I used to be a law enforcement dispatcher/comm center supervisor. For dispatch purposes the parish (county) was split up on a map. We had 3 districts. Each district had a common radio "channel", dispatcher, and group of deputies complete with their own ranking officers. Each district was divided into smaller increments called zones. At the start of every shift each district's rank sent us a list of zone assignments so we knew who was working what area. You don't want all the units in the same area. You gotta spread 'em out. Sooooo, if we didn't have anything to dispatch to a unit in his/her assigned area they sorta found a place to sit up where they could get in & out quickly. When you see police units sitting on the side of the road they aren't necessarily shooting radar. They could be sitting & waiting in case they get a call to go to in that area which they are assigned to.
Also, when officers/deputies work accidents or other calls that require them to submit actual reports they don't always have the chance to do that right when they are on that call. Often they wrap up what they are on with detailed notes they took so they can move onto the next call they need to go on. At some point they have to go back into their mobile computers, fill out all the required forms for their reports, type things up as needed, and submit everything. Well, a good time for them to do all that is when they aren't assigned to a call. They often sit where they can get in or out quickly and work on reports. An advantage to sitting next to a second unit assigned to that same area is that other unit knows that you're working on reports so they'll catch any calls so you can finish.
That's just a couple reasons other than shooting radar that units might sit on the side of the road. If you're worried about paying a ticket don't speed. It's an assumed risk. Do I speed? Yep. My window tint on my car is illegal, too. If I get a ticket I'll pay it. I accept the risk.
The argument that traffic units are wasting time when they can be busting "real criminals" is hilarious. First of all, law enforcement agencies have units to cover all their needs. There's narc units for investigating & busting drug trafficking. There's street crimes units who investigate street-level crimes like theft rings, prostitution, etc. Guess what? There are also traffic divisions whose purpose is to handle traffic related incidents including people who break the law speeding. That's their job. Calls don't get neglected and criminals aren't being given free passes so officers can pick on motorists. This I can assure you. ((Sorry, that's one of those phone callers we used to get all too often....along with the "I-pay-taxes-therefore-pay-your-salary-so-you-have-to-do-my-bidding" people. Hello! We pay taxes, too. Guess we pay our own salaries, huh? :hammer

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