They can get you but it may not be particularly accurate given there can be traffic or other objects that reflect.

A few years back when I was younger and thought I was impervious to tickets, I was doing somewhere north of 90 in a 35 around a bend when a cop came over the hill the opposite way. He was way too close for me to slow down so I just coasted to the stop and watched him pull an emergency U. He said I was doing 62 which was a bit more likely to be the minivan that was behind me speeding since I was blazing the trail with a bright red Prelude. I even asked the cop to show me his calibration and he broke out the tuning forks.

Went to court with an armful of documents that I researched, talked to the solicitor and pled it down to 12 over w/a $40 fine.