Motorway
M74 (extension
Actual cost = £692,000,000
Length (km) = 8
Cost/km = £86,500k
A-road (dual)
Aberdeen western peripheral route (estimated)
Actual cost = £395,000,000
Length (km) = 29
Cost/km = £13,621k
A-road (single)
Average based on costs of A737, A75 and A82 bypasses;
Actual cost = £58,000,000
Length (km) = 8.57
Cost/km = £6800k
I know nothing about building roads, but assume if you had to move a lot of earth to build cuttings / embankments or had bridges to build, then the cost goes up exponentially.
The A82 Crianlarich bypass is 770m of single carriageway and comes in about £5,700k per km.
Borders railway (upper estimate) comes in at £6,020k per km.
Edinburgh tram the last time I calculated it was coming out at the eyewatering £60,625k / km (or put another way, it costs £1 million to built 16.5 metres of it)