Googling turned up a couple of threads including this six-year-old one from the SDMB - but none of them answers the question I am interested in. Which is, assuming these temporary strips of rubber lying across the road are measuring speed as well as traffic count, is it best to go slowly over them to try and convince the authorities there is no need to lower the speed limit, or should one go as fast as possible over them to get the average speed up and convince the authorities the limit should not be lowered? Has anyone worked in the traffic or legal department at a local authority and knows the factual answer to this? If not, opinions are also welcome!
I have an opinion. My opinion is that regardless what you personally do, it isn’t going to be statistically significant to the problem being studied.
Obviously not, but it’s still a good question.
My civil engineer friends have stated that speed limits are determined from a menagerie of different variables like sight distance, road type, proximity to residences/schools, etc. I doubt that the speed at which an average person travels on the road has much bearing on their decision, but I’d be interested to hear otherwise.
I don’t follow the logic there. How does driving fast convince them that everything is okay?
I used to live on a small, back road that was being studied to see whether the township could justify applying for funds to resurface the roadbed. A neighbor knew the details and told all of the residents (maybe 30 or so) what was going on. I modified my routine to hit the sensors as often as possible. Whatever my destination, I detoured to add another car to the tally.
The road was resurfaced.
Are your friends involved in setting speed limits? On these boards and elsewhere (PDF file), engineers have stated that speed limits are set to the 85th percentile of actual traffic speed.
I didn’t know those strips can detect speed. How could they possibly do that? You would have to be able to track individual vehicles from strip to strip and if there is heavy traffic I don’t know how you would do it.
Two strips placed about a foot apart. Not too difficult to determine speed of vehicle from first thump to second thump.
But the actual traffic speed is factually controlled by the speed limit, and as far as I know, every road in this country has a speed limit already in place. If the present speed limit is 25, it is pretty obvious that the 85th percentile of the actual traffic speed is not going to be 65, no matter how logical it would be for every sensible driver to drive at that speed under the prevailing conditions. The threat of arrest is a pretty formidable element in dictating the “actual traffic speed”, much more so than the actual motoring conditions.
A multi-lane road presents a challenge: multiple first and second thumps. Two vehicles pacing each other in different lanes at speed limit but with a half-car-length offset would present as two vehicles traveling at double the speed limit with no overlap, unless the sensors are lane-restricted, and that seems a lot more complicated.
It also doesn’t explain single-lane anomalies like cars of varying lengths, 18-wheelers, motorcycles, cars towing boats, etc. They’d have to record the number of thumps on the first strip and the timing between each, and compare that to the data set of thumps on the second strip, and make sure they’re the same. Then they could calculate speed.
Put me in the camp that they’re simply counting cars.
Then there would be no need for two strips or more.
Even with anomalies, analysis of the impacts can determine most vehicle speeds pretty accurately. The multi-lane ones often have different sensors for each lane, with the other lane crossings using noncompressible tubing.
The posted speed limit has far less an impact on driver speed than the average perception of what is a safe speed. I’ve heard countless stories from traffic engineers about when lowering posted speed limits resulted in an INCREASE in the average speed for that road.
For a speed limit to be effective, it must be considered ‘reasonable’ by most drivers. Otherwise, they ignore it and drive like there’s no speed limit at all.
But, acceptance isn’t the only (or even the bieggest) factor in determining the speed limit. If a lot of drivers are travelling at an unsafe speed, the city/county/state will likely modify the road to encourage slower speeds by adding landscaping, installing speed humps and/or roundabouts, or otherwise making the road seem more dangerous at high speed.
So, drive whatever speed you’d normally travel at. This gives the designers data about actual road usage, so they can optimize the road for that usage.
Fair point, but you can make the same argument about 1 vote not making a difference in an election. If everyone thought that way… Also, if I can convince enough people to do the “right” thing (as in kayaker’s example, then maybe it will eventually make a difference.
See the comments about speed limits sometimes being set to the 85th percentile of the average speed. So if the speed limit on a particular road is currently 40mph, then they set up one of these devices and this shows the average speed is actually between (say) 25mph and 35mph, they may decide there is no problem in reducing the limit to 30mph for no reason other than this. Which is a pain for people like me who actually want to get somewhere in a reasonable amount of time.
Don’t get me wrong, I like to drive fast, but I’m not the type that travels everywhere as fast as the laws of physics allow - I try not to break the speed limit in urban areas, and like all sensible drivers I aim to drive at a speed where I could come to a stop if I rounded a corner and met with an obstruction. But the speed limits on a lot of roads are set at a lower level than this for no good reason, so I don’t want to see them lowered further.
Exactly, this is what I’m talking about.
Yes, this is what I’m talking about. But regarding your last paragraph, as I explained above, I am not particularly interested in “optimising the road for that usage”, I’m interested in travelling as quickly as safely and legally possible. The speed limit often means the latter is the limiting factor, when it shouldn’t be.
I think on balance, from the responses so far I’m leaning towards going as fast as possible over the measuring strips, unless a highway engineer can come along and tell me that this is likely to cause the authorities to decide the road is obviously dangerous and the speed limit needs to be lowered.
For the record, I’m in the UK and have seen a few of these devices round my local area recently, so I suspect it’s an area-wide survey covering a few specific roads. I also suspect they will use any excuse they can to try to lower the speed limit wherever possible.
The pressure sensors are spaced closer than the distance between a car’s axles. The front wheels would trigger both sensors before the rear wheels did the same, so all the device has to do is measure the time between. The number of axles makes no difference.
Actually, they’re counting axles. Dividing by 2 will give an approximate number of vehicles unless you’re near a truck stop or a boat launch.