It is built as a perf-board Arduino and programmed to know the speed limits in the areas where I drive. When I speed, a police lights LED display in my rear windshield lights up and makes it look like there is a police car behind me.
Kinda nifty and humorous at the same time! All that's needed is a siren!
I typically avoid speeding by looking at my speedometer occasionally, knowing the area I am driving (and checking the signs); if I am lucky, I am driving a car with cruise control, so I will just set that at the speed shown on speed limit signs, and not worry about it.
It may not be "cool", but it beats a speeding ticket any day...
I put the GPS receiver/Arduino unit on the center console next to the driver's seat (to the right of my right leg).
It's sensitive enough to work accurately in my house, too, as long as I'm not in the basement. If I get a fix while upstairs or outside, I can then go into the basement and maintain the fix.
I talked about accuracy in the conclusions section at the end of the project post. Position is very accurate, speed is off a bit (1-2mph). Published specs for the EM406A are that it is accurate within 10 meters and that's been my experience with it.