This used to be a route for long vehicles but is no longer used, the long vehicle could not negotiate the roundabout and the strip was there to allow them over it instead of round. This route is no longer used and the strip serves no purpose anymore.
This was specifically for the extra long vehicles from Bruce Peebles works at East Pilton carrying electrical transformers to the docks for export. Some of them even went the whole way by road. Very. Slowly.
And a whole fascinating set on Flick here; https://www.flickr.com/photos/cagiva1994/5518389504