The Tollway’s Technology: Will Wonders Cease

My idle mind wandered over the weekend during a family sojourn to Frisco. You know those big, huge dot-matrix-y signs that tell commuters how long it’ll take to get from where they are to, say, the George Bush Turnpike? How do the signs know it takes six minutes? I mean, do they gauge how fast a sampling of cars are going, run the numbers to find the average, do some Train-A-Leaving-Chicago-Train-B-Leaves-New-York computations, then spit out the answer? Or do they invisi-tag one car to time it and use that one as the standard? Or do they just post random guesses, like hostesses at restaurants who invariably say it’s either a 15- or a 20-minute wait? I’m curious. But not so curious as to track down the answer. I also like developing theories.