Reply #65: I've seen that implemented.

[Igel]

The library at university my wife's currently at is actually open something like Sunday noon to Friday midnight, and then Saturday daytime. The doors are unlocked from something like 8 am to 10 pm, and a card must be swiped for entrance--or ID presented and authorization granted by a security officer. During the other hours circulation services are shut, but the library can be accessed by swiping your university ID.

I think it's risky, but haven't checked what security is on patrol at the time. Perhaps they have somebody there making sure that materials don't walk out the door and students don't let outsiders in.