Here in NY we are told that when you are going lights and sirens its 5 over on city roads and 10 over on expressways. No one really follows that but usually if you get into an accident, you are screwed. I am not sure of the particulars of firefighters going hot because I am strictly vol ambulance. However we have four priorites (1-4) and 1 and 2 are always lights and sirens and always ALS. Priority 3 can be lights and sirens but not neccessarily and priority 4 is no light/no sirens and is usually a BLS call. Priority 2 and 4 are the most common, at least where I operate.