While it is technically wrong to ease off/brake for other vehicles to join a motorway or indeed any dual carriageway it does put you in control, you make the decision as they might not have bothered to look, might not have seen you, you might be in their blind spot or they may well be in an unmentionable brand of car that does actually own the road, after 18 years in industry of 40000 miles a year with only two rtcs, one my fault and one not my fault this technique worked for me, but I agree that its not what the great and good highway code says, just don't get me started on illegal reg plates dave, see you for dtm after my week at le mans.