Original sentence: This required a government rethink regarding marker posts and by early 2007, after experiments on parts of the  M25 and  M6, a driver location signs erection program was under way in England (but at the time of writing, not in Scotland, Wales or Northern Ireland),, at about 500 metre intervals on many motorways
Edited sentence: This required a government rethink regarding marker posts and by early 2007, after experiments on parts of the  M25 and  M6, a driver location signs erection program was under way in England (but at the time of writing, not in Scotland, Wales or Northern Ireland), at about 500 metre intervals on many motorways