I had a spooky encounter a week or so ago, and it made me curious about the workings of radar guns. My question is in the last two paragraphs if you want to skip all the peripheral details and backstory...
We were coming down the Florida Turnpike in the central part of the state, working on setting a new record for Champaign, IL-->Miami (smashed our old record by over an hour, by the way...) when I got a sudden strong Ka alert on the V1 as we approached a bend in the road. Very shortly thereafter, a FL trooper came around the bend.
I don't know if he was hitting someone else with I/O or if I simply had a very short warning time due to the topography and curve in the highway, and while I was able to slow considerably, there is no doubt that at some point during our passing he must have gotten a reading that was still a fair bit above PSL.
He was ~3/4 of a mile away from a median passthrough, which he gunned it to and then flipped a U turn to get in behind us. By the time he was behind me, I was cruising at exactly PSL in the right lane and being passed by everyone in the left lane. He was probably a mile (maybe more) back from me, he was blasting the Ka for a good spell with nobody directly between us at first.
He wasn't standing on it to catch up to me (I'm not certain he knew which vehicle had been the source of the big ticket number, and now all of the choices were basically going PSL, plus I drive a pretty inconspicuous looking car) and I eventually was able to put a few cars between the two of us without any undue or unusual moves.
All of this happened in the ~10 miles before a Turnpike Oasis, and as we approached it, we debated pulling in to top off the tank one last time but ultimately decided that if he was in fact interested in us, that it might appear (rightfully so, I suppose...) that we were trying to give him the slip. It was the right decision, as he pulled in and after slowing to confirm he wasn't giving us the rope-a-dope false exit trick, he was out of our hair and we could resume making good time.
I was never sure if he was after us or not, or if he even knew which vehicle it was that may have given him a noteworthy readout. I suspect he didn't, as he stayed back and painted everyone with Ka from far behind after he turned around.
Anyway, it got me wondering. At what point does a radar gun return a meaningful reading? Must it be during a direct line of sight, or can it register a speed of anything within it's broad beam of signal, as in around a corner or bend in the road? That is, if my V1 can pick him up, does that mean that he can pick me up?
If I'm being unclear, consider two flashlights (one representing the gun, the other the V1), one on either side of a corner arranged perpendicular to one another but out of direct "view" of each other, but each illuminating a shared intersection of the two beams. Can a reading result from any situation where there is some overlap of beams, or must there be a direct line of sight between the gun and the car?