High volumes of traffic from Flemington Markets, a factory outlet and Olympic Park make the spot where Homebush Bay Drive connects with the M4 motorway a nightmare for motorists. Credit:Fiona Morris But according to Professor Hensher's latest study, the modelling that has underpinned all previous and current tolled motorways has never looked at the cumulative impact on motorists of driving on multiple tolled motorways. In his study, based on a survey of 600 respondents, Professor Hensher found that when the cost of using multiple motorways was added together, motorists were significantly less likely to want to pay to travel. "This is a very important finding, raising serious concerns about the estimates of VTTS [value of travel time savings] associated with all current studies used to forecast patronage demand for new tolled links when there are existing tolled links in place," says Professor Hensher's study, prepared with colleagues Chinh Ho and Wen Liu. "It may also have implications for the frequency of use of the existing tolled links."

"Many commuters in Sydney are already close to their toll budget threshold": David Hensher. Traffic forecasters have been notoriously bad at estimating the number of people who will use toll-roads, and have sometimes been out by as much as 100 per cent. But in the past in Sydney, the financial risk of overestimating the number of people who will use a toll road has fallen on private investors. That was the case with the Cross City Tunnel and the Lane Cove Tunnel. For the NorthConnex motorway, to be built between the M1 to Sydney's north and the M2 at Pennant Hills, the risk will fall on private operator Transurban, though risk has been offset by concessions on other routes.

All of the financial risk of the WestConnex motorway, meanwhile, is expected to fall on the taxpayer because the government intends to sell off sections of the project only after they are open to traffic. According to Professor Hensher's study, a typical industry estimate of the value motorists put on saving travel time is about $22 an hour. That is, motorists will pay to use a motorway provided the cost does not go beyond that rate - so, motorists will tend to be prepared to pay a $5.50 toll per person to save 15 minutes. But in his empirical study of Sydney motorists, drivers value their travel time at no greater than $12 an hour and, for new motorways, possibly closer to $6 per person per hour. That would mean a motorist who is already paying tolls in Sydney would have to save an hour of travel time before being prepared to pay another $6.

"The evidence suggests that many commuters in Sydney are already close to their toll budget threshold, which when reached will flatten the commuter's willingness to pay to save extra commuting time," the study says. Professor Hensher identified Sydney as an ideal "laboratory" for his study because of the city's unusually large number of toll-roads. Financial documents released by WestConnex show the Baird government has already agreed to increase tolls on the motorway much faster than the current rate of inflation - by at least 4 per cent per year. By the time the entire motorway is planned to be finished early next decade, motorists will have to pay as much as $11 a trip. Professor Hensher's study was funded by the university, and took place after his work for WestConnex had finished. According to a spokeswoman for the Sydney Motorway Corporation, the entity in charge of WestConnex, the motorway's traffic modelling "considers the impact of multiple tolls and different willingness to pay studies undertaken in major cities including Sydney, which demonstrate that, if people recognise value in good infrastructure, they will use it".

"WestConnex will save motorists a combined 100,000 hours each day through reduced traffic congestion," the spokeswoman said. Clarification: Professor Hensher's study concerned the motivation of commuters only and not business related or truck travel.