It's tempting, I know. With the flip of a switch you can go from lumbering along in traffic to king of the road. Those lights and that siren; man, it's hard to resist...and let's face it, it's fun!
Whether you call it "running hot" or going "priority" or "code-3" or anything else, driving with lights and sirens is serious, dangerous business. In many cases, it is also very necessary. So how do you know when and when not to light it up?
Like with almost any other decision you make as an EMS provider, you must ask yourself: "How would I explain this in court?" The trick is to consider how the answer would sound to the person who is suing you, or a jury who knows nothing of your job — not to another EMS provider.
For example: "The patient was bleeding profusely. We were doing our best to control it, but what he really needed was a surgeon. His blood pressure was falling and his pulse was increasing, both signs of shock, so I knew the clock was ticking..." That is a solid reason to use lights and siren.
On the other hand, neither "I had to pee really bad!" nor "the patient was puking all over the ambulance!" (Read: and I didn't want to have to clean it all up) are going to convince a plaintiff or a jury that the lights were justified.