20-minute wait on the GW…

How does the traffic reporter on the radio know how long the wait to get across a bridge or through a tunnel is? Do people collect data on this? Is the reported wait time merely a function of how long the “line” leading to said bridge or tunnel is? Or are other factors (maybe time of day or the general badness of traffic at the time) involved? Has anyone ever investigated whether these waiting times are accurate? Just wondering.

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2 thoughts on “20-minute wait on the GW…

  1. I don't know that it's the same in every city, but I believe a lot of major cities use some variation of a traffic counter. These are frequently coils of wire embedded in/around roads that measure the amount of time a car takes to pass over the wire (through measures of inductance). From the amount of time a single car passes over the wire, as well as the number of cars passing over in a certain amount of time, and between which loops (again, I think), they can extrapolate the amount of time it takes to pass, say, 1 mile of a road. Couple that with knowing that the "Can of Worms" or "Route N" juntion near the "Whatever Toll Road" is X miles from some common point (usually downtown, or another junction, as Chicago likes to do — "from the I-95 junction to the Post Office…") and you get a reasonable measure of travel time.

    Places like traffic.com are using roadside sensors and traffic cameras, too.

    Note, though, that is all dimly recalled. There have been some studies on the accuracy, I believe, but I don't know how they fared. Even if the reports are way off, I would not expect to see them disappear. California installed more electronic signs on the roads to give people more information about upcoming congestion. Of course, they admit that these signs slow things down because people slow down to read them. The major reason, I'd argue, for the signs, is to reduce frustration over traffic patters (revving up, only to slam on the brakes around a bend because everyone is stopping) and hopefully ward off road rage. But that's just a pet theory…

  2. Caltrans use buried loop sensors (the same type used for traffic lights). The resolution varies but they can apparently be as close together as 1/3 of a mile. They measure # of cars and the amount of time a car spends on top. How they do the speed estimate I don't know offhand–my guess is they just take the average time spent and an 'average' car size and ignore the fact that semis are large.

    Anyway, you can watch it in realtime at http://www.dot.ca.gov/traffic/

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