More cars will pile onto Parramatta Road after the first stage of the WestConnex is built, despite the government's stated intention to beautify the much-maligned corridor.
By reimposing a toll on the M4 motorway between Concord and Parramatta, the government will drive traffic onto alternative free routes such as Parramatta Road and Victoria Road, official analysis shows.
This means buses travelling along – and trying to cross – the western stretch of Parramatta Road will also be slower, while surface air quality on some sections of road will suffer.
There is also more likely to be more trucks on suburban roads as they attempt to avoid the tolls.
These details emerge from the environmental impact statement for the first stage of the WestConnex motorway – two extra traffic lanes to be added to the existing M4, which are to be built by 2017.
After building the two extra lanes, the federal and state governments plan to reimpose a toll on the M4.
They will use the toll revenue to help fund the rest of the WestConnex, such as a tunnel further east under Parramatta Road and another M5 East tunnel.
The EIS predicts that travel times for some motorists using the widened M4 will fall by as much as 74 per cent compared to if the project was not build.
But free routes will slow. Traffic modelling for the project predicts that traffic numbers on Parramatta Road will increase from 3210 vehicles per hour to 3350 vehicles per hour in the morning peak.
The impact is expected to be more pronounced outside of peak hour, when motorists are more likely to try to avoid the new toll.
"The model predicts that the project would increase traffic during the day and the evening on Parramatta Road at Auburn and on Victoria Road at Rydalmere," the EIS says.
The number of vehicles using Parramatta Road, however, would be less than before tolls were dropped from the M4 before 2010.
"The benefits of WestConnex will be fully realised when the entire motorway is operating, because each section is designed to operate as part of the broader network, rather than in isolation," Roads Minister Duncan Gay said.
The document also details slower bus times. Journey times on the Metrobus M92, for instance, will increase up to 3.4 minutes one way as it travels on Parramatta Road.
Air quality studies show that while there would be an improvement along the M4, there would be an increase in emissions along Parramatta Road.
Labor's roads spokesman, Walt Secord, said: "Families will be getting more heavy vehicles and trucks as well as more pollution."
The widened section of the M4 will attract a toll of between $1.50 and $3.90 in 2013 dollars.
Greens transport spokeswoman Mehreen Faruqi said widening the M4 would shift a traffic bottleneck.
"Even if motorists have a faster run on the widened and tolled M4, they will still be sitting in a paid car park once they reach Parramatta Road," Dr Faruqi said.
The design and construction contract for the M4 widening is planned to be awarded later this year.