I keep seeing two long rubber hoses being laid across local roads. They’re attached to a box at one end. Sometimes both hoses go across both lanes, sometimes one hose is short and runs across one lane only.
What are they? Do they measure speeds or number of cars?
Yes and yes. Primarily # 2.
For planning purposes.
I’ve also heard that they can be used for marketing research; for example, how many people will see a billboard at this location.
Note: My Dad’s worked for the Mississippi Dept of Transportation for 30+ years, so I know these things.
They count axles. When you drive, say, a car over it, it counts that as two. When an 18-wheeler goes over, that’s 5 more. A motorcycle counts as two also. You get the total at the end of the period, and divide that by two to get an average. It won’t be 100% accurate, but is good enough for traffic planning (Do we need a light here?) and Marketing (Is this a good spot for a billboard?)
Depending on the distance between the boxes, and if they are newer models, it is possible to figure out the speed people are going between two sets of boxes. Not that you could get a speeding ticket from a box, mind you. It would be more for planning purposes (We designed this for 45 and these dimwits are taking this curve at 70! We’ll need a wall here.)
Next Question: How do they work? I would guess that they are pneumatic, and there’s an air pressure switch in the control box?
The reason that there are two hoses is that the speed at which the vehicle crosses them can also be measured.
I doubt that they would be useful for measuring speed except for getting a gross approximation. To get an accurate measurement the distance between the hoses would have to be constant (which it won’t be since the hoses are rubber and will creep around a bit) and the distance would have to be known very accurately (which it won’t since these things are set up quickly by your average road crew).
The hoses I’ve seen (over here in the UK) are always stapled to the road, and always about two feet apart.
Yes, the hoses work pneumatically, just like in gas stations of old.
The two hose system can be used for speed detection, but there are way too many variables in terms of two cars approaching the hose from opposite directions, depending on the distance between hoses, wheelbase of the cars.
The reason you see one hose across two lanes and the other hose across one is so the same box can be used to count cars in each lane. Imagine hose one (the short one)registers 500 cars while hose two (the long one) registers 750. We can tell that during the test time 500 cars traveled across the lane with two hoses and 250 traveled across the lane with one hose.
Um, sorry to nitpick, but I can’t see that the wheelbase of the cars makes a pile of difference (you’d be calculating the speed based on the time difference between one wheel activating the first hose and the second; you’d do the same with the rear wheels, regardless of how far they were from the front set.
Now admittedly a truck with more than one set of front/rear wheels is going to get tricky and if the hoses cross both lanes, then it’s going to get confusing when cars travelling in opposite directions cross the hoses at roughly the same time.
This is way the hell off topic, but it reminded me of something.
When I was a kid there was one of these hoses across the road nead our school. There was a group of about 10 kids that used to walk home together. When we got to the hose, all the kids would stop and jump up and down on it for a few minutes. The little box near the sidewalk would start clicking like crazy.
We joked that is was a misplaced service station hose and we all giggled at the thought of some service station several miles away with a bell going “dingding” everytime we jumped.
Ok, I’ll shut up now. 
We did that too.
I finally got around to doing the math for this, and the accuracy problem isn’t as big as I thought in my previous post. Assuming the hoses are 2 feet apart, a car traveling 30 MPH will cross them in 0.045 seconds. If the distance between the hoses increases by one inch then the speed indicated will be 29 MPH. So it seems this system is fine for road planning purposes, but I don’t think it could be used as evidence for a speeding violation.
Another possible reason for having two hoses is to determine the direction of traffic flow. You could tell by which hose was hit first which way the car was going. Cars hitting the hoses at the same time would mess this up, but you could just throw away the confusing data points.
When ever I come across these hoses when I’m out cycling, I wheelie over them so it only records one axle. Maybe that will cause the council to add a unicycle lane to the road 
Sorry 'bout that, I wasn’t clear in my post. The I was considering the wheel base being a problem in a one hose system or in a two hose system if the hoses are set further apart than the wheelbase of the car. I mentioned deparation of hoses in my post, but didn’t relate it to wheel base.
I also should have said that while such a system could be used to determine speed of vehicles, these systems are owned by Highway Administrations or other companies, not police departments (so far as I’m aware) thus aren’t used for these purposes.