The islands are part of "traffic calming measures" that may eventually encompass the whole A4 Kennet Longbarrow/Silbury Hill/Beckhampton/Avebury area.
The Beckhampton roundabout is a 'fast' roundabout, as most of us on bikes know, but there have been a number of minor car collisions there. When someone in a BMW lost it, and demolished a bus shelter, used by schoolchildren, the locals started a campaign for a speed limit.
These islands are the result.