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"The electronic highway signs on Loop 410 and other major highways in San Antonio now display estimated driving times. How does TransGuideTM determine estimated time between two locations?" While driving the city's streets, you've probably noticed areas at stoplights where it looks like rectangles have been cut into the pavement. In places like that, wires have been embedded in the road that can sense the presence and motion of cars, trucks, and other vehicles. TransGuide has similar "loops" embedded in the freeways every half mile or so. You can see them if you look closely at the road while driving by, assuming you don't cause a major wreck by staring at the pavement. These loop detectors, as they're called, record the speed of vehicles that pass over each lane and send that information back to the TransGuide operations center. From there, the data is made available to the processes that compute and display travel time information. The computation is a fairly basic process. The time that's displayed on a sign is based on a computation from the exact milepost of the sign to the milepost of the destination freeway or street. The travel time system finds all the loop detectors on that stretch of highway that are between the sign and destination, then starts the hard work of crunching numbers. First, the speeds of each lane at one location are averaged together, to get an idea of the overall speed of traffic at each detector. Now we have speeds at points in between, but need to translate that to time over that entire distance. The highway is broken down into segments by splitting it up at each loop detector. The speed for each segment is then taken as the lower of the loop detectors on each end. (Sometimes this leads to a higher time, but the thinking is that people will be less upset about "beating the clock," so to speak.) Once the speed for a segment is known, its distance is computed, based on the mileposts of the loop detectors, and those values are used to come up with a time for the segment. Adding together each of the segment times yields a time for the entire stretch of highway from sign to destination. Usually a two-minute interval including that time is then selected, and the result is sent out to the signs, although for times over 20 minutes, it's stretched to a three-minute span, and above 30 minutes, all bets are off, and the system gives up trying to make any more specific prediction. All this happens for every sign displaying travel times in the system in about 10 or 15 seconds, and a minute later, the whole process begins again. No predictions are involved in the process; just computation based on conditions no more than a minute ago. So the next time you see one of those signs blazing times overhead, you can think about all this and know just how those numbers are being generated. Just don't think about it too much. You might cause a wreck, and that makes those times even longer. This month's Whizard is Roger Strain, an analyst in the Intelligent Transportation Systems Section (Automation and Data Systems Division). The Lighter Side
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