There have been studies done, or so I have heard about when responding ambulances don't use their lights and sirens they make it to their destination in the same amount of time or even quicker. The problem is to me, that people see the lights coming up behind them, and just stop. That leaves a lot of potential for other incidents to occur. Given, that doesn't always happen, but for the most part, people act like morons trying to make room for the ambulance, or they just plain ole ignore it.
Second of all, in the state of PA, not sure about other states, the vehicle code says that when operating an emergency vehicle, when the lights are on (emergency rotating lights) there must be an audible warning device on as well. I will look for this in the vehicle code later when I have more time and will quote it.