If I may point out, In L.A Mod you can select a police cruiser and pull over a civilian vehicle of your choice, however there really is no scripting for the officer to get out of the car, do a alchohol test, and if the guy blows the legal limit then the cop would take the guy/girl out of the car, cuff them, and put them in the cruiser. In L.A Mod you get the officer getting out of the car, but doesnt do anything.
I am sorry what are you getting at here? Because goog and CFD might be able to create a mission like that. But there is no current missions in free play that is like that.
I can't remember who I heard talking about this but it was a reputable source, it would cause more harm than good to do this because all the cars driving around follow preset paths, if they were scripted to say move off the road when an emergency vehicle was behind them it'd force them off of these paths, and them trying to get off and back on to these paths would just make them go crazy. So basically you'd just have cars all over the place anytime you sent out a unit on a code run. Maybe there's a way to get around it with the scripting or the way the paths are set up but I couldn't tell you how that would be able to happen.
Unfortunately not, there is no way to script it for the exacts reasons you are saying. It would be nice but unfortantly it won't happen. The only real issues that I had with traffic is when you had tons of traffic around a scene. Other than that is it.