Unless I'm missing something, there isn't really any reason to choose one emergency vehicle over another, is there? So the challenge is just noticing when a type of vehicle that you need becomes available.
My first time playing I ended up doing so well that I wasn't sure it was ever going to end (it seemed like it was always giving me situations that were possible, though maybe I was just getting lucky; or is there an ending?) and I gave up, which is a thing I've noticed in the past as a possible issue with endless games (which is why I put a limit in one mode of my own game).