On my commute its not that uncommon to see a marked car sitting behind an unmarked car when checking HOV compliance.
He didn’t, hence the “finished” writing the ticket. 
Cop pulls Car A over, cites them, finishes paperwork, is getting ready to pull back out onto the road when Car B blows by and gets picked up. Unless of course the cops are finishing their paperwork after they’ve already pulled back onto the road.
While I see what you’re saying, your assumption is faulty. Cops don’t always have handheld radar/laser guns; some vehicles have the radar or laser mounted and pointed backwards. These also make a noise at a frequency corresponding to the speed of the car it has targeted. I would argue a cop writing a minor ticket could easily hear the high-pitch squeal of a speeder and give chase as the speeder might be a higher risk to the public than the guy he pulled over for not signaling.
Also, there are other traffic citations they can write even if they can’t get a speed. Broken taillight? Oops! Failure to display tags? Gotcha! Two speeders? No problem, first for speeding, second for “failure to maintain an assured clear distance” (tailgating). In city traffic almost no one follows at the proper distance.
Often traffic will slow when a cop has somebody pulled over on the shoulder. It can be helpful to know that this is the reason and not some type of hazard to be avoided.
…Or texting on your smart phone to report a cop on the side of the road – while driving… :o
Hey sosb, I just noticed your first post after only 6 days as a Waze member. Congratulations for joining and even more important for being bold enough to post in the forums so early.
Welcome.