We compare accident severities on roads with differing speed limits


By using official figures for road casualties for 2001, this page examines the percentages of killed and seriously injured road users in personal injury accidents by speed limit. The "Speed Kills" proponents would have us believe that risk of death climbs very sharply with increased speed. The facts tell a rather different story.

We've simply calculated percent killed and percent seriously injured by dividing the numbers killed by the total number injured. This gives us a measure of average accident severity on different roads with different speed limits.

Data from "Road Accidents Great Britain", 2001 figures, Table 13. (click here)

Figure 1

We've calculated what percentage of road users are killed in accidents on roads with differing speed limits. The percentage who die is represented by the red bars so that, for example, 2.8% of injury casualties on 60 mph limit roads are in fact fatalities. In this graph (figure 1) we see that higher speed limit major roads do indeed have accidents with more serious consequences. The 70 limit roads are dual carriageways and motorways, and they are (presumably) relatively safer because it's harder to come into direct conflict with oncoming traffic.

Figure 3

It's immediately stunning to note that the risk of death in a personal injury accident is less on a 70 mph minor road than it is on a 20 mph minor road. Assuming that the percentages at 20 and 70 mph might perhaps be some sort of anomaly, consider also the range from 30 mph to 60 mph speed limit roads. We note that the increase in fatality rate is roughly linear with speed limit, and certainly not the much steeper 4th power relationship that the speed kills lobby would have us believe.

Also note that a larger proportion of casualties in 20 mph zones are fatalities compared with 30 mph zones. We find this astonishing. We don't know why it happens, but one credible possible explanation is that driver attention is less in 20 mph zones; they see things later and brake less before impact. 

Figure 4

Here the relationship between speed limit and serious injury likelihood is not subject to any square or exponential rules. In fact it's less than linear. Put another way you are less than twice as likely to be seriously injured despite travelling at twice the speed. Looking at the 70 mph roads (dual carriageways and motorways) the risk of serious injury is about the same as the risk of serious injury around town at 30 mph.

To clarify, when comparing 30 mph limit and 70 mph motorway and dual carriageway accidents the risk of serious injury in a personal injury accident is similar. Speed does not make much difference on these different road types.

Figure 5

We're not sure that there are sufficient "minor 70 mph limit" roads to make for a valid comparison.

But looking at the other figures we can see that speed limit has little to do with accident outcomes. 20 mph limits have "bigger" accidents than 30, 40, 50 or 70 mph limit minor roads.


Attention, skill, observation are big important driver characteristics which have a huge influence on accident outcomes and on the fact that accidents take place at all. As we've always claimed these characteristics show as much more important than speed.

Obviously getting into a crash at a higher speed has some tendency to increase the severity of injuries, but not to anything like the extent that the anti speed brigade would like you to believe.

We can't draw any large or earth shattering conclusions from the figures presented, but just present them because it's interesting and illuminating to see that accident severity does not always rise very sharply with speed limit. 


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