The Greenfield Bridge in Pittsburgh

A speaker in my department reminded us about the bridge-under-the-bridge in Pittsburgh, which involved building a bridge to catch falling debris from traffic-carrying bridge above. The speaker posited that perhaps the resources that went into building the second bridge would have been better spent fixing the broken bridge.

The bridge is finally being replaced (link here). The article actually provides numbers we can use to decide if, in hindsight, the secondary bridge was a good idea.

The bridge is projected to cost 19 million dollars in today's dollars to build, while the secondary bridge cost $625,000 in 2003 dollars. Based on the CPI, $1 in 2003 would be $1.29 today, and therefore the bridge would cost about $800,000 in 2015 dollars. But it turns out that is pretty irrelevant since $800,000 / $19 million = 4.2%. Assuming building new bridges wasn't wildly cheaper back in 2003, this was an obviously good investment since the bridge lasted 12 years, not one.

I will add one caveat: I don't know a lot about how these kinds of capital projects are funded. The replacement bridge project page mentions that 5% of the funding is coming from the city, 15% from the state, and 80% from the federal government. The above analysis in some ways assumes that the breakdown is the same for the secondary bridge. However, it appears likely that the city of Pittsburgh covered the full cost of the under-bridge. That puts their investment in 2003 for the past 12 years of bridge as about the same as for an entirely new bridge which will hopefully last 100 years. From that perspective, the city's best decision is a little less clear.