I've been wondering that. There's one on the A52 (I think) not far from JN25 of the M1 where traffic turning off to the right goes through the centre of the roundabout so I wondered if it was a way of speeding up the flow of Derby bound traffic heading for the A38 which is where most traffic leaving the M1 southbound seen to go. About 20 years since we had anything to do with the motorway though.