Case Western Reserve University in Cleveland makes its academic Wi-Fi network open to the public: It's an ambitious project that allows the public to take advantage of an expensive, but bursty and abundant service. The university has over 1,200 access points, and unless it's a unique case, there must be businesses, apartments, and houses sprawled all around and on top of it that can take advantage, as well as visitors to the campus. The project is labeled OneCleveland.
Via email, I asked Lev S. Gonick, the university's chief information office, how the network separated authenticated traffic and public traffic. The 1230+ APs are segmented into 6 VLANs that ride external to the Enterprise (OneCleveland) infrastructure. Users (guests) are assigned an IP range outside the enterprise space and can VPN to any network, including those of course in University Circle with proper authentication. Otherwise it's [using] shared bandwidth on the VLAN.
The university is also using packet shaping to control traffic flows. Dr. Gonick said that the group would put up network diagrams and other information. Case could become a case study for how universities and communities can work together, and might extend on some of the public/private partnerships for municipal Wi-Fi currently in the works.
The key, of course, is keeping private network traffic separate. As more hardware and software emerges that makes this simpler, or just a basic part of running the network, extending public access becomes a no-brainer: you give the people what's not in use, which is almost always going to be plenty.