The availability requirement raises cost considerations. Equipment that costs less is more likely to be available. But cost is not the sole, or even the primary, determinant. If some technology meets the other criteria described below, it will be made widely available, despite the cost. Think of elevators again, ATMs, and electrical lights.
Good technology is always on.
Or if not always on, can be turned on with a single one-stroke command. Better yet, it can start automatically when needed. One thing that makes the telephone useful is that users do not need to boot up the operating system before making a call. A weakness of motor vehicles is that they are not always on. This can cause endless frustration for motorists who need transportation on a freezing February day.