Since some people didn't quite get my brilliant idea, I was marveling at the number of people who otherwise agree with me about most transit and livable streets issues, but don't see any problem with putting highways in tunnels. And this is not the first time: I've seen otherwise reasonable people support the Gowanus tunnel, and even lament that Westway never happened.
I started writing an explanation of why highway tunnels are problematic. Then Streetsblog highlighted the fact that I'm not alone in this. Over on the other side of the country, Dan Bertolet is having very similar discussions with fellow Seattleites who don't see any problem with putting the Alaskan Way in a tunnel.
So here is your answer: highway tunnels suck for many reasons. As discussed in the comments on Ben's blog post, they are expensive, draining money from other worthy uses. They require ventilation structures that spew carbon monoxide into the air, in much larger quantities than subway vents. As Dan Bertolet observes, if they free up road capacity it's only temporary. And here are three more from me:
The cars still come from somewhere. Ben recently had a post about the never-completed Nostrand and Utica Avenue subway lines. A number of commenters observed that the area is now very car-dependent and full of transit NIMBYism. I'll go further and point out that the people here regularly vote for anti-transit politicians like Lew Fidler, Alan Meisel and Carl Kruger. Their car-oriented lifestyