"Pulling the plug" - for however long - on the Melksham (and Westbury, and Beanacre bypasses) does not reduce the amount of freight travelling south-north through Wiltshire. 

It helps make the case for transfer to rail - where that's appropriate, and furthermore it's far less intrusive, less locally controversial, far environmentally better, and far cheaper to put in the infrastructure for rail.   If it can be done BEFORE our population explodes as planned, and with a "metro" frequency rail service in place ahead of time, then our economy and good housing stock will grow with people ready, willing AND ABLE to use rail where appropriate - and that in turn lessens the load and congestion on roads in the area from what it would otherwise be.