Monday, July 6, 2009

Sign Placement Takes an Engineer

Traffic SignI was driving into work this morning and I noticed a sign off to my right. It was a temporary sign which evidently was warning us drivers of some upcoming event. It was too small to contain the entire message so they broke it down into pages which displayed for about 3 seconds each. The first page I saw read:
July 12, 2009
From 6:00 AM
To 7:00 AM
Interested, I eagerly awaited the next page. About 3 seconds later the sign read:
Exits 6 and 7
But, before I could find out what was happening during that block of time, I passed the sign. What in the world did that third page read? Should I plan on staying home? Should I leave home? What's happening?

Well, I started thinking about it and it occurred to me that it was pretty dumb to put that sign on the side of the road right after a curve. Why not put it after the curve in the ensuing half mile straight away?

So, I looked into it. If you take a look at the image above, you'll see the approximate location of the sign. I drew a line segment starting at the sign, passing the west most occluder, and ending at the median between the northbound and southbound lanes.

Then, I traced the median all the way up to the approximate angle where I could no longer see the sign because the visual angle was too narrow.

Next, I traced the fast lane (the longest route within the "polygon of visibility"). I measured it out in google maps and it turns out that the sign was visible for 448 feet.

The speed limit at this point on GA 400 is 65 miles per hour (approx. 95 ft / s). That means that the sign was visible for just shy of 5 seconds.

I'm sure by now, you've done the math and come to the same conclusion. If the sign pages rotate at a minimum of 3 seconds per page and the maximum visible time at or above the speed limit is 5 seconds, then it is only possible to read 2 pages of data. However, the message must be at least 3 pages in order to be complete.

There's no way to reasonably expect drivers to read the entire message and therefore most people passing that sign today will have no idea what the full message is. Evidently, when it comes to sign placement, it takes an engineer.

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