Not incoherent, just wrong.
Since Queen’s Birthday weekend 2010, the tolerance has been lowered for speeding drivers to only 4km/h for public holidays, which police say has led to a drop in fatal crashes during these periods.A police spokesperson told the Dominion Post crashes during holiday periods had been cut by 46 per cent.
Clive Matthew-Wilson, editor of the Dog and Lemon Guide, … accused the police of “massaging the statistics to suit their argument”. “When the road toll goes down over a holiday weekend, the police claim credit. When it rises by nearly 50 per cent, as it did last Christmas, they blame the drivers. They can’t have it both ways.”
In fact, it’s not at all impossible that the reduction in deaths was due to the lower speeding tolerance, and that the increase over last Christmas was due to unusually bad driving. The police argument is not logically incoherent. It is, however, somewhat implausible. And not really consistent with the data.
If the reduction during holiday periods since the Queen’s Birthday 2010 was down to the lowered tolerance for speeding, you would expect the reduction to be confined to holiday periods, or at least to have been greater in holiday periods. In fact, there was a large and consistent decrease in road deaths over the whole year. The new pattern didn’t start in June 2010: July, October, and November 2010 had death tolls well inside the historical range.
The real reason for the reduction is deaths is a bit of a mystery. There isn’t a shortage of possible explanations, but it’s hard to find one that predicts this dramatic decrease, and only for last year. If it’s police activities, why didn’t the police campaigns in previous years work? If it’s the recession, why did it kick in so late, and why is it so much more dramatic than previous recessions or the current recession in other countries? The Automobile Association would probably like to say it’s due to better driving, but that’s a tautology, not an explanation, unless they can say why driving has improved.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »
If they switch to a permanent 4kph margin of error, I’m going to have to get a radar detector. If I had to watch the speedo closely enough to make sure I never drift up by 4 kph where roads have lots of hills, I’d likely wind up in the ditch for not seeing a curve ahead.
13 years ago
4kph is within allowable speedometer error for passing a Warrant of Fitness. You can get that amount of error by changing the inflation in your tires.
I always use those highway odometer tests when I pass them. There’s a good one on the straight between Kaitaia and Awanui. The odometer can be off by as much as 5% (measuring 5.2 km over 5 km). Given that the odometer and speedometer are working off the same measurements, I’m the guy holding up traffic.
Other examples of tolerance and the problems it causes:
– Speed camera outside school on Onewa Rd. Onewa Rd is downhill, and outside a school there is a 0kph tolerance.
– Lasering cars going down Howe St outside Auckland Girls Grammar School. Quite a steep hill. 0kph tolerance.
Just hope your speedometer is correct!
13 years ago
The WoF says:
“3. The speedometer must be in good working order and operate while the vehicle is moving forward.”
http://www.nzta.govt.nz/resources/virm-in-service-certification/docs/gen-7-vehicle-interior-v4a3.pdf
No mention of 4% error at 100kmph or 8% error at 50kmph.
13 years ago
Speed cameras themselves have a 2km/h margin of error.
I got a speed camera ticket for doing 61 in a 50km/h area. I requested a calibration certificate for the camera and went to court to challenge on the margin of error. They accepted my argument that my actual speed may have been 59 and I got my fine reduced to $30 from $80.
13 years ago
If it were up to me you’d get a ticket for doing 51 in a 50 zone. Zero tolerance.
Don’t like it? I have some sympathy. Really. So let’s debate appropriate speeds, and stick to them. Over by 1kmph and you get nailed. Don’t like that? Just drive at 5kmph under the limit.
The trouble with the tolerance is that it becomes an excuse to exceed the limit. An expectation. What’s the point in having a limit if there’s a tolerance? Accuracy of measurement? See above. It encourages people to drive to the “new” limit. I’m amazed a court let you off.
If you think 110kmph on the open road is safe, then campaign for a limit of 110kmph, but this tolerance crap is just an excuse to break the rules because, go on, you’re a safe driver, right??
13 years ago