Being timed out every 15 minutes is really inconvenient, especially if your front desk is manned primarily by volunteers. On slow days, they will have to be logged back in constantly. You should allow users to decide what time-frame they'd like for that time-out. I understand security reasons, but there's no security if I need to keep a username/password written down at my front desk for every volunteer who is being logged out repeatedly during the day.
Customer references | Front end staff, all staff |