Some interesting maps released by the DfT yesterday:

Page 10 of the link below shows the TransWilts corridor as one of the top 50 commuting trips into and out of Local Authority Districts in the South West.
http://www.dft.gov.uk/pgr/regional/strategy/databook/annex3.pdf

Page 12 of the link below shows West Wiltshire towards the top in terms of forecast growth in population 2006 to 2026 in the South West (Swindon is right at the very top.)
http://www.dft.gov.uk/pgr/regional/strategy/databook/annex6.pdf

Page 12 of the link below shows the TransWilts corridor right at the very top in terms of forecast growth in numbers of households 2006 to 2026 in the South West.
http://www.dft.gov.uk/pgr/regional/strategy/databook/annex7.pdf

Page 12 of the link below shows the TransWilts corridor right at the very top in terms of forecast growth in car ownership 2006 to 2026 in the South West.
http://www.dft.gov.uk/pgr/regional/strategy/databook/annex9.pdf

Page 21 of the link below shows West Wiltshire towards the top in terms of forecast growth in trips (productions) 2006 to 2026 in the South West (Swindon is right at the very top.) Page 22 of the same link shows the TransWilts corridor right at the very top in terms of forecast growth in trips (attractions) 2006 to 2026 in the South West.
http://www.dft.gov.uk/pgr/regional/strategy/databook/annex10.pdf

There are also high forecast future delays on relevant sections of the A350 (Page 3 of the link below.)
http://www.dft.gov.uk/pgr/regional/strategy/databook/annex14.pdf