If they wait on the side of the road for you to drive by, while they may see you speeding, they usually have to have an official speed in order to serve a ticket - they may pull you over however.
The main risk is LIDAR guns, however they must always be still and usually window has to be down (ruling out rainy days, cold days, etc. usually). They may also use "instant on" Ka Band, but in general they will key it up and you'll get the alert. I have indeed been pulled over by a cop who was sitting still while I sped by, but they will typically pull out and come up on you while keying up the radar, at that point you will be alerted and usually slow down before they can get a speed.