The lord mayor, Clover Moore, is so incensed with it she has called a Town Hall meeting for Monday night.

The state government says it will crush congestion through the city, and the Prime Minister says it will make motorists so happy they'll be singing in our cars.

But what is the WestConnex motorway? Is it the right thing to be building? And why do so many in the transport profession seem so cool on it?

What is WestConnex?

"If you think of it in terms of the Snowy Mountains project, it's double the Snowy Mountains project," the Roads Minister, Duncan Gay, said last weekend.

Gay's been a champion of the project since it was proposed by Infrastructure NSW in 2012. The plan for the road has changed a few times since then, but Gay has stuck thick.

"Hard to believe, in today's terms, the Snowy Mountains would be about $8 billion," Gay said. "This is about a $15 billion project to fix the roads of Sydney."

This "fix" comes in a few parts.

There's a widened M4 from Parramatta to Homebush; a new M4 East tunnel from Homebush to the City West Link at Rozelle; a new M5 East Tunnel from Beverly Hills to Tempe; and then a new tunnel from Rozelle to Tempe to link it all up.

What are the benefits?

The 33-kilometre WestConnex will make it quicker and easier to drive between parts of Sydney.

But while the motorway will cut through the inner suburbs of the city, the main benefit would go to people from further out.

The motorway would double the road capacity for someone driving from Campbelltown to the airport. And it would remove 52 sets of traffic lights for someone driving from west of Parramatta to the city – provided they used the tollroad.

"If you look at the population growth, the job growth, from Penrith in through Parramatta, through to the city, down south to the west, there's lots of people that are going to be moving and lots of jobs people will need to go to," the chief executive of the project, Dennis Cliche, told an audience at Enmore Theatre this month.

The other big benefit, according to the WestConnex Delivery Authority, is that local roads will be quieter once hundreds of thousands of cars shift to the tunnel. This may enable better public transport to be added to surface roads like Parramatta Road, though there is little detail on how this would work.

What are the problems?

The two main arguments against WestConnex are that it will encourage more people to drive more often, and that it will encourage those people to drive to the wrong areas.

"The WestConnex project in Sydney adds significant extra road infrastructure to the city and therefore makes travelling by car more attractive," says Michiel Bliemer, from the Institute of Transport and Logistics Studies at the University of Sydney Business School.

"This encourages travellers to take the car and as such increases the total number of cars on the entire road network."

According to Bliemer, this extra traffic will worsen congestion throughout the city, even if the motorway helped on some routes.

Indeed, this effect could be heightened by imposing tolls on roads that do not have them. The government's own figures for the first stage of WestConnex show that re-imposing a toll on the current M4 will push thousands of cars and trucks onto alternative routes – Parramatta Road, the M2 and Victoria Road.

Some of the criticism of WestConnex is based on the premise that governments should not be building more roads. But there are also road builders who doubt the WestConnex roads go to the right places.

"You should spend money on roads, no question about that," says a former director of the Roads and Traffic Authority, Ken Dobinson, who opposes most elements of the WestConnex project. "But spend it on the right things for the right reasons – that's common sense to me."

Dobinson's opposition to WestConnex reflects a common stance of city planners that "good" motorways run around the circumference of congested areas. Bad motorways radiate into and out of them - such as the proposed new M4 East and M5 East tunnels.

"For access into Sydney's CBD and other dense centres, road-building solutions - whether road widenings or new roads  - are destined to fail, because these dense centres, by their very nature, simply do not have room for everyone's car," was how The Sydney Morning Herald's transport inquiry of 2009 and 2010 framed the issue.

"Any major new road investments in Sydney, beyond maintenance, should only be in the form of circumferential rather than radial connections, enabling travel between lower density areas," said that inquiry, chaired by former NSW roads and rail chief Ron Christie.

How much will it cost and who will pay?

We will pay, through tolls and taxes.

The latest figure put on the total WestConnex project is $15 billion, a 50 per cent increase from the $10 billion first quoted when the project was proposed by Infrastructure NSW.

At the moment governments have come up with about one-third of that total (a $1.8 billion from the state government, $1.5 billion from the federal government, and a $2 billion "concessional loan" from Canberra).

They say they will raise the rest by progressively selling off the motorway sections to private bidders.

But there's some scepticism WestConnex will be able to raise that extra $10 billion from the private sector.

The Herald has already reported on leaked Macquarie Capital analysis showing plans to sell off the rights to toll other motorways to pay for WestConnex.

And recent motorway transactions in Sydney suggest the government may struggle to make up the numbers. The Cross City Tunnel, with about one-tenth of the hoped for 300,000 to 400,000 daily users of WestConnex, sold last year for $475 million.

Similarly the Lane Cove Tunnel, used by about 80,000 people a day, sold in 2010 for about $700 million in today's money. If these transactions represent the going rate, WestConnex would be struggling to raise $5 billion from the private sector.

And then there are the tolls. Someone who currently drives from Penrith to the airport for free will, using WestConnex, pay about $9 each way.

Similarly someone who currently drives from Campbelltown to the city for the $4.41 M5 toll will, when WestConnex is built, be paying about $10 a trip.

What do the parties say about WestConnex?

