What do you optimize for?

Person on scene?

Time to hospital for patient?

Time to arrival of a fully equipped ambulance to deal with emergencies?

Risk and time from dealing with a complex handover of the patient from fire truck crew care to ambulance care?

For me it seems like in the vast majority of cases optimizing for getting a fire truck on scene fast is the wrong solution. Waiting a few extra minutes for the ambulance would improve outcomes.

And it is of course possible to differentiate on calls.

A life or death situation like a cardiac arrest or someone bleeding out would select from all available units. Including the fire department, ambulances and police. But those are a tiny sliver of 911 calls.

Yes, in some places there may be more fire stations than ambulance locations. But that's not always the case. In the modern age you would think that either the ambulance or the fire truck could be routed to the destination based on the their GPS locations depending on which is closest.

But also, I don't understand why first responder ambulances don't co-locate with the fire stations (in places where there are more of the latter). If there's an emergency that is not a fire, the ambulance goes.