Agree with Morningsider and SRD.
People scaled towns and cities is the way forward. Doesn't this all become a bit moot eventually anyway as petrol costs increase and people turn to active and public transport, reserving the car for journeys of a long distance.
If this is likely to be the direction of travel, then the eventual need to redo infrastructure in towns and cities and how they link up, will make the cost of the Forth bridge pale by comparison. The remedial work would cost more than placing the infrastructure there in the first place.