But according to David Hensher, founding director of the Institute of Transport and Logistics Studies at the University of Sydney, no one in the world has yet studied when motorists might hit their "saturation point". That is, when motorists start to value the time they save driving on a toll road less because they are no longer willing to spend that much money driving. "I think we need to understand where's the pinch point where that might occur and at the moment we just don't know," Professor Hensher said. Professor Hensher's team will start to survey motorists in the next few weeks, and he views the residents of Sydney's west and north-west as ideal for his purposes. "We are simply saying, and people agree with us, this is a topic that's been ignored and Sydney is just a classic global example of where you can test this," he said.

"We have almost more metropolitan toll kilometres than any other city in the world," he said. "We've got a laboratory in Sydney to test this stuff." For James Fiander, the former chairman of the Hills Transport Working Group, the cost of driving on toll roads between his old home in Kellyville Ridge in Sydney's north-west to the city eventually led to him and his partner moving to inner-city Pyrmont. "It worked out to about a round number, about $500 a month," Mr Fiander said. "And that was just to get to work, before you do weekends, and that pushed us out of the Hills." We have almost more metropolitan toll kilometres than any other city in the world Mr Fiander's old commute would take him through one of the distance tolls on the M7 Motorway, the full $6.41 Hills M2, the $3.13 Lane Cove Tunnel, and the $4 Harbour Bridge toll southbound, for a daily total well over $20.

The toll burden will spread further south through western Sydney when WestConnex opens. Two existing motorways which do not have a toll – the M4 and the M5 East – will start to charge motorists when WestConnex projects that significantly expand their capacity are built. Mr Fiander agreed that there could come a point when motorists stopped driving on toll roads because of the cost, though it would depend on the motorist. "We were commuters to the city, so we were career-driven city folk. You probably wouldn't find a student going to Macquarie University going on the M2 every day, it would break them," he said. Professor Hensher, who has worked on the WestConnex project as a peer reviewer, said his study would look at the trade-offs between saving time on toll roads or using free roads that might be slower. He said that, depending on his findings, it could lead to predictions that fewer motorists would use toll roads.

"Once you adjust your value of travel time savings for the toll saturation effect it is expected there will be a reduction in the use of toll roads," said Professor Hensher, who should have results in a few months. A spokeswoman for WestConnex said the traffic modelling for the project did "consider the impact of multiple tolls and people having a different willingness to pay to use toll roads". "WestConnex will have a distance-based tolling system, similar to the M7, with a cap on the toll paid," she said.