The fact of the matter is that lights and sirens actually impede your progress. I have driven emergency equipment for years, although not in the last few years, and it is amazing what you observe. The majority of the people when they finally realize that you are approaching them, hit the brakes! I believe there was a study circulating about 10 years ago from UNC Charlotte to this effect.
We were always trained to never trade a life for a life. If you don't get there, no good can come of the effort. Consider a patient already in distress in the back of a loud and bouncing ambulance, would a nice and easy priority 2 transport not get them there less stressed than they already are? Every driver should be back boarded and driven to the hospital as part of their training. When you experience the anxiety with no injuries, it provides an entirely different outlook on piloting an emergency vehicle.