The Independent reported at the beginning of this month "Tory strategists have identified a "golden ribbon" of marginals along three busy motorway corridors: the M62, M6 and M5." The M62 links together the northern conurbations of Liverpool, Manchester, Leeds and Hull. The M5 runs from Birmingham to Exeter - close to the SW of the Birmingham conurbation - and the cities (smaller towns than the post industrial revolution population areas - but ancient cities) of Worcester, Gloucester & Taunton - and the Bristol conurbation.
The M6 actually starts in South Leicestershire (in the seat I stood in twice - which has always enjoyed a 'healthy' Conservative majority) through (after 1/2 a mile) the super marginal of Rugby - and then through the conurbations surrounding Coventry; Birmingham; Stoke on Trent - between Manchester & Liverpool - Preston & Carlisle. Blackpool is not too far away.
I would also add that the M1 passes by key marginals in northern London; Luton; Milton Keynes; Northampton - and through the super-marginal area around Nottingham and Derby. The M1 continues to Leeds.
The M4 runs from West London - through another area of supermarginals - to Slough; Reading; Swindon; Bristol; Cardiff and Swansea.
To get interactive maps of motorways in England (and the latest traffic news!) visit the Highways Agency website.