Portland has really only one helo provider for air ambulance service - Lifeflight. There are several local and regional fixed wing providers that we also work with.
Having said that, our rule is that to call the bird, you have to have a critical patient, and it has to save at least 15 minutes from the scene to the hospital. That includes the flight time to the scene... Guideline is that you have to be at least 45 mins out by ground to even really consider it.
One of the counties has a protocol that any crash on rural highways, or other serious call based on initial triage by the 911 center will automatically put the bird on standby - this means the crew gets their gear, loads up in the helo, they preflight it, and get the engines warmed up. That saves at least 5-10 minutes on the response time.
There are the situations where even fairly close, it makes sense to get the bird inbound.... We had a rollover crash where the patient was ejected from the truck that was only 8 or so miles away from the nearest trauma center. BUT.... It was rush hour, and we'd have been fighting traffic all the way in to town. The bird was put on standby based on the ejection. And we were only 5 minutes away by air from their base, so as soon as we got on scene and evaluated the patient, we activated them. They landed before we even got the patient fully packaged, right on the freeway, so we didn't have to transport him to a separate LZ, loaded up, and were off the ground 10 minutes later. Driving to the hospital, even with lights and sirens, in that traffic would have taken a minimum of 45 minutes. He landed at the ER 6 minutes after takeoff, and was in the OR 10 minutes after that for a flail chest, hemopneumothorax, and a basilar skull fx.
Then there's the opposite of that - we had a call at a youth camp for troubled youth. 15 y/o girl had fallen off a horse.... Onto sawdust.... In a padded horse arena.... At least 6 inches of sawdust.... :) She claimed she hit her head, and had "blacked out".... Well, that got the volunteer firefighter/EMT all worked up, and he activated Lifeflight even before we were on scene. She was conscious, alert, oriented, with a GCS of 15. No sign of trauma on her head, or anywhere at all for that matter. Basically a case of Status Dramaticus. The funny part (if there is one) of that story is that as the helicopter was inbound, they radioed for a patient report. And my partner and I let the Danger Ranger handle it - we weren't touching this one with a 10 foot pole.... So sure enough, he's giving report to the flight nurse, and says the patient has a GCS of 15. Which is absolutely perfectly normal, no impairment.... And the nurse comes back on the radio, sounding incredulous, saying, "Just confirming, that GCS was 15???".... When they landed, she asked my partner and I why we activated them - we just pointed at the fire guy who was now trying to hide behind his fire truck... And that poor kid's parents got a bill for something around $10k because some volunteer EMT had a hard-on for watching the helicopter come in......