20 mph casualty figures - another failure to properly evaluate risk

Chris Peck's picture
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The number of road casualties on streets with 20 mph limits have increased by a quarter in one year. Unfortunately those figures fail to take into account the change in the number of 20 mph streets, which have been increasing steadily, particularly in the last year.
20 mph limits - are they really less safe?
20 mph limits - are they really less safe?

In July the Department for Transport published its annual report of road casualties, which revealed startling increases in casualties amongst all road users, including serious injuries to cyclists and pedestrian fatalities.

Buried in that report was a table that revealed the numbers of casualties that occurred on roads subject to different speed limits. Apparently serious injuries on 20 mph streets rose 35%, while all crashes increased 25%.

But what those figures don't show is the number of streets that were designated 20 mph in 2011. If the increase of the length of streets is equal to, or more, than 25%, then all these figures suggest that the risk of injury is actually declining on these streets. Figures on the length of road subject to different speed limits are not reported by the Department.

Many local authorities have been implementing 20 mph schemes - including across whole cities, such as Newcastle, which finished installing the last of 2,758 20 mph streets at the end of 2011. 90% of Newcastle's streets are now subject to 20 mph zones or limits. Even if the risks on lower speed streets are substantially reduced, some injuries will still occur.

Interestingly, the only other category of road that has seen a significant upward increase in all forms of casualty are 50 mph rural roads. Again, like 20 mph, more roads have become subject to 50 mph limits over the last few years as a result of the reviews of speed limits which local authorities have carried out. Many local authorities have been imposing 50 mph limits on rural roads that used to be subject to the national speed limit (60 mph for cars), and, again, even if this makes those roads safer, it transfers the casualties from one road type to another.

These figures - re-released in August - were covered by the press with the erroneous and utterly disingenuous conclusion that 20 mph were unsafe. Many media outlets ran with these conclusions, including taking quotes from road safety organisations to support them. 

Only a few commentators have seen through this nonsense. Unfortunately these issues will reoccur as long as the Department for Transport continue to publish data on casualties without regard to the rate of injury per mile, per trip or per hour.

The damage from this episode is likely to be ripple out and used by the small minority of people who oppose 20 mph limits to block further moves to introduce lower speed limits. 

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