The new road proposed through the Ashton Park development IS the diversion route or link road that is central to reducing traffic through the Yarnbrook roundabout.
The point of it is to allow traffic from or going to the West Wilts Trading Estate and/or leaving / arriving via Southwick / North Bradley (actually a heavily-used lorry route) to go via the A350 to/from Melksham without using the Yarnbrook roundabout. It is a stand-alone scheme and is justified without a Westbury Bypass dealing with traffic going / coming further south on the A350.
But in fact as currently planned without a strategic bypass for the other areas, outlined in my previous post, the Yarnbrook pinch point is simply moved some 150 metres North to the area of the Yarnbrook viaduct.
Traffic coming from Westbury via the A350 will meet traffic from Trowbridge, Southwick and North Bradley, together with vehicles from the West Wilts Trading Estate via Hawkeridge Rd and Phillips Way.
The simple, if costly solution to this problem would of course be a road from the A36 at Standerwick linking the West Wilts Trading Estate
bypassing the villages and joining Phillips Way at Yarnbrook, a route favoured by at least one Conservative Wiltshire Councilor.