Transportation analyst and Streetsblog contributor Charles Komanoff is out with a piece in Reuters today that examines the traffic impacts of adding 2,000 new yellow taxis to Manhattan streets, and it’s not pretty.

As part of the grand bargain struck between Mayor Michael Bloomberg and Governor Andrew Cuomo that will create a new class of hail-able livery cabs, NYC will auction off 2,000 new yellow taxi medallions. The city is expected to haul in a billion dollars from the auction, but Komanoff calculates that in the bargain, central Manhattan streets will be overrun with even more traffic:

No one mentioned traffic when the taxi deal was rolled out last month at City Hall and in Albany. After all, with 800,000 motor vehicles already entering the Manhattan Central Business District (CBD) each weekday, what difference could a mere 2,000 additional yellow cabs possibly make?

Plenty, it turns out. Yellow cabs spend three-fourths of each shift, around seven hours, plying CBD streets and avenues. (And of course some are active for two shifts a day.) Most private cars driven in Manhattan don’t do so for long. Even at the CBD’s notoriously labored traffic pace — now averaging 9.5 mph, up from 8 mph before the recession — the two to three miles per day logged by the average car below 60th Street occupy 15 to 20 minutes.

Adding one new medallion is thus equivalent to adding 40 private cars. Adding 2,000 of them — as the City now intends to do during the next three years — would be the traffic equivalent of adding 80,000 cars, a 10% increase in volume.

Some form of congestion pricing would be just about the only way to mitigate the impact of all this additional traffic, Komanoff writes. You can see the analysis underlying his conclusions in this PDF.