Environmental campaigners have taken the Scottish Executive to court hoping to halt Glasgow's M74 extension.
Green MSP Patrick Harvie is supporting FoE's challenge
Ministers have said the link is vital to the area and pressed ahead with the £500m plan despite the recommendation of a public inquiry reporter.
It would result in a six-lane elevated motorway across the south of Glasgow.
Lord Nimmo Smith, Lord Philip and Lord Clarke heard submissions on behalf of Friends of the Earth Scotland and Joint Action Against the M74 (JAM74).
An inquiry into the scheme ran from December 2003 to March 2004.
On 24 March, 2005, the Scottish Executive announced it had overturned the independent reporter's decision to recommend refusal of the proposal.
Friends of the Earth Scotland has argued the five-mile stretch of road would bring more traffic and pollution.
At the time of lodging the appeal, legal representatives stated their grounds as "failure by ministers to give adequate reasons where they disagreed with the reporter's conclusions" and "introduction of new evidence by ministers to support their rejection of the reporter's conclusions".
Friends of the Earth Scotland launched its legal challenge at the Court of Session on Tuesday because it does not believe the executive has fully justified going against the inquiry recommendation.
The proposed motorway extension has caused controversy over the last three decades.
The current M74, the main link between the west of Scotland and England, comes to an abrupt end in Cambuslang, five miles east of Glasgow.
The new link would extend the motorway to connect it to the M8 near the Kingston Bridge over the River Clyde.
Speaking on BBC Radio Scotland's Good Morning programme, FoE Scotland director Duncan McLaren claimed the extension would waste nearly £1bn of public money.
He said: "This is a motorway which would add to congestion, add to pollution and add to the economic injustice faced by some of Glasgow's poorest communities.
Protesters are opposed to the road being built
"The building of roads has been shown consistently, particularly in urban areas, to add to traffic levels.
"All these things do is move the congestion round the network. The road is forecast to add significantly to traffic, carbon dioxide and other pollutants."
Mr McLaren said FoE Scotland had a strong legal opinion that on four of five different counts ministers did things outside of the due process.
The cost to FoE Scotland of bringing the case would amount to tens of thousands of pounds, he added.
Transport Scotland, the national transport agency which implements Scottish Executive policy, said the new motorway was a "vital" investment that would create jobs.
A spokesman said: "We believe that completing the missing link for the M74 will have benefits not only for Glasgow businesses and communities but also the wider west of Scotland."
The hearing is set to resume at 1000 BST on Wednesday.