Gas company? wow, i hope I never see a gas company flying down the road with red lights and sirens. some emergency personel drive scary enough when responding. I'd hate to see a gas employee responding.
I think you need to think of the call you are responding to.
for EMS calls, I would go:
1. EMS/Ambulance
2. Police
3. Fire
4. Gas Company
for Fire/HazMat Calls
1. Fire
2. Police
3. EMS
Shooting/Stabbing/weapon injuries
1. police
2. Fire
...
72. EMS (I'm not going to rush to get there before there are a lot of PD already on scene

)
car accidents
I think this is one of the situations where you have the greatest possiblity for multi-agnecy response. Personally, if i'm in a rig, and I see an engine or rescue pull up behind me with it's lights, sirens, Q2B (or whatever you call the manual siren), and air horn blaring, i'm going to pull over and let it pass.
1. fire (if they respond)
2. EMS
3. PD
of course, we all know the PD will find a away to get past you, whether you want him to or not.