Montreal and Boston have done it. Seattle's is under way now.

Should heavily congested Toronto and Vancouver also dig tunnels to mine the underground for traffic solutions?

Going underground is not for the timid. The costs and engineering challenges are immense. But so too are the potential benefits:

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a weather-free conduit with no visual distractions.

the city above left unscarred by intrusive at-grade or elevated expressways.

A glance at Boston and Seattle may give boosters pause and critics ammunition.

Both cities undertook to bury elevated freeways. Boston's 5.6-kilometre job took 15 years (nine longer than planned) and the cost of $14-billion more than doubled estimates, though some bridges were included. Seattle's 3.2-kilometre dig is under way, sort of. The $80-million borer got stuck last December, one-tenth of the way along its route before breaking down. They need to rebuild the bearings and dig a shaft to reach the machine, a projected 16-month process that is expected to delay the dig until March 2015.

Then there's Montreal. In the 1970s, Montreal narrowly escaped a plan that would have demolished 40 per cent of Old Montreal to build an elevated expressway. Instead, planners routed the Ville Marie Expressway directly downtown, sending it underground for three kilometres. It was built not by boring but by digging down and then covering. A mid-section is still open to the air, but is below-grade and out of the way. Today, more than 100,000 cars a day drive underneath downtown.

Vancouver, tightly packed on a peninsula, would seem like an ideal candidate for subterranean solutions. Louise Yako, president of the B.C. Trucking Association, an organization with a keen eye on traffic conditions and road networks, says there is not much appetite in Vancouver for big digs.

"Over the last decade the governments have done a good job of addressing the bottlenecks that existed," says Yako, explaining that projects like the ones in Seattle and Boston aren't likely to find much favour.

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With Toronto facing an unending overhaul of the Gardiner Expressway, thoughts of tunnelling are never far away. The idea gained particular prominence four years ago when mayoral candidate Rocco Rossi suggested running the truncated Allen Expressway underground from Eglinton Avenue to downtown. That route proposal wasn't taken seriously, but Rossi, now president of Prostate Cancer Canada, maintains it's a strategy that Toronto should pursue.

"We have ignored transportation infrastructure for so long that incrementalism is not really an option," says Rossi, arguing that revenue from Highway 407 shows major road projects can be financed through tolls because people will pay for options.

Brian Garrod, executive vice-president of Hatch, an engineering project management firm in Toronto that is involved with the partially underground Eglinton Crosstown LRT, says tunnels are more expensive to construct than elevated expressways. But over the long run, the costs converge, owing to the lower maintenance costs and longer lifespans of underground roads.

As for the geo-technical issues at play below the Gardiner, Garrod says Toronto's waterfront is built on eight to 10 metres of fill material, under which lies shale bedrock. "The conditions are good for tunnelling," he says.

Solid bedrock or not, whether the practical conditions are good for large-scale tunnelling is something Dr. Eric Miller is skeptical about. The director of the University of Toronto's Transportation Research Institute confesses he's not a fan. That being said, he's more enthusiastic for small-scale projects.

As Miller puts it, a road network is a massive queuing system. "If you solve a bottleneck in one area, you might just be adding to another down the road," he says.

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He cautions that downtown's capacity to absorb cars must be considered before thinking about increasing the roadways that funnel into it. As far as building a tunnel to replace the job the Gardiner does, Miller says "those billions could be better spent adding capacity on transit and other roads."

Where Toronto might make better use of below-grade space is to smooth transitions at interchanges such as the one where Highway 401 meets Highway 404 and the Don Valley Parkway. Miller says the city could look at building shorter tunnels to bypass perennial trouble spots where the road capacity on either side is sufficient.

"Anything that can be done to reduce turmoil at interchanges would be very helpful," he says. "There are three levels to a city, underground, at-grade and above," he says. "As cities grow, we have to be imaginative and use all three levels. You need to bite the bullet and spend money. Because it will be worthwhile."

If you have questions about driving or car maintenance, please contact our experts at globedrive@globeandmail.com.

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