Towards the end of Monday’s Police Commission meeting there was a short discussion about the value of “your speed” signs that tell drivers both the posted speed limit and the speed they are actually travelling. Deputy Chief Bill Moore cited the before-and-after experience of the Bridge Commission, which has put such signs at the approach to the MacKay Bridge toll plaza. The evidence shows that the signs work, said Moore, and so therefore the police department’s traffic division is considering putting them in other locations.
I’ve long been a fan of such signs, because they’re a gentle reminder to drivers to slow down without heavy-handed police enforcement. But do they actually work? To find out, I called Alison MacDonald, the aptly named communications person for the commission, to get the stats Moore had referenced.
The speed limit on the Circumferential Highway is 80kph, but most drivers are going closer to 100, while the speed limit in the plaza is 50. The speed limit on the bridge itself is 70.
Unfortunately, the stats aren’t so clear. “For several hours a month a member of the corps commissionaires records the speed of people coming across the bridges from a couple of different locations and take an average,” says MacDonald in an email. So the available stats don’t measure speed right at the signs, but still, wouldn’t the mere presence of them be a reminder to drivers to slow down even before and after they pass the signs? Apparently not. Recall that the bridge speed limit is 70; here are the numbers MacDonald provided:
May 2013: average speed was 70
June 2013: average speed was 71
Signs installed in Fall of 2013
May 2014: average speed was 70
June 2014: average speed was 68
MacDonald says that the signs themselves record the speed of each car, but that data has yet to be analyzed. But without “before” data, I don’t see how the sign data can tell us anything about the effectiveness of the signs. Everything at this point is anecdotal.
If the police department does decide to use the signs, I hope they collect a lot of “before” data before installing the signs, so we can get some reliable stats.
Average speed at random hours doesn’t tell us much. V75 and V90 are the measurements used in professional studies I’ve seen. i.e.: What speed (V) do 75% (90%, respectively) of vehicles not go over?
For example: If V75 in a 30 km/h zone is 45 km/h or less, the zone may be deemed a succecss by traffic planners. Definitely at V90.
Additionally: Any measurements must be taken under similar conditions. Same day of the week, same time of day, similar weather situation, similar daylight, and both have to be outside school holidays or similar important events. If everybody is watching the Soccer World Cup Finale or the Ice Hockey Olypmics Finale in period A, but not in period B, you can’t really compare them. Same if there is an accident, construction, etc. You also have to make sure that the speed indicator was actually on and similarly visible in one of the periods, but not the other.
Hence, you need to do a lot of measurements until you have comparable periods. It is not economical to have an officer take these many measurements. But there are speed indicators that record the data (without taking pictures), wether there is any indication or not.
Further: Note that these speed indicators usually are not calibrated to law enforcement specs They typically allow a larger deviation (fluctuation margin) than speed guns. And speed guns already allow several km/h deviation of the measured speed from the actually driven speed.
Having said all that: Studies in other countries have shown that speed indicators do work. I don’t see why they wouldn’t work in Halifax, where driving habits are generally slow and very relaxed.
The most dangerous thing I see in Nova Scotia (apart from DUI) is not speeding, but tailgating. Tailgating is what law enforcement needs to focus on. “Vehicle distance measurement guns” are available and should be deployed.