What are the black cords strung across roads for?

I know these are for gathering traffic flow data but in particular: Many times you see 1 cord, other times you see 2. Is 1 for only counting the cars? Is the 2 cords for speed data and count? Doesn't seem like speed data could be derived from one very acturately. Anything else I haven't mentioned?
Additional Details
Well, I was thinking after seeing your answers so far... I think I always see 1 or 2 streched across both directions of traffic so that wouldn't tell you the flow in either direction as all cars would hit both cords.

fedupoftheeu

The chords are rubberised switches that connect to a data-logger, one chord measures traffic numbers for flow data , two chords measures speed (maybe to determine if they will put up a speed camera). if they only want it in one direction then it will only go half way across the road.

dubyazilla

I've always assumed that one cord is a count of total volume, while two cords permit the count in each direction.

Paul

Cities are required to periodically do a "survey" of traffic flow. If traffic is going faster than the speed limit, and no reason such as schools, shopping centers etc can be found for keeping a low limit, they will maybe raise the speed limit a little.

I once saw a guy beat a speeding ticket by pointing out that the required yearly survey had not been done on that street for more than 5 years, and he felt that his speed was well within safety requirements.

flackjack

actually if you were to have two cords with a given distance you could calculate the speed of the cars but that information would really not do much good. mu guess is that they are for two separate peoples or for redundancy.

Michael M

counting vehicle unit crossing daily, weekly
it is for stats keeping

W_Howey

The cords actually count number of axles crossing a point on the road in a given time.

You can figure a very close estimate of how many cars by looking at the number of axles.

But if you see two sets on the same lane, within 80 feet of each other they are attempting to split out car vs. semi traffic. The front axle of the semi will cross the second set about the same time as the rear axles are crossing the first set.

If you look at a car, you have 2 axles (one front and one rear) a commerical truck may have as many as 8 axles depending on the set-up. With 8 axles the commerical truck may look like 4 cars using a road rather than the single truck that it really is.

Skip

Had the same thought cross my mind two days ago. I'm guessing you saw the two cords on a two-way street? I would hypothesize that one records the number of cars going uptown, while the other records the number of cars going downtown...

UCANTCME

TRAFFIC COUNTERS.

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