A topic that comes up almost daily for any traffic engineer that’s a public servant.
Speeding is a very common complaint from the public. The road in question could be the ally behind someones house, the local road in front of it, the major road down a few blocks, the freeway, or the highway heading out of town….but speeding is the problem – a big problem. Boy, it’s a topic that can really get the blood boiling. But I’m guessing I don’t have to tell you that…..
People drive too fast – way too fast. The speed limit should be reduced. No, for the love of the children, the speed limit must be reduced. It makes perfect sense. I mean, if we’ve got a speed limit of 80 km/h (50 mph for my American friends) and ‘everyone’ is going over 90 km/h (55 mph), than if we reduce the speed limit to say 70 km/h (45 mph), then speeds will drop by the same difference (10 km/h (5 mph)) to the more “reasonable” 80 km/h (50 mph), right?
It’s simple math. If 80=90, then 70=80.
Makes sense. Makes common sense.
But it’s wrong. Very wrong. Scientifically proven wrong.
Study after study (they are called before and after speed studies) have shown that the actual posted speed limit (that black and white sign on the side of the road) has very little effect on actual measured operating speeds.
A before and after speed study is quite simple and it looks like this:
- Don’t change a thing and measure speeds (with a radar gun, for example). This is called “the control” and is what makes this whole thing scientific. (c:
- Change a speed limit (either raise it or lower it – both are commonly measured and different times)
- Measure speeds again
- Compare the results
- Draw conclusion – which is almost always an insignificant change. I say almost because there are certainty times when a reduction in a speed limit is warranted. Most likely, it’s due to a change in environment (like a change in land use – say, a big new development comes online) which motorists will naturally lower their speeds for (cause of added access/conflict points, parking, etc.)
Sometimes, the science is not enough to hold back the will to make the change. A speed limit is reduced and now you have a speed trap. And now a whole new group of people are upset.
OK fine. So what does the science really tell us?
Would you agree that most drivers are reasonable, law abiding, safety conscious people who are not out to get themselves or anyone else injured or killed on the road?
OK, don’t answer that or we’ll open up a major philisophical can of worms that I’m not prepared to get into at the moment…
So let’s assume that most people drive their car at a speed they feel is safe and appropriate – regardless of the speed limit – based on the environment they encounter on the road they are driving. The environment I’m talking about is the road geometry (curves and hills), the presence of pedestrians, trucks, cyclists, intersections, accesses, etc. Sure you can throw signs into that list – but judged against all that other stuff, signs just aren’t as important. And that’s what all the speed studies prove. Signs just don’t matter as much as the “visual cues” of the environment that are picked up by a driver. It’s accepted in the industry as very sound science.
So if all that is true, then what does lowering a speed limit really do?
More often then not it just makes more people brake the law. It does not slow things down and it almost always does not make things safer. It just makes more of the reasonable, law-abiding, safety conscious people get more speeding tickets.
It also makes people not trust signs – even important ones like speed limit signs. :)
There’s more to the story of speed and speed limits, but I have to stop somewhere. I’d love to hear your thoughts on this topic. Please leave me a comment below and give me a piece of your mind.