On Wed, 10 Aug 2016 09:11:43 -0400, Pat wrote:

On Wed, 10 Aug 2016 03:09:05 -0400, wrote:

I've always wondered how they work, and
why they dont detect cars going the opposite direction.(I assume they
work on radar). ???

Yes, they work on radar. Cars going the opposite direction DO get
detected, but they show up as going a negative speed. The built-in
software just ignores those signals and displays only positive speeds.

That makes sense....

I have also noted that those signs are always showing about 2mph slower
than my speedometer on my pickup truck. But vehicle speedos are never
100% accurate because of factors in the vehicle, gear ratios and so on,
but tire size is a huge factor. I have heard of guys changing the speedo
gear, just because they put on larger or smaller tires. As long as I
know that my speedo is showing "30" when I'm actually doing 28, I know I
can legally go a little faster. I actually like those signs, because
they do "test my speedometer.

I dont know what size tires came with my truck, but I tend to run
heavier tires than most people, because I haul farm equipment, hay
bales, etc. I'm sure that makes up the difference. My tires are all
10ply, and the quarter or half inch of extra thickness does matter in
the long run.