Scotland's road network must not be expanded further, according to the Scottish Green Party's campaign director, Mark Ruskell, who is standing to regain his seat as Mid-Scotland and Fife MSP.

Mr Ruskell wants to oppose the M74 extension through Glasgow, the Aberdeen ring road and a possible new road opening up development potential near his Dunblane base.

"If there's a safety case for a particular road, that's fine," he said. "What we're against is more and more trunk road capacity as a way to build our way out of congestion."

But would the Greens not also have argued against road-building in the past that is now essential to keep Scotland moving, such as the Edinburgh ring road? "We have a road network that is in place and serving the Scottish economy quite well," he replied.

"The problems is that in areas like Perth, Stirling and Glasgow there's a call for additional trunk road building. We're saying you can't continue building more roads if you're serious about tackling climate change."

Mark Ruskell, 34 and one of the seven Green MSPs from the last parliament, is very serious about tackling climate change. His party wants you to take fewer flights, reduce your consumption and carbon footprint, travel by car less, cut down on food air-freighted across continents, and halt the march of the supermarkets.

But don't accuse the party of being against the things people have come to value. "I don't know where that idea comes from. I drive a car, I use it at weekends. It's not about saying to people don't drive'. It's about saying let's put in an alternative to help you get to work'.

"The other parties are trying to introduce more taxation, putting penalties on people, with more taxes on aviation and on fuel, but the money raised isn't giving people an alternative."

The Green alternative is a carbon credit card, with chip and pin, used when you buy petrol, pay a gas bill or purchase a flight. If you use up your personal quota, you would have to pay the government for more. But if you don't, the government will reward you.

"If you want to use your carbon credits for a flight, that's great, and if you want to fill up your car and drive round Scotland, that's great," said Mr Ruskell. "It's up to you."

Most things about the Greens are alternative, including their leadership structure. This interview could have been with joint leaders Robin Harper or Shiona Baird, but the way the party operates, it could as easily have been with their five Holyrood colleagues.

They want to be different in power as well, influencing government in a loose alliance that would vote through the budget and support the major party leadership on crucial votes, but would otherwise force the minority administration to make the case for each law.

"We haven't ruled out coalition," said Mr Ruskell. "We'd have to consider the model that delivers the most Green demands. We would bring a Green backbone to government."

Shiona Baird is heading a group assessing other parties' manifestos, and preparing negotiating tactics.

Greens would only talk to those willing to rule out new nuclear plants, and would then argue for a £100m fund to help communities develop green transport schemes, reduce local business pollution and improving quality of life, rather than "piddling little pilot projects with small amounts of money".

Amid the council taxation debate, they want to see a homes and business tax on land value rather than property price. Allied to radical changes in planning law, the idea is to provide incentives to bring unused shops and brownfield sites into use.

For Greens, this is a precarious election, with polls leaving it unclear if they could push on to 10 seats or lose out to the main parties, which have pushed climate change up their manifesto concerns.

For Mr Ruskell the glass is half full. "This is a real opportunity. This is the first election since the European election in 1989 when the environment is very important. The challenge for us is to get over the fact that the other parties are very good on rhetoric, but the reality is very different.

"Voters can see the parties are calling for massive road building programmes, and can see they are not serious about the environment."