Vastly inflated traffic estimates have plagued all three tunnels - Clem7, Airport Link and Legacy Way - and the Go Between Bridge since opening, costing investors millions when two early operators collapsed.

But new figures reveal the shortfall between projected traffic modelling and the number of cars on the road is still growing, with one tunnel - Airport Link - hosting little more than a quarter of the estimated trips a day.

That led to calls for the network of toll roads to offer more incentives, such as bundling trips that include more than one tunnel or frequent user discounts, to attract more drivers.

Those calls, from multiple quarters, came as updated figures showed the gap continuing to widen between traffic forecasts and the reality of Brisbane's so-called "TransApex" of toll roads. In 2016, the Clem7 tunnel was meant to have 67,053 trips a day, according to Brisbane City Council's "conservative" pre-construction estimations. In reality, the average daily usage was 33,443 - less than half the expected patronage.

The Go Between Bridge between Hale Street and West End was projected to have 18,347 daily users by 2016, but only 13,034 motorists used it daily.

Even Legacy Way's conservative estimation of 31,000 daily trips in 2016 was a long way off, with only 21,044 motorists on average using the tunnel between Toowong and Bowen Hills.

The figure was starkest, however, with the Airport Link tunnel.

The project's original projections by traffic modellers Arup predicted a total of 221,087 vehicles would be driven through at least part of the Airport Link tunnel every day by 2016.

In reality, state government figures showed a little more than a quarter of the expected motorists - 57,500 - used the tunnel every day.

Both were subsequently bought by Transurban, giving the Melbourne-based company a monopoly of all Brisbane's toll roads.

RACQ principal transport economist Susan Furze said while the roads themselves were good facilities, the cost of the tolls ensured many "price-sensitive" drivers kept to the free alternative routes.

Ms Furze said that meant Transurban needed to explore incentives to get people underground.

"When Airport Link was free for three months, you had 81,000-odd daily trips and when the toll went on, it dropped to 50,000," she said.

"Yes, it's climbed up now to about 57,000, but it's well off that 81,000 figure when it was a free tunnel.

"That shows what putting a toll will do, and how price-sensitive motorists are."

Ms Furze said incentives such as "bundling" trips, where more than one toll road was used in a single trip, needed to be explored.

"These days, it's the same operator, so that would make sense," she said. "It's a very logical option."

Griffith University road and transport researcher Matthew Burke backed that approach.

"We thought we might start to see some of that as they became basically the monopoly operator in the Brisbane setting, but that's yet to appear," he said.

"And we're not seeing enticements to regular users, which some toll operators have done elsewhere in the world."

Former premier Campbell Newman, Lord Mayor Graham Quirk and Deputy Mayor Adrian Schrinner at the Legacy Way opening. Photo: Legacy Way

Lord Mayor Graham Quirk, who as the Brisbane City Council infrastructure chairman in the Campbell Newman administration oversaw much of the TransApex implementation, said bundling was worth considering.

"There's nothing to prevent those sorts of initiatives and incentives being created and that may well evolve in time," he said.

"...If there is an encouragement for the use of more than one facility, where there is a discount when two facilities are utilised, that would be an incentive.

"But that is fairly and squarely a decision for Transurban and they would be doing and they would be doing a far closer assessment of the numbers and the impacts of that than I've done.

"It's not within my domain to determine."

When asked for his thoughts, Transurban Queensland group general manager Wes Ballantine appeared open to the idea.

"Like any other business, we're always looking at new ways to improve our service offering for our customers," he said.

A model problem

At the heart of the public perception of failing Brisbane toll roads was the fact the tunnels had failed to perform to the levels predicted ahead of their construction.

Cr Quirk said the council knew private sector modelling of the city's first toll tunnel project, the Clem7 between Bowen Hills and Woolloongabba, was "ridiculous" from the start.

"With the Clem7, there was an enormous gap always between what council was saying and what the private sector was saying in terms of modelling," he said.

The council's projections had the Clem7 averaging 38,862 trips a day when it opened in 2010, a little more than 7000 above the actual 31,749 average.

Cr Quirk said the council did not make its projections public "because we wanted the private sector to bid and bid on its own forecast", a decision that ultimately led to private investors in RiverCity Motorway losing millions of dollars.

"Because the private sector had expected and bid on a 100,000 vehicle start-up, that was the number that was invariably used in the media and, of course, Clem7 in its very first year did about 32,000," he said.

"We were obviously much closer than what we thought were the ridiculous forecasts being put forward by the private sector at that time."

But even the council's own 2016 Clem7 projection was more than double the actual number of vehicles that used the tunnel daily last year.

Airport Link under construction in 2009. At $4.8 billion, it was the most expensive of Brisbane's toll tunnels to construct. Photo: Glenn Hunt

Traffic modelling was "not an exact science", Cr Quirk said, so it should not necessarily be taken as gospel.

"It's been my experience that traffic modelling makes a lot of assumptions, but at the end of the day, it is that - it is a model only," Cr Quirk said.

"What people actually do and the habits they adopt is a matter for them.

"What we can safely say, however, is that all of the bits of infrastructure in their entirety ... if you close these bits of infrastructure down, people would realise how important it was to have them in the network."

Dr Burke said the crux of the issue was the public-private partnership model that was pursued to get the tunnels built.

The PPP model meant competing firms worked on traffic projections and, invariably, the most optimistic model would come out on top, he said.

"The modelling of traffic numbers was done by the same firms that stood to gain design contracts of many tens, if not hundreds, of millions of dollars for these projects," Dr Burke said.

"That's something we need to consider putting some Chinese walls around in future, because it's not a good look."

But Cr Quirk said it was still good news for Brisbane ratepayers, except for those who invested in the tunnels.

"It wasn't good for private investors in the infrastructure, but from a ratepayer investment point of view, it meant that the contribution of ratepayers to build those facilities was much lower," he said.

Slow growth

While none of Brisbane's TransApex toll roads have matched expectation, there has been growth in all bar the Go Between Bridge, which has had daily averages in the 13,000s since 2011.

The Go Between Bridge has seen little growth since it opened in 2010. Photo: Glenn Hunt

Both the Clem7 and Airport Link have reached the levels they saw during their initial discounted periods.

"These are 50-year assets and not necessarily a short-term play," Dr Burke said.

"They've certainly offered congestion relief in the city at peak hours and greater accessibility for certain user groups to get to key destinations, including the airport."

In a written statement, Mr Ballantine said it would be a smooth ride for Brisbane going forward.

"Brisbane is now benefiting from the significant private investment in tunnel infrastructure over the past decade," he said.

"The improved inner-city connectivity that the tunnels provide contribute to the high level of liveability that our city currently enjoys.

"We are pleased to see traffic volumes growing steadily across the Airport Link, Clem7 and Legacy Way as motorists enjoy the benefits of their travel time savings and trip reliability."