You've got to hand it to Boston. In 30 short years, it's built a highway that's launched countless corruption scandals, seen record cost overruns and even killed someone.

The $15 billion Big Dig is truly a gift that just keeps on giving. And now it seems it isn't even doing what it was intended to do - reduce congestion.

An analysis by the Boston Globe found the 3.5-mile tunnel under the center of town has actually doubled commute times in some areas, proving two things: First, major road projects need parallel investments in mass transit. And second, it's all but impossible for cities to pave away traffic problems.

The nation's single most expensive underground road construction project has gained international acclaim as a highway project gone wrong. The Big Dig was designed to replace the notorious Central

Artery, a double-decker elevated highway that bisected the city and routinely saw traffic jams that lasted hours. One study concluded the Central Artery would be gridlocked 16 hours a day by 2010 if something big wasn't done, and done quickly.

That something was the Big Dig.

The project faced huge obstacles from the moment it was conceived in the 1970s, not the least of which included tunneling through a huge expanse of landfill and doing it without disrupting any of the 190,000 or so cars that traversed the Central Artery each day. And then there were the myriad neighborhood concerns about the Dig's impact on ordinary Bostonians.

It was a Herculean task, which is why it took the better part of three decades to complete. When the bulk of the project opened to traffic five years ago, the Dig's budget had swelled from an $2.8 billion (inflation adjusted) to $14.6 billion. And it wasn't long before the Massachusetts Turnpike Authority discovered thousands of leaks in the tunnel walls. Things went from bad to worse in 2006 when a three-ton panel fell from the ceiling, killing a motorist.

Despite being a national embarrassment, advocates say the Big Dig has alleviated traffic, and to a large degree they're right. Getting across town on the Central Artery could take as long as 15 minutes; these days you can do it in three minutes or less. Back in the day the Artery might be bumper-to-bumper for 14 hours. Nowadays, the Dig is a breeze.

But get out of downtown and things don't look so rosy. The Globe examined reams of state highway records and concluded that congestion has gotten worse north and south of the city - in some cases a lot worse. One 11-mile stretch of highway just north of Boston saw commute times double from 12 minutes to 25. Similar delays were seen on other highways leading to the tunnels.

It's easy to see why. The Dig has encouraged more suburbanites to drive, which might have been OK had the highways leading to the tunnels been expanded to handle the added traffic. But of course they weren't. When the Central Artery still stood, traffic flowed through an hourglass, with the roads leading into and away from the Artery wider than the Artery itself. The Dig reversed that - the roads leading into and out of the tunnels are narrower than the tunnels, so everyone creeps toward the Dig, speeds through it, then creeps away from it.

In other words, the Dig didn't solve the bottleneck, it just pushed it out of central Boston.

It hasn't helped that funding for public transit hasn't kept pace with the need. The MBTA, which runs Boston's subway and commuter rail network, is spending money where it can on new vehicles and tentatively extending lines further into the suburbs. But it's also drowning in debt, limiting its options. In 2005, then-Gov. Mitt Romney proposed a transit funding plan that provided money for maintenance but little in the way of expansion. It's no wonder Boston area motorists saw the completion of the Big Dig as an opportunity to get behind the wheel and start driving into the city.

The Big Dig is a telling case study. Build new roads to alleviate traffic and you invite more drivers to hit the road. That causes more backups, which leads to increased demands for more highways. It's a vicious cycle.

Don't get us wrong – the Big Dig has done some great things for Boston. The giant green monstrosity that bisected the city is gone, reconnecting neighborhoods and creating new space for pedestrians, bikers, and greenery. The new suspension bridge built as part of the project is world class. And if you're only trying to get across town (as opposed to trying to get into town), the Dig has made it a breeze.

But the Big Dig was first and foremost meant to alleviate congestion. If it has failed to do that, as the Globe analysis suggests, then it is $15 billion down the tube.

POST UPDATED 11 a.m.

Photo by Flickr user davedesign**.