Expound, good sir!

Heh. I said it was rare.

It's when they're using a time/distance trap to measure your speed. I don't know if these are even used at all any more, they were rare even when that old book was written.

It's a little device hooked into the odometer on the cop's cruiser. They roll the distance between two landmarks and punch the start and end points into the device. Then they watch cars going between the two landmarks and hit a start/stop button for each, and it reads out their speed. If you think you're in the middle of one of these traps (i.e., you've crossed the start but haven't crossed the finish), then the only way out of the ticket is to put flat spots on your tires.

The advantage to the above system is that it's entirely passive, and radar detectors won't do speeders any good. They were reasonably accurate, if I recall correctly, and stood up well in court.

Honestly, though, I've found that here in California at least, my only threats are from pacing (that's where the traffic scan is critical, a good pacing is ironclad in court), aircraft (easy to spot), and from KA-band radar. When I travel in Nevada, I also have to watch for K-band. I've never had an X-band warning that wasn't a door opener and I wish I could kill all X-band alerts on my detector (I can only reduce the x band sensitivity, not kill it completely).

I'm afraid of laser, since laser detectors are useless (always instant-on, and you don't get backscatter from other cars), but I'm crossing my fingers that those are still too expensive to be in widespread use by the CHP.