Monday, June 25, 2007

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.


Sir Henry said...

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 said...

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 said...

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?

Robert said...

I wonder how we will explain away this years huge jump in the road stat's in SA?

For years we have accepted report after report that presents as "fact" a wider range of computational estimations that present as Proof that this speed equals that risk!

These are NOT facts, they are by and large "probabilities". These probabilities are also highly questionable. By example here is a simple one:

"Increasing the speed traveled by 5Kph effectively doubles the proportion of vehicles that crashed"

Wow you might say that is huge .. BUT wait think about it ... drivers that tend toward risky behavior where do you think that they will be most represented statistically ... Yes you guessed it more of them will appear at the higher speeds.

So what this shows is there are "other factors" that contribute and speed is just one. The problem is it is easy to measure and very profitable to enforce.

I would instruct you to consider a very much under considered factor here as we continue to create ever more complex speed and speed enforcement regimes.

It is called Receptor fatigue. This describes the behavior of any animal include humans to a repeated stimulus that does not have any immediate effect. After a certain amount of "reaction" the animal or persons reception of the stimulus begins to decline and ultimately we ignore the input completely.

This is what we are doing to our drivers! It is a very dangerous thing indeed as we are warning people of horrific risks resulting from a difference between 50 and 60kph when in fact when driving sensibly and to the conditions the driver who moves between 45-65 across an ordinary suburban road environment is executing quite safely.

Personally I was horrified by last years "Creepers" adverts in SA which suggest that 5kph over a speed limit is some how exposing you as both to huge risk and denoting you as a bad driver. That is absolutely a case of miss use of statistics and also demonstrating a very poor grasp of behavioral science as well.

What it did do nicely is set up an argument to defend the SA Government from the huge wind fall of revenue bagged from Drivers continuing to drive (by habit) at 60 in zones now either marked on by omission set at 50Kph.

Lastly IF those in positions of authority seriously think that we the ordinary driver do not notice and consider these issues then they are also seriously deluding themselves. We do we notice and we by and large grow a bit less attentive year by year.

What I can tell you have learned to do VERY carefully is read my Speedo every hand full and seconds and check the sides of the road for signs. Arguably NOT the safest use of my valuable driving attention .. which rather confusingly is directed by another State advert blitz to "drive" as good drivers just "drive" apparently! Shame about that new 50 zone I just missed!