Speed Kills? Really?
December 22nd, 2006 by Stephen
In a previous article I wrote about a correspondent, MW, who argued (against the evidence) that most people break the speed limits - relying on anecdotal evidence.
He also wrote:
2) People who don’t break speed limits tend not have accidents
My answer:
Are you sure? I am sure I read an article somewhere that showed that people who tend to drive well below the speed limit are much more likely to have accidents than people who travel at around the speed limit (whether a little below or a little above). This was, I think, in an old issue of Advanced Driving magazine, although unfortunately I have no reference to hand.
Munden (1967), reported some interesting results for drivers in the United Kingdom who habitually drive at deviant speeds (speeds well above or below the average speed for a road.) The speed of selected drivers were observed and compared to the four preceding and four following vehicles. For drivers observed more than once, those traveling more than 1.8 standard deviations above or below the mean traffic speed had significantly higher crash rates. This from:
J. M. Munden, “The Relation Between A Driver’s Speed and His Accident Rate,” Report LR 88, Transport and Road Research Laboratory, Crowthorne, England, 1967.
But that survey was one of a number that discovered an interesting U shaped curve in accident rates against deviation from the mean speed by drivers (when excluding changes of speed for purposes of maneuvering). What is shown quite clearly is that both very slow moving vehicles and very fast moving vehicles are much more likely to have accidents. Strangely we do not see a move towards a simplistic “slowness kills” message, because we understand that other factors are at play here.
What is very interesting was some follow up research [2] by David L. Harkey, H. Douglas Robertson, and Scott E. Davis. “Assessment of Current Speed Zoning Criteria.” Transportation Research Record, 1281 (1990), p. 51.
“Speed at which accident risk is minimized occurred at the 90th percentile of the travel speeds observed.”
Thus those who exceed the speed limit a little are statistically (in that survey) the least likely to have accidents, and in fact all the U curves in all the studies show accident rates at their lowest at about 10% over the mean speed, which is usually a little over the speed limit.
Of course there are other surveys that show those who seek compliance with speeding laws are at less risk of having accidents, and that there is often a direct correlation between excessive speed in a driving situation and higher accident rates. I do not want to excuse habitual or excessive speeding, nor suggest that speeding is really acceptable. What these data do demonstrate, however, is that MW was wrong to say:
“People who don’t break speed limits tend not have accidents.”
Again the evidence shows otherwise. The statement is too simplistic and ignores the real data.
What we can say is that those who habitually and/or excessively speed are statistically more likely to have accidents.
Notes:
[1] The Relation Between a Driver’s Speed and His Accident Rate, Report LR 88 JM Munden - Road Research Laboratory, Crowthorne, England, 1967
[2] Assessment of Current Speed Zoning Criteria DL Harkey, HD Robertson, SE Davis - Transportation Research Record, 1990
[...] I have been detailing a reply to a correspondent, MW, about speeding. He argued (against the evidence) that most people break the speed limits, and that those who do not break speed limits tend not to have accidents. On investigation, both claims were clearly false - but MW preferred anecdote and prejudice to colour his views. I am reposting MWs message now, to show how easily we can make errors in our decisions if we base those decisions on anecdote and hunches. [...]
My pastor is constantly getting on us about this “pet sin” that “everyone has.” I, for one, endeavor to maintain the speed limit, although I’m not always successful. Thanks for the logic and hard facts around this practice!
Yes, I agree with your pastor about the fact that we tend to overlook speeding as a sin. I don’t think we should, but the danger is that those of us who try to keep to the posted limits at all times may then be lulled into a false sense of security that our driving is necessarily safer.
Thanks for the comment. I hope you had a happy Christmas (and didn’t spend too much of it online!)
Stephen