Speaking at the Transport for Tomorrow conference in Edinburgh last week, the Executive's head of transport group, John Ewing, said one of the challenges in building the new crossing was to ensure construction would not create additional congestion in the network.

"There is no easy solution to that. One of the things we are having to wrestle with is, do you produce any kind of increased capacity at all, or do you provide something equivalent to what we have already got, given there's a continual growth in transport? That's one of the difficult choices for the coming administration.

"We are not looking at the Forth Crossing in isolation - we're not just looking at how we build a new bridge, or a new tunnel, or a new causeway. We are looking at it as past of a broader strategy for the Forth estuary as a whole."