[Updated 2020] Back in 2012 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.
But similar circulation plans have since been implemented in places such as Ghent (in 2017) and proposed (in 2020) for cities such as Birmingham and Auckland. And the Connecting Oxford proposals from the Oxford and Oxfordshire councils come close to implementing a full circulation plan (needing only a couple more bus gates).
- Groningen - 5km inner, 22km outer
- Oxford - 5.5km (using High St), 9km (Donnington Bridge), 25km (ring road)
- Leuven - 8km ring
- Houten - 9km (larger loop), 14km (full Rondweg figure-8)
- Birmingham - 10.5km (A4540 Middleway)
- Ghent - 12.5km inner, 40km outer
- Amsterdam - 12.5km (S100 inner), 35km (A10 outer)
- Utrecht - 16.5km inner, 28km motorway
Groningen's inner ring may be only 5km around, but Ghent has implemented a circulation plan with a 13km ring, and Houten was built inside a 14km figure-eight (with one cut-through for motor traffic; the larger loop is 9km around). After Amsterdam finishes its latest changes (starting in 2018) it will have something like a 13km ring. And Utrecht has a policy of trying to push private motor traffic to its outer (28km) motorway ring.
One problem in Oxford is that its ring road 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.) In contrast, Houten is more isolated and Ghent has an outer 40km outer 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 a complete circulation plan. But can we wait on that?