The federal and state governments say it is the most important thing around. "Every day motorists will be rejoicing," Prime Minister Tony Abbott said last week.

"They will be rejoicing, they will be singing in their cars, frankly, because their cars will be moving," he said.

Labor, however, says WestConnex is a brand they are not buying into.

Labor would build the M4 and M5 east sections of WestConnex, but will not commit to a route.

Labor would also scrap the tunnel between them that the WestConnex Delivery Authority says is the most important element of the project.

The Greens would dispense with the entire motorway.

Is the project popular?

The answer, it seems, depends on who you talk to.

"Absolutely," says Charles Casuscelli, the Liberal MP for Strathfield and a former project director of the NSW Transport Management Centre.

"I would say 98.5 per cent of people see it as a wonderful thing and the sooner we get it the better its going to be for everybody."

But Casuscelli must be doorknocking different people than the challenger for his seat, Labor's Jodi McKay.

"I haven't had any positive comments about it,"  McKay says. "I cannot remember one and I've doorknocked most of the electorate. In my electorate when people talk about WestConnex they talk about … overdevelopment on Parramatta Road."

There seems a greater consensus in the Drummoyne area, where the incumbent John Sidoti is up against Labor's Jason Khoury.

"From my electorate's point of view the overwhelming majority are supportive," says Sidoti, "because of the benefits it will bring in returning local streets back to the local community."

Khoury says he is personally in favour of the M4 East section of the motorway, as are many of the residents he talks to.

"But the big thing is the lack of information about where the exhaust stacks are going to be," he says.

Further east, and the issue emerges as one of the main fault lines in the inner west seat of Newtown, to be fought between Labor's Penny Sharpe and the Greens' Jenny Leong.

"The impact of the St Peters interchange that is not popular at all in our area,"  Sharpe says. "It will simply flood our local roads and people in our area know that, which is why it is so unpopular."

For all that, the majority of voters might have more pressing daily matters to think of.

According to a poll by ReachTEL for Channel 7, just 22 per cent of people surveyed knew WestConnex was a road project.

The Member for Parramatta, Geoff Lee, who says the project will be great for his area, also says that there is not much discussion about it.

"I haven't had a lot of feedback from people in the street about it," Dr Lee says. "I don't even think anyone's sent me an email about it."

Are there alternatives?

Sydney has no shortage of alternative transport plans – if only you could drive on them.

The Herald inquiry proposed a major expansion of the public transport system in lieu of any new motorway building.

It proposed new rail lines between Parramatta and Epping, another harbour rail crossing, and eventually another metro-style rail between Westmead, Parramatta and the CBD.

But plans also exist for much smaller motorway expansions, aiming to make it easier for trucks and commercial vehicles in particular to criss-cross the city.

Indeed Infrastructure Australia, proposed an idea along these lines in 2012. It suggested a narrower tunnel to run from the end of the M4 at Concord to the port, limited to commercial vehicles and buses, all of which would pay a heavy toll.

This is an idea that has also attracted Garry Glazebrook, one of the authors of the Herald inquiry. Glazebrook says the more narrow tunnel could be built for about a third of the price of WestConnex, and be opened up to use by general car traffic on weekends

"You would scale it right back and run it from the end of the M4 through to the airport and port area," Glazebrook he says. "It would go nowhere near the city. You don't want to encourage another car going to the city."

As well, Glazebrook says vast interchange car parks could be built at major train stations on both the Western Line and East Hills line so commuters did not have to drive all the way into the city. The metro line from Parramatta to the CBD should also be built.

"You've got to soak off the traffic before it gets into eastern Sydney," he says. "But the bit you can't soak off is the commercial vehicles."

Dobinson's group, meanwhile, the 10,000 Friends of Greater Sydney, would scrap the component parts of WestConnex but build a new western bypass of the CBD around King Georges Road.

"This extends from the M1 Freeway to Beverly Hills connecting the M2, M4 and M5, provides a western bypass of the CBD, offsetting growth of traffic feeding towards the CBD," says that group's 'Sydney Manifesto', released last week.

Should you believe what the government says about it?

Some of the initial claims for the motorway, the glossy brochure ones, appear over-cooked.

The government has claimed that it expected the travel time by bus between Burwood and the City on Parramatta Road to "almost halve". But freedom of information requests show that assumes buses will somehow double in speed on sections of Parramatta Road where there is no upgrade planned.

And official scrutineers are also sceptical of the proposed benefits. The state's auditor-general reckons the benefits are likely to be over-stated in the project's business case, which has not been released publicly.

Infrastructure Australia, the federal government advisory body, also raised questions about the business case, though both organisations say the motorway would do more good than harm.

Tim Williams, the chief executive of the Committee for Sydney, says the city needs a better and more informed civic dialogue, not just about WestConnex but about the management of congestion in the city.

Williams says residents would welcome the project if it could be shown it would help revitalise Parramatta Road, provide better mass transport and urban design, lower congestion, cement Parramatta as the city's second business district and enhance the productivity of Western Sydney.

"If that is what WestConnex enables its advocates have a strong case and need only do one thing: share the evidence and the appraisal on which the project is based with the community so we can have a big city dialogue about our future based on facts – and not ideology, whether its pro or anti such grand projects," Dr Williams says.