Some Sydney motorists driving further west than Barry O'Farrell's WestConnex will face worse traffic when the motorway is built, the government's own figures show.
The first elements of WestConnex to be built will be a widened M4 between Homebush and Concord and a tunnel under Parramatta Road between Concord and Ashfield.
But even on the government's own estimates, which show drastically reduced travel times for motorists using the $11.5 billion WestConnex and on most nearby roads, not everyone will be a winner.
Traffic modelling published in WestConnex's ''Strategic Environmental Review'' last week shows travel times could increase for motorists using alternate routes to the new tollway.
The modelling shows travel times on the Great Western Highway, which runs parallel to the M4 and which will not be tolled, will be 5 per cent greater between the Cumberland Highway at Wentworthville and Concord Road compared to a scenario in which WestConnex was not built.
"On some routes, such as the Great Western Highway (Cumberland Highway to the M4 exit at Concord) travel times may actually go up due to additional congestion on these arterial routes," the document says.
A spokesman for the Sydney Motorways Project Office confirmed travel times on this road could increase "marginally" as more drivers tried to use free routes.
To help pay for the project, the government will re-impose tolls on the M4 between Parramatta and Concord.
To trumpet the benefits, the O'Farrell government has been touting impressive travel time savings.
"We'll take about 40 minutes off a trip to the airport from Parramatta," Roads Minister Duncan Gay said.
Outside peak hour, it is in fact possible to make that trip in less than 40 minutes.
But that prediction of a 40-minute saving between Parramatta and the airport is based on a projection of peak-hour traffic in 2031, almost 20 years into the future.
The government also says WestConnex will halve the driving time between Parramatta and the CBD, from 50 minutes to 25 minutes.
Again, the prediction is based on a peak hour drive in 2031.
Releasing segments of the business case underpinning the WestConnex last week - the full business case will not be released on the grounds it is "commercial in confidence" - Premier Barry O'Farrell defended not releasing estimates for how many cars will use the new tunnels.
"I think this is a project that's necessary," Mr O'Farrell said. "No one's suggesting it's not necessary."
Nevertheless, an umbrella group of environmentalists and public transport advocates called NoW [No WestConnex] Public Transport held its first general meeting over the weekend and is planning a public meeting on October 10 in Leichhardt Town Hall.
Labor's roads spokesman Ryan Park criticised the lack of detail released by the O'Farrell government.
"It is very hard to see what assumptions underpin the claims within the document," Mr Park said.
A spokesman for the Motorways Office said: "The traffic modelling … indicates the motorway will ease congestion, reduce travel times and improve reliability on many key east-west and north-south routes."
READ MORE: Work to start on WestConnex