It is the start of traffic counting season in Ohio. Each year, we get about 7 months to count the cars on the road. With my involvement in this type of work, I hear a lot of horror stories. First off, I wanted to discuss how these things work and how the data is used and cannot be used, and then show some of the war stories.
Traffic Counter on side of road
First off: how these things work
Those that have been around for 30 or more years may remember when some gas stations had a hose that rang a bell to call a station attendant to pump your fuel. Those that don't should watch Back to the Future. This is the same basic concept for most traffic counters. There are hoses that go across the road, and based on what the sensors feel and the time between them, these little green (or sometimes gray) boxes calculate the number of axles, distance between them (which can be used to derive the type of vehicle), and the speed.
I know that speed is a big issue with a lot of people. After all, some of these counters are being used for speed studies to see if they want to put a cop on a road at a certain time. This does happen, despite my wishes that cops and others would use less-detectable methods for enforcement. There are two other ways that counts, with speed, can be taken. One is by RADAR (the same thing they use for active speed enforcement). Mind you, for speed sampling, RADAR is pretty useful when installed correctly, and the boxes can be hidden quite well. The other is using magnetic loops. There are portable models of these that sit in the lane and are difficult to see (and also susceptible to theft). There are also permanent models that can be completely hidden from view.
One thing I can say with ALL hose counters: WE CANNOT USE THEM FOR SPEED ENFORCEMENT! The units do not have any cameras (etc), so if you speed while going over them, we know you did, but we don't know who you are!
Second off: How We Use The Data We Get From These Things
This one differs by jurisdiction, but most use it for traffic studies. Speed, count, and vehicle type are very useful for roadway improvement design. Another use is for travel model validation. We (specifically me, since it is my job) use this to ensure that the region's travel model is accurate so when we use it to plan billions of dollars in improvements, we know we're not just guessing, which would be a waste of money.
Law enforcement will use the number of speeders per unit of time to plan when to run patrols. As I indicated, I wish they wouldn't use hose counters for this, but they do, and the data they get is accurate. However, hoses are pretty conspicuous, which is why I wish they wouldn't use them.
We cannot use the data in court. You cannot be detected to be going 45 MPH in a 25 MPH zone based on a traffic counter. The counters do not have cameras in them, and none that I know of can connect to a camera. A camera would be required to prove who was speeding. Without the connection, it would be difficult to prove, since the times would have to be the same, the counter has to be operating perfectly, and the hoses have to be measured very precisely. Some states also forbid the use of cameras for passive law enforcement (a cop can actively use a RADAR+camera, but not mount one on a pole and get every car that is speeding).
The War Stories
I have two, both given to me by a salesperson for Jamar Tech, one of the leading traffic counter manufacturers.
City of Boston Thinks a Counter is a Bomb. This one is proof that some cops don't use hose counters, else they would have known what this unit is.
Counter burned, likely by an accelerant. PDF from Jamar, which the salesperson sent me just after I bought 8 counters from him.
Don't Mess With Them!
It amazes me that 1 month into the season, I've had to replace several hoses because of cut or stolen hoses. This is your tax dollars at work. The more hoses we have to replace, the less money we have to improve the roads.