Traffic on the East West Link in the morning peak is expected to have slowed to 20-30 km/h by 2031 as worsening congestion pushes the road close to capacity just 12 years after it is due to open.

The East West Link is forecast to carry 80,000 vehicles a day on opening in 2019, increasing to between 100,000 and 120,000 a day by 2031, modelling shows.

Speaking at day four of the planning panel hearing on the major project, traffic expert Stephen Pelosi said the toll road would reach morning peak capacity by 2031. The most effective way to ease congestion would be to apply a higher peak-hour toll to reduce the number of discretionary trips.

''If it's reaching 120,000 we're at a position where we're reaching capacity,'' Mr Pelosi said. ''Unless you intervene in some manner and manage the toll rate to influence demand, you get a situation where you're near capacity.''

Mr Pelosi, a transport planner and traffic engineer who has worked on several major road projects in Victoria, was giving an expert witness report on behalf of the Linking Melbourne Authority, the state authority in charge of delivering the $8 billion link.

Cross-examined, he said it would be reasonable to conclude that average speeds would have slowed to about 30 km/h by 2031 under what VicRoads rates a ''level of service of D''.

This means average travel speeds begin to decline, freedom to manoeuvre between lanes is restricted and minor disruptions cause queueing.

But he said these slower speeds would happen only in the peak and that the road could be expected to provide free-flowing traffic about 70 per cent of the time. ''Peak hour represents 15 per cent of trips … a lot of travel is made outside the peak,'' Mr Pelosi said.

Labor's shadow planning minister, Brian Tee, said the prediction that the link would become congested within 12 years proved it was a poor investment for the state.

''This tunnel is not a long-term solution and is not value for money,'' Mr Tee said. ''For generations Victorians will be burdened by an $8 billion debt for a tunnel that will have long passed its use-by date.''

But Transport Minister Terry Mulder said the toll road was essential to stop Melbourne traffic grinding to a halt in future years.

''It seems that the commentary on this project is that either no one will use it or too many vehicles will use it,'' he said.

''The reality is that more than 100,000 vehicles will use the East West Link each day, freeing up congested inner arterial and local roads and providing a continuous free-flowing connection between the Eastern Freeway and CityLink.'' The road would help reduce congestion for decades, he said.

''Imagine Melbourne today without CityLink. In decades to come, without East West Link traffic growth will see the Eastern Freeway and a significant part of our road network grind to a halt.''

CityLink opened in 2000 and parts of the toll road are already at capacity, with heavy congestion in the peak. The panel hearing, before an assessment committee of six planning experts, will sit for 26 more days.