This is where experience, well trained dispatchers, crews, good communication skills, and good old fashion common sense from all comes into place. There's no way you are going to ba able to sit down and put into polocies and / or procedures that you must/must not use L&S.
I'm not going to go on a rant and rave on it here. All I'd be doing is repeating and going in circles.
I will mention that when I first started there was an unusual "unwritten" policy in place. Where it came from I had no idea. You have to remember it was a very small volunteer service. And no one had honestly worked with or got first hand knowlege from an actual EMS professional. But there were a few that talked of how much they did know. And one of those facts was that "it was the law that anytime you had a patient on board, emergency or not, you had to have at least flashing lights on." This was actually practiced for awhile. We'd be sitting in traffic at a stop light with flashing lights on waiting for the traffic light to turn green. All it did was cause confusion. People with the green light wouldn't go waiting for the ambulance with "emergency lights" on to go, others trying to get out of the way of the ambulance to give them room to get through traffic. It was a true mess.
Finally the coordinator and a couple of others asked the right people (which they should have done in the first place) and got it straightened out. I was there and I felt embarrased for the "verterans" I was running with. Come to find out that the guy who came up with the 'rule", who tended to sound important was just trying to think of an excuse to use the lights. And of course he started backing up saying he was misquoted.