In 2001, Wiltshire County Council chose a scheme of an Eastern Westbury Bypass combined with Northern (ie on the Trowbridge side) Bypasses of Yarnbrook and West Ashton.  You can see all these routes here.

An Eastern Westbury Bypass would not actually work with a Northern Bypass around Yarnbrook, because the A350 traffic would still go through the bottleneck of the Yarnbrook cross-roads/mini-roundabout.

Wiltshire County Council's consultants at the time discreetly acknowledged this rather fundamental failing.

WCC's Far Western Route around Westbury would work well with the preferred Northern Yarnbrook Bypass; it is on the correct alignment and would provide a genuine bypass of the Yarnbrook A350 bottleneck.

In 2003, traffic relief for Yarnbrook and West Ashton, which have the worst traffic congestion on the A350 in Wiltshire, far worse than in Westbury, was illogically omitted from Wiltshire County Council's road scheme.

The Eastern Westbury Bypass, by itself, would actually have the effect of severely worsening the already worst congestion at Yarnbrook and West Ashton, plus at North Bradley, Southwick, Rode and Melksham, because it would push more heavy traffic, especially more HGVs, onto these other local areas.

Footnote for nerds only:  After already doubling-back west, from the proposed Eastern Bypass, to the West Wilts Trading Estate, via Wiltshire County Council's intended additional Glenmore Link road, as in the map, it would then be possible to carry on along the Hawkeridge B-Road to the north of Yarnbrook.   Consider the long loop extra journey distance involved.   But, local highway authority Wiltshire County Council is not proposing this as a strategy, nor is WCC identifying upgrading the Hawkeridge Road.   Without a Yarnbrook bypass, most on the A350 would continue to join the original Yarnbrook tail-back.