Tuesday, 14 August 2012

Accidents in 20mph zones

Fullfact has been looking at various media reports on accident statistics in 20mph zones. In essence official statistics show that the number of accidents in 20mph zones has been rising. The conclusion drawn by some media is that this shows that 20mph zones don't work. In reality Fullfact finds that the figures take no account of the expansion in the use of 20mph limits. So an increase in the number of accidents tells us nothing about  the effectiveness of 20 mph zones.

To make any real sense of the raw data we would need to know how many roads have a 20 mph limit, and how much traffic they carry. It seems that the DfT can't help, because they don't know.

I wondered if the OSM data could provide any more information.

OSM contributors have added speed limits to about 9% of the UK road network. As a result, about 3,500km of road currently show a speed limit of 20 mph. That represents just under 1% of the whole road network, and just under 3% of minor urban roads.

We must bear in mind that some 20 mph zones will not yet be added to the database. And OSM data isn't always accurate, so it's also possible that some roads have been marked incorrectly as having a limit of 20mph. We could make some assumptions about roads that don't yet carry speed limit information, but it doesn't help a lot here. It is normally safe to assume that OSM data is incomplete, but less likely that data is inaccurate. So we can make a reasonable assumption that the true extent of 20mph roads is going to be higher than the recorded figure. In other words, we can use the data to estimate that at least 1% of the UK road network now has a 20mph limit, and 20mph limits now represent almost 3% of minor urban roads.

Meanwhile, according to ONS/DfT data, 20mph zones account for just over 1% of all accidents, and 1.6% of accidents in built-up areas. In other words, the proportion of accidents in 20mph zones seems no higher than the proportion of 20mph roads. If we just look at built-up areas then the accident rate in 20mph zones is about half what we might otherwise expect.

Perhaps more importantly, the DfT data on its own shows that the mix of accidents is different under different speed limits. With a 40mph limit, one in 100 accidents involves a death; at 30mph it is one in 200, and at 20mph it better than one in 300. In 20mph zones the number of deaths on the road could be as little as a tenth of the level elsewhere.

All this doesn't get us to complete and reliable figures, it can take no account at all of traffic volumes, and there are surely some more complex causes and effects underlying the data. Nevertheless, if the limited numbers that we have tell us anything, it is that accidents in 20mph zones are in line with the obvious hypothesis that lower speed limits are associated with fewer, less serious, accidents.

More here - http://www.sustrans.org.uk/resources/in-the-news/are-20-streets-really-less-safe


Shaun McDonald said...

Here's a map of the speed limits in the OSM data: http://www.itoworld.com/map/5

Tom Chance said...

The other important point is the severity of the incident. 20mph zones serve two purposes - to try and reduce the number of collisions and to make them less severe. The stopping distance difference should mean a lot of potentially fatal or serious collisions only result in slight injuries. So you might also look at the incidence of "Killed and Seriously Injured" casualties, a subset of the DfT data.

Chris Hill said...

20mph limits seem a very good idea in some places. They are rarely enforced, indeed try driving at 20mph around some residential roads and see how you get tail-gated or overtaken. To automatically enforce the limits many have traffic-calming measures such as ramps, humps or cushions. All of these inverted potholes are unpopular with motorists and have caused real problems for ambulances for example. I believe cushions are a very bad idea.

Cushions do not cover the whole width of the road, so some idiots drive around them. This makes the use of cushions more dangerous to me than not using traffic-calming measures at all. I think they are cheap to install.

If OSM could help to demonstrate the benefit of 20mph limits it would indeed be useful.

gom1 said...

Thanks to all.

In response to Shaun - good point: a useful tool to find places that need fixing up.

In response to Tom, the DfT data does show the proportion of deaths declining with speed (as you would expect). The balance between serious and slight injuries is more complex though. At 20 mph the proportion of slight injuries is quite low, and the proportion of serious injury quite high. I assume this is because the number of "no injuries at all" cannot be measured.

In response to Chris - all you say is true. I'm not sure, though, that lack of enforcement is the same as lack of benefit. If lower speeds mean lower risk, then "virtually all" drivers obeying a 20mph limit because of enforcement is presumably safer than "most" drivers voluntarily obeying 20mph limit. On the other hand, "most" drivers doing 20mph in a built up area can still be safer than "virtually all" doing 30mph.

I suppose that my own preference, based only on my own experience, is for more consistency across larger speed zones. Too many changes of speed limit get confusing (e.g. round here we have 20mph just where the road runs past a school gates, and 30mph elsewhere).