This report from FT.com shows that the Government's Highway's Agency, part of the Dept of Transport, thought new roads schemes since 2002 would produce an extra 11,240 tonnes of carbon emissions. The actual increase turned out to be 21,870 tonnes - this is clearly a whapping inaccuracy not just an underestimate, given that its almost twice as much!
Conclusion: their method is rubbish and they dont know how to work out what carbon emissions will be. How can we have decent trust in any official estimates of carbon emissions that will result from major developments, local or national, eg road traffic due to expanding Heathrow or building a South Bristol ring road??