The parking brake generally isn't capable of locking the wheels, it is too small.

There is such a thing as burnishing the parking brake linings. This is where the parking brake is lightly applied then released while driving over a period of time to wear the linings in. As with regular brakes, new linings don't work as well as linings that have been in use for a little while. This could offer better holding power but still, the parking brake is pretty small.