Anything over 150 miles (240km), I'll fly, so my 250km YEG-YYC flights are always flown. Its usually same day return trips, so thats 5 hours in a car, which can be alot, especially in the winter on a busy highway.

That being said, I will drive if i absolutely have to. For example, last march I had a very important meeting in YYC but could not for the life of me get a return flight because of spring break. I ended up driving that day, so it happens, albeit rarely.

The other exceptions to this rule are mountain resort towns where there is no real air service (Banff, Jasper), and YQR-YXE, which is only served 5 or 6 times daily by expressair at about $400 round trip. Its an easy drive, and because the number of flights is so limited, its not worth it to wait around in the afternoon if I am done early.

I also know YEG-YMM (Fort McMurray) is interesting and that a lot of people who would usually drive that distance (400km) end up flying because the highway is notoriously dangerous and stressful to drive.