The Plain received its name after St Clement's Church was demolished from here in 1828 and a new church was built near the other end of St Clement's, at the southern end of Marston Road. This left a wide open space where the roundabout is now located.
For a long time this roundabout was notorious for being dangerous and difficult to negotiate, especially on a bike. It is one of the few large roundabouts in central Oxford (a city known for its many cyclists), and therefore the only roundabout that cyclists would often have to negotiate. The layout of the exits in particular made it difficult to choose the correct lane and signal unambiguously. In early 2006, the Plain was redeveloped to improve this situation and make the junction safer.
The 2006 work introduced pinch points between the motor and cycle traffic, especially on the Iffley Road entry to the roundabout.
The 2007 redevelopment work (finished in late September) was mostly resurfacing, but there was some movement of the traffic islands. There was supposed to be a "2-speed" cycle filter going over Magdalen Bridge: either go in the "slow cyclists" lane and get cut up going over the bridge or ignore this and come out very wide and block motor traffic. However, as of early October 2007, this has yet to be painted on.
It does not address the inherent problem signalling for entry to the Iffley Road. The Cowley and Iffley Road mouths are so close together that timely signalling is not possible. While there are individual marked lanes for the two roads, so many people are confused / in the wrong one, that simply observing the lane someone is in is insufficient to identify their destination with certainty.