fact it's clear that the "arterial routes at 30mph should clearly be reduced to 20mph as well.

Dave Robertson said...

Seems a rather big assumption you are making there Alex. I don't think it's unreasonable for those of us against this change, to ask for actual stats to back up the change, rather than assumptions.


Anonymous said...

Point 11. Driving slower will create less wear and tear on our streets. These potholes are not caused by pedestrians and cyclists. Buses and many cars are now heavier and you can see the result on our streets. The loaded impact at 20mph is a lot less on the street, neighbouring buildings and vehicles - one reason why many support this. Ultimately pedestrianisation is the way to go - will give people a chance to get out their cars and get some fresh air. Works in most cities and Edinburgh has a long way to go. 20mph is a compromise. In Jersey they have 15mph limit on many rural roads.

Unknown said...

I find it amazing that there are people out there who don't think reducing the speed limit to 20 mph is a good idea.

I suspect the people that object are the same drivers that shout abuse at me when I'm cycling my bike - presumably because I dare slow them up as they get on with their busy day.

Unknown said...

Why do you hate cyclists so much Dave Robertson - do you think it is reasonable to hate so many people you have never met because they choose to ride a bike rather than use another mode of transport ?

Morningsider said...

Dave Robertson - your "simple question" does not have a simple answer. It isn't currently possible to know the precise speed a vehicle was travelling at on point of impact with a vulnerable road user. Accident investigators can usually establish a reasonable range - but not an exact figure. The data you demand to see simply doesn't exist.

That said - there is ample evidence for the positive impact of 20mph limits. A 2010 Department for Transport study (1) found that the risk of fatal injury to pedestrians rose from under 1% at an impact speed of 20 mph to 5.5% at 30 mph, to over 30% at an impact speed of 40 mph.

Another study found that on the types of urban road likely to be considered for a 20 mph speed limit, accidents could be expected to fall by between 4% and 6% for each 1 mph reduction in average speed. The greatest reductions were achievable on “busy main roads in towns with high levels of pedestrian activity”

I don't think it is too much of a leap, given the current urban speed limits and the research quoted above, to assume that almost everyone killed or seriously injured on Edinburgh's roads was hit at between 20 and 30mph. Unless they were one of the unlucky people involved in 11% of all reported accidents and 22% of fatal accidents where drivers travelling too fast for the conditions or at excessive speed is reported to have contributed to the collision. (3)

I am intrigued by your question though. How many people killed or injured at between 20 and 30mph would you consider appropriate before lowering the limit?

(1) http://webarchive.nationalarchives.gov.uk/20120606181145/http:/assets.dft.gov.uk/publications/pgr-roadsafety-research-rsrr-theme5-researchreport16-pdf/rswp116.pdf

(2) Taylor, M. C., Lynam, D. A. and Baruya, A. (2000) The effects of drivers’ speed on the frequency of
road accidents.

(3) http://www.transportscotland.gov.uk/statistics/j340611-07.htm

Unknown said...

If you want some wishy washy statistics you should check out the Say No To 20 Facebook page - apparently 85% of cyclists think the roads will be MORE dangerous when the speed limit is lowered. Just how scientific or independent this study was is unclear !!!

Stephan Matthiesen said...

12. 20mph areas are a first step and prerequisite for other changes to create more space for people and also for more residential car parking (!), because at lower speed the carriageway can be narrower.

For example, in the residential area in Germany where I grew up, the conversion from 30mph (50kmph) to 20mph 30kmph) in the 1990s at first did not change much. Over time, however, lower speeds enabled the council to place planters on the road to make it nicer, and some parallel parking was converted to perpendicular parking, creating considerable additional parking space without reducing pedestrian space.

I remember well that at first people were moaning a bit because long, wide roads encouraged higher speeds. Now, 2 decades later, nobody would want to go back to the higher speeds, as it would require the removal of parking, benches, planters to make the road wider again.

Stephan Matthiesen said...

An example of what I said in the last comment, here is a Google Streetview of one place where the introduction of lower speed limit enabled the council to convert some road space into additional parking. Show that to the critics!


Anonymous said...

"Noise: Reducing traffic speeds by 10 km/h, a noise reduction of 2-3 dB is achieved." Do you mean to say that decreasing the speed limit by 10 Miles per hour will decrease the noise pollution?