Six years ago I was inspired by a story about how Groningen had controlled its motor traffic by blocking routes through its centre, and suggested the same thing be done in Oxford. Richard Mann pointed out that the Groningen "ring road" was 5km long where Oxford's was 25km, however, and there didn't seem much enthusiasm even in CyclOx for the idea.
The steady increase in city centre congestion, and its unpleasantness for walking or cycling or indeed anyone, has kept this idea alive. Geography leaves the only ways to gain space as bus tunnels or cable cars; parking restrictions and congestion charges could help, but seem likely to bring only small improvements. And now there's a new organisation Oxfordshire Liveable Streets advocating the same idea, inspired by the same example. Their starting point was a Guardian story about Groningen, and at their launch event we spoke to one of the councillors who had pushed through the changes there in the 1970s.
Groningen has an outer ring (the N370-N7-N46) which at 20km approaches the size of Oxford's, but has a network of routes inside that providing connectivity. (Unexpectedly to me, Groningen sprawls considerably more than Oxford does, with a good chunk of the city outside that ring.) But there are plenty of examples from other cities with circulation plans involving disconnected sections within quite large rings: Ghent for example has a 13km ring, and Houten a 14km figure-eight. After Amsterdam finishes its latest changes (starting in 2018) it will have something like a 13km ring.
One problem in Oxford is that the ring road here is not actually designed as such, but has been cobbled together piecemeal. It consists of two major through routes, the A34 and A40, which don't even have a proper junction, augmented with the A4142 "Eastern Bypass"; and only the A34 is proper freeway, the other sections have multiple traffic lights and roundabouts on them. Congestion and delays can make it unattractive to use the ring road rather than cutting through the city centre, despite the problems there, and pushing more traffic onto the ring road is only going to make that worse. (Though this cuts both ways: if we can shift commuters into Oxford from cars to buses, that will improve things for through traffic, not just inside the city.) Groningen's 20km ring also carries through traffic, Houten is more isolated, and Ghent has an outer 40km ring that should take most bypass traffic.
The point of a ring-road or bypass should be to allow almost all traffic to be removed from the city centre, not just to provide an alternative route, as Mark Treasure explains in "Bypassing the Bypass". If the Oxford ring road is ever upgraded, that should definitely be part of a package including Dutch-style restraints on through routes. But can we wait on that?