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Fatal car accidents & car speed

Tim Blair criticises the idea that small reductions in speed can greatly reduce the probability of a car accident. I think on this occasion he is incorrect.

I forget much of my high school physics but isn’t it true that impact is proportional to mass and speed squared? Thus increasing speed from 60 to 65 km per hour – by about 8% – increases the collision impact by 17%. This is quite apart from difficulties of stopping a faster car, problems with lower reaction times and so on. Taking all these into account it has been found that increasing speed as suggested does in fact double the accident probability.

The impact of speed on accidents is discussed here. Their suggestion is that reducing a car’s speed from 60 to 50 km per hour reduces the probability of killing a hit pedestrian by half although you are still almost certain to injure them. A more ccomplete bibliography is here.

Of course reducing car speeds to zero would reduce traffic accidents to zero. The question then is what are appropriate speeds from the perspective of trading off reduced travel times against reduced accident risks. There is quite a literature on this (see here for example) . My understanding is that current speed limits on urban residential streets get it about right.

3 comments to Fatal car accidents & car speed

  • Sir Henry

    The link to Jeremy Wolley, Harry is very informative but it only tells us, I think, that there has been observable reduction in casualty crashes. In other words, if you are going to have a prang, one at a slower speed is more likely to have you walking away with nothing more than soiled undies.

    Probability of an accident at a higher speed – presumably because the driver loses control – is dependent on conditions of the road, weather and the driver’s skill and experience.

    Under some circumstances the probability of an accident is inverse to the speed because the faster you go the less input movement to the steering you need, for example, to avoid either an oncoming car or an obstacle.

    The problem with studying statistics is that they don’t tell you how well trained the accident drivers were.

    This is very dodgy stuff (no pun intended).

  • hc

    Sir Henry, I think the argument in your third last para while sound is of limited importance.

    I have also heard the argument that if you drive faster you take more care.

    Yes, dodgey.

  • Bill

    Regarding lowering speed limits for safety, our Village is planning to lower their speed limit from 30 mph to 25 mph because “it appears to be a good idea.”

    Is there any data showing this would actually be “safer” and we should spend the money to do it?

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