City desk

Transport Bodies Need to Look Again at How They Model Traffic, Some Say

When the numbers for some journey times didn’t seem right, a consultant appointed by An Bord Pleanála travelled the route himself.

It took 17 minutes at peak hour to go from Shelbourne Park to Heuston Station, said figures from Dublin City Council and the National Transport Authority (NTA). Off peak, it took 20 minutes on average, they said.

But that didn’t seem to chime with what consultant Daniel O’Connor experienced. It took him 47 minutes, he said in his report. “The modelled journey times could not be accepted as accurate.”

Problems with modelling came up time and again in inspector Breda Gannon’s report from An Bord Pleanála, rejecting plans to make a giant plaza at College Green in the heart of the city.

It’s the same modelling that the city uses for all major developments, says Green Party Councillor Ciarán Cuffe, who is chair of the council’s Transport Strategic Policy Committee.

“It opens up the question of if the modelling is fit for purpose,” Cuffe said.

Big Picture, Small Picture

Cities and transport agencies use traffic modelling to predict what will happen if they make changes. College Green was no different.

For that, the National Transport Authority used the East Regional Model (ERM) with software called SATURN. It has used it before to model the entire eastern area of the country, with a focus on Leinster and the greater Dublin area.

It’s a macroscopic model, says David O’Connor, a transport-planning lecturer at Dublin Institute of Technology, who also writes a column for Dublin Inquirer.

Those are used for large areas, and look at overall traffic flow rather than the finer details. In its submission, Dublin Bus said that was the wrong model to use – that they should used a microscopic model, says O’Connor. “So you can see it in action.”

Microscopic modelling “looks like video games”, says O’Connor. “It’s based on numbers. You use the actual street layout.”

You add in traffic “and can create a video of what it would look like”, he says. “It’s very powerful.”

An Bord Pleanála’s concerns around journey times would be address with a microscopic model, says O’Connor. “I would expect the NTA to use a much more specific model.”

“The background level of traffic is really what’s at issue here,” says O’Connor. “The SATURN model didn’t really demonstrate that a high volume of traffic was a problem.”

There’s also the fact that some streets, such as Winetavern Street, were left out of the modelling despite plans to reroute a number of buses there under the new plans, which consultant Daniel O’Connor said in his report.

These missed-out streets, along other concerns,”cast significant doubt on the reliability of the model to accurately identify the significant effects of the proposed development on traffic”, wrote inspector Gannon in her report.

Best Way

Not everyone agrees that a microscopic model would be more accurate, though.

“It’s like a crystal ball to see what would happen,” says Brian Caulfield, assistant professor in the Department of Civil, Structural and Environmental Engineering at Trinity College Dublin.

A macro-model fits the project, Caulfield says. “A micro-model wouldn’t work. The NTA are more qualified than anyone in the state to say what modelling works.”

Neither Dublin City Council nor the NTA got back to queries about why the model might not have been accurate, whether it could be done differently in the future, or concerns about the same methods being used for other projects in the city. Dublin Bus didn’t respond to queries either.

The SATURN system has been in place since the 1990s, says Caulfield. It was used for the Luas and also for BusConnects, he says – and An Bord Pleanála were happy to accept the model results for the Luas. “I don’t know what the NTA could have done better or differently.”

Green Party Councillor Cuffe says the issue needs another look. “We have to go back and reconsider how we’re going to do it,” he says.

Zuzia Whelan portrait
Zuzia Whelan

Zuzia Whelan is a city reporter for Dublin Inquirer. You can reach her at zwhelan@dublininquirer.com.

 

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