Campaigners against the £500m extension of the Glasgow M74 extension have been backed in calls for the scheme to be scrapped and money diverted to health.
Protesters donned gas masks outside Transport Scotland
A letter, signed by more than 20 health experts and local GPs, condemning the initiative was delivered to ministers.
The letter calls on the Scottish Executive to commit to transport plans that would improve rather than harm the health of the city.
The executive said the letter ignored the health benefits of the extension.
The M74 northern extension will create a six-lane elevated motorway.
A letter of objection was handed into the Glasgow headquarters of the executive's transport agency Transport Scotland.
A group of GPs and members of Friends of the Earth Scotland staged a peaceful protest outside the office.
Dr Peter Cawston, a GP at Drumchapel Health Centre, has signed the letter.
He said: "Some of the areas through which this motorway is to be built already have among the worst health in Western Europe.
The M74 completion will cover five miles across south Glasgow
"We desperately need a healthier environment for Glasgow's citizens.
"It makes no sense to spend more than the cost of the Scottish Parliament on five miles of motorway.
"Let's start by diverting this huge investment into public transport, cycle paths, traffic free zones and safe roads."
An inquiry into the scheme ran from December 2003 to March 2004.
On 24 March, 2005 the executive announced it had overturned the independent reporters' decision to recommend refusal for the M74 motorway.
Along with community representatives, Friends of the Earth is taking the executive to court over the decision.
A court date has been set for 27 June at the Court of Session in Edinburgh.
Friends of the Earth Scotland's Chief Executive Duncan McLaren said: "The M74 would be bad news for the health of many in Glasgow and bad news too for the environment.
"We welcome this intervention by some of the city's leading health experts and we hope that ministers will sit up and listen."
A spokesman for the executive said the letter ignored the health benefits of reduced congestion and reduced air pollution in Glasgow city centre as a result of the extension.
The new motorway was not expected to have "a detectable effect" on the health of local residents, he added.
"The M74 scheme will also bring a major boost for the area opening up the way for much needed regeneration with studies suggesting that more than 20,000 jobs could be attracted to the area," said the spokesman.
"This devolved government is committed to protecting the environment and improving Scotland's health by minimising emissions.
"That commitment is signalled by our spending 70% of our transport budget on public transport. In addition, we are providing significant funding to local authorities for safer routes to schools, cycling and walking projects."