Everybody’s favorite transportation geek, Charles Komanoff, has a fascinating new paper out on the economics of New York’s new Tappan Zee Bridge. The old bridge is decrepit, and needs to be replaced — everybody agrees on that. And the replacement is now in the works, at a cost of $5.2 billion. But does it need to cost that much? Komanoff makes a strong case that it doesn’t.

I won’t try to summarize Komanoff’s paper here. Instead, I’ll just point to one fact which is buried there. The new bridge comes with a combined bike/pedestrian lane, 12 feet wide. And the cost of building that lane — the amount that the cost of the bridge would decrease if you simply built it without that lane — is an astonishing $400 million.

To put that number in perspective, Komanoff tells me it would cost roughly $40 million, in the same 2015 dollars, to build two bike/pedestrian lanes on the Verrazano Narrows bridge — lanes which would get vastly more traffic than the one lane on the new Tappan Zee.

As for the cost of the first three years of New York City’s ambitious bike program under transportation commissioner Janette Sadik-Khan, that was just $8.8 million, 80% of which was paid by the federal government.

In other words, for the $400 million which governor Andrew Cuomo is planning to spend on a white-elephant bike lane almost nobody is going to use, you could utterly transform the bicycling infrastructure for millions of New Yorkers in all five boroughs.

Oh, and I almost forgot — it looks as if the old Tappan Zee bridge is going to be converted into a bike/pedestrian walkway anyway, making such a facility on the new bridge even more superfluous.

But this is how big projects always work: it’s weirdly easier to raise billions for something huge than it is to add millions to an annual budget somewhere. “Gridlock” Sam Schwartz, for instance, in his clever new congestion-pricing plan, is proposing three new massive bike/pedestrian bridges: one from Jersey City and Hoboken, in New Jersey, would span the Hudson River and land just north of Chelsea Piers. A second would go from Long Island City and Hunter’s Point, in Queens, and would cross the East River to midtown Manhattan. And the third, and most ambitious, would start in Red Hook, in Brooklyn, head over to Governor’s Island, and then continue on to the Financial District.

These are utterly wonderful ideas. If beautiful new pedestrian bridges can be built by Santiago Calatrava in Venice or by Norman Foster in London, there’s no reason New York can’t follow suit. Still, it’s a bit depressing that we don’t seem to have the mechanisms to take the billions available for vanity projects, and use some small fraction of that money for things which would make a huge difference to the daily lives of millions of New Yorkers.

This phenomenon isn’t confined to government, of course: anybody working in a big corporation has seen some huge acquisition made, using money which was never available for smaller projects from existing teams which had much clearer benefits. And there are hundreds of museums around the world which never have money for important things like conservation, but which somehow manage to find enormous sums for glossy new starchitectural projects. Basically, people want to be able to see where their money is going, in the form of something large and grand and headline-grabbing. Even if there are much more sensible uses for it elsewhere.