Average daily traffic: 104,000, give or take.
Length of the bridge: 340 feet.
Cost: $50 million.
Cost per linear foot: $147,000
Cost per linear foot per person*: $1.18
(* assumes 1.2 passengers per car)

A bit closer to the city, there’s another bridge which whisks commuters in and out of Boston every day. It’s twice as old as the Route 2 bridge, and carries more people (but fewer vehicles). I speak, of course, of the Longfellow.

Average daily traffic: 130,000, give or take (20,000 cars, 100,000 Red Line, 10,000 bikes & peds)
Length of the bridge: 1850 feet.
Cost: $255 million.
Cost per linear foot: $138,000
Cost per linear foot per person*: $1.04

From Andy Singer.

The Longfellow Bridge, which is in a dense urban environment (harder to access with materials), over water (necessitating floating in many materials), includes significant historic elements (so it can’t be replaced with an off-the-shelf box-girder design), and is longer and higher than the Route 2 bridge, is actually cheaper to rebuild overall, and even moreso when you factor in the number of daily users. Sure, you could get five Route 2 bridges for the cost of one Longfellow, but those five bridges, end to end, wouldn’t reach across the Charles. So when we talk about transit subsidies versus highway subsidies, we should remember that, at least in this case, it’s cheaper to rebuild a multi-modal bridge than one used just by cars.

I’m not saying that we shouldn’t rebuild the Route 2 bridge. We can’t let our infrastructure crumble. But when it comes to rebuilding infrastructure, it’s important to know that every time you see cones on the side of the road, there’s a real cost involved. I would not be surprised if there are a lot of people who look at the Longfellow as a bloated project—a quarter billion dollars!—and don’t give a second thought driving by the orange barrels on Route 2.

Yet if we measure return on investment by how much new bridge we get per person traveling on it, it’s the little bridge replacement project out in Lexington that costs us more. And while the Longfellow is one of only a few transit-centric replacement projects (and even it contains a majority asphalt), there are scores of bridge replacement projects across the Commonwealth that chew up a lot more money. No one ever blinks an eye at a proposal to rebuild a decaying highway bridge. That’s necessary maintenance. But more transit projects are a much harder sell, even if they’re more efficient.