I use the standard siren (wail) and switch to the two-tone when I approach an intersection. if it's a particularly busy intersection, it's been red for a while, or anything else where I'm there for more than a split second, I'll then switch to the phaser just to make sure I have everyone's attention before moving on. One of our ambulances has a device that shoots bass into the ground (thumper? pounder? I always forget the name) so that cars around you will vibrate and it's so cool I use it whenever I can. I only ever use the airhorn when I want to make extra sure everyone knows I'm around, but I mostly do 911 at night so I try and lay off. I will use the airhorn to ensure that a driver who blazes through an intersection after I've arrived knows that he done f'ed up though.