Along Houston freeways there are numerous programmable displays that are used to alert motorists to accidents, Amber alerts and most memorably, admonitions to look for space shuttle debris. When not displaying these types of messages, they generally list driving times from one location to another; for example “Driving time Woodlands to Beltway 8 – 17 minutes at 8:40 AM”. My question is, how are these times determined? Does someone from the highway department drive the stretch and phone the transit time in, or is it a, er, more elegant solution?
It is my understanding that they use the chips of those automobiles that have EZTag. As cars with EZTag cross through unmarked detectors they feed a computer that finally displays the time on the signs. If you exit before reaching the second detector, your signal gets lost. If you pass through both detectors your signal becomes part of an average time from here to there.
I don’t know about Houston, but they recently installed a similar system in Salt Lake City. According to the news reports at the time the system was launched, there are sensors buried in the freeway at various strategic points which count the number of cars passing per minute. Computers use this data to calculate expected drive times.