Scotland's infrastructure secretary has insisted that upgrading the A9 road is not incompatible with climate targets.
Michael Matheson said the £3bn project was vital to the economies of the areas it serves.
The government has been warned by its advisers that difficult choices will have to be made.
The Infrastructure Commission for Scotland said the government should make big spending decisions based on whether they help reduce emissions.
In a major report just published, it said public projects should look first at re-using existing structures.
The government has backed a plan to make the A9 dual carriageway all the way from Perth to Inverness.
Last year the Scottish parliament voted for a legal commitment to have all emissions offset by 2045.
An interim target will see emissions reduced by 75% between 1990 and 2030.
Mr Matheson, speaking on the BBC's Good Morning Scotland programme, denied that projects such as upgrading the A9 road, were incompatible with the net zero target.
"Even in looking to reduce our carbon output we will always have to have - in moving to a zero carbon economy - we'll always have to have good road infrastructure," he said."
The A9 is the largest infrastructure project ever in Scotland at some £3bn.
"It's critical to supporting the Highlands economy and the economies of Perthshire that are dependent on the A9."
Recommendations for the creation of what is called "an inclusive net zero carbon economy" from the Infrastructure Commission report include:
• A new charging system to replace fuel duty and road taxation
• The "future proofing" of existing roads in preference to building new capacity
• New incentives for homes, public buildings and commercial premises to use low-carbon heating technologies
• Members of the commission said "the nature, purpose and focus of infrastructure investment over the 30 year horizon is likely to change fundamentally".
Source: BBC