If on standby we are required to be able to get to the crew room within an hour of the call. At some bases this includes clearing security to get to the crew room.
Generally we should be within an hour of our base, but some seem to get away with up to an hour and a half commute.
You've got to consider early reports, like 05:30-07:00, if you have an hour and a half commute before that you're leaving home at 4am, is it really safe for you to then work an 8 hour shift and still drive home? I certainly wouldn't want to do that every day.