This month, our new associates have had the opportunity to discuss their upcoming Test with one of the IAM RoadSmart examiners. I was reminded of the Meet the Examiner I attended when I first joined the Bolton and District Group of what was then the Institute of Advanced Motorists. I found it invaluable and I’m sure that my fellow associates felt the same. One of them was obviously concerned about keeping within the speed limit. This was at a time when satnavs were coming into general use and he had compared one with the reading of his speedometer. He was surprised to find that his speedometer consistently read a few miles per hour higher and he wondered which was correct.
He was worried that he would not be driving at the limit during his Test. Could the examiner help? The answer was quite simple. The examiner explained that he would not be bringing a satnav to the Test and he and the associate would have to rely on the car’s speedometer. So there was no need to worry, he should just follow the instrument on the dashboard and all would be OK.
Satnavs and speedometers were also compared in The Daily Telegraph recently but now, of course, we know much more about the former. A reader’s letter described how the writer had received a speeding fine although he had been monitoring his speed by satnav. He felt that the GPS technology should be more accurate than the speedometer and yet, getting a ticket for speeding would seem to indicate otherwise.
I was most impressed by the reply given by Alex Robbins, The Daily Telegraph journalist. He initially agreed with the reader saying that (s)he was correct that GPS is usually more accurate than a speedometer. By law manufacturers have to make sure that car speedometers don’t underread. So firms set theirs to overread within the legally permitted margin of error.
While satnavs may provide a more accurate indication of a car’s speed it’s important to remember that they are not infallible either. Garmin, for example, says its GPS receivers are accurate to within 15 metres (49ft) 95 per cent of the time. Therefore, five per cent of the time it is not accurate; perhaps the signal is obscured by atmospheric conditions or electromagnetic radiation. The accuracy of the readout is also reduced during acceleration and deceleration. So, as advanced drivers, we know that we can’t rely on the GPS speed displayed on a satnav.
In fact, this is why GPS is not deemed in law to be an acceptable replacement for a factory speedometer. The law considers that although speedometers aren’t accurate, their inaccuracy is at least consistent, meaning that, as long as they are functioning properly, drivers shouldn’t end up going faster than they intended. And they cannot use a faulty speedometer as an excuse if caught speeding. So, while it’s interesting to compare speeds measured by GPS and by car speedometer, it’s wise to rely on the latter. Otherwise they could, in Alex Robbins’ words, land themselves “in hot water”! A speed awareness course may beckon!
Gary Whittle