Zombie Cliché Lookout:
Safe zones are great in theory, but you need some sort of system to ensure that the infected don’t get in and turn your safe zone into a breeding ground for the living dead. The solution is a thorough search of everyone entering, and a quarantine of anyone who looks suspicious (i.e. has bite marks all over them).
The problem is that if no one can get in, no one can get out.
Think the Berlin Wall here.