I don’t take the highway very often, but occasionally I have to do so for schoo or, as with today, work. I have begun noticing large electronic signs over the past year or two which list the various local interstates/ highways and the congestion time (or something). For example, as I was leaving a meeting today, the sign said “I-64 8 mins I-370 19mins I-55 22 mins” with, of course, varying roads and times (I live in the St. Louis metro area, if that helps). What exactly are these numbers/ times, and how are they derived?
Those are the estimated travel times between you and that landmark. They arrive at those figures by observing the average speed of the vehicles on that section of highway. THe average speeds are determined by metal detectors buried in the road or by cameras mounted along the route.
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Ah… I had always assumed that those signs were indications of the relative traffic levels on the roads indicated. If you’re right, then I’m glad that I don’t travel higways often, as I’ve clearly messed up the strategy.
In the San Francisco Bay Area, those times are also determined in part by tracking, in aggregate, the time taken by individual cars with FastTrak transponders (the electronic toll collection system).
Ed
I hate those bloody signs. Used to be, the Freeway Condition sign only lit up to report a major incident, or the occasional Amber Alert. Now the signs are on constantly, telling you how many minutes it will take to reach downtown. AND EVERYBODY SLOWS DOWN TO READ THEM! Gahh!!! As if L.A. traffic wasn’t bad enough…
I’ve got to agree with that. Blank sign=normal traffic seems reasonable, and if anybody can’t work out the connection between distance and time, that’s their problem. (The exception being that I do like positive confirmation that the M6 Toll is clear before opting to spend £4 on it, and I presume other Brits agree.)
I do indeedy.
I noticed these appearing much more frequently these days - particularly noticeable along the M40 and M25 this weekend. They say “JCT 7-10 15 miles 16 minutes” and the like. Quite handy, I reckon, though inaccurate as I usually do somewhat more than the speed limit.
Hah, yes, I like the way they let you think ‘ooh, I’ve arrived early’ 
It’d be nice to see them be honest about the fast lane.
J14 Stafford
18 miles
11 mins
(And it’s not just motorways, either: there’s a few appearing on the A14, too.)
I have to respectfully disagree on this one. A particular stretch of freeway I use can be anywhere from 20 minutes (for the stretch pertinent to me) to 45 minutes regardless of whether there are any accidents or not. No rhyme nor reason to it, really, so the term “normal,” sadly, doesn’t apply (not during rush hour, anyway). So, the readerboards are invaluable wrt telling me whether I need to take a different route or not.
(And, to appease the OP, I don’t need to slow down to read it, because traffic is always crawling where the board is anyway.)
So a prediction based on the past thirty minutes’ traffic provides an accurate prediction of your journey…how?