Margins of error on Atlanta-area traffic signs

Every work day, in the evening on the way home, I pass a sign on Georgia 400 a few miles north of I-285. On a good day it will read something like:

“I-285: 4-6 MIN / I-85: 11-13 MIN”

and on a bad day it’ll read something like

“I-285: 10-12 MIN / I-85: 32-34 MIN”

I figure this sign is somewhere in Sandy Springs, Georgia, although it may be in Roswell, the next city north; see a google map. There are other, similar signs that I also pass on my commute but this is the one I pay attention to.

But what’s interesting about these signs is that, no matter how long they claim the drive will take, the range is always two minutes wide. And you’d expect that the error on the distance from my sign to I-285 would be smaller than my sign to I-85 – the drive to I-285 is a part of the drive to I-85, and it would be quite strange for errors on estimates in the segment from the sign to I-285 to be negatively correlated with errors on the segment from I-285 to I-85. Presumably these one-minute “errors” are purely cosmetic, in order to remind people that these estimates are not always correct.  I assume that there is some internal estimate of the margin of error in the system doing the estimation, though, presumably calibrated on past estimates – why not just use this?  Although this would perhaps be a level of sophistication beyond what people are used to handling.  In weather forecasting, for example, we regularly see probabilities of precipitation, but not error bars around temperature forecasts.

Another, less mathematical, thing about these signs: where Georgia 400 crosses I-285, in the morning (when traffic is generally heading towards 400, not away from it) a sign often reads “I-285 SPEEDS : EAST 55+ MPH WEST 55+ MPH”. I suppose they don’t want to just come out and admit that people go at least 70 when there’s no traffic; the speed limit is 65 in good conditions.

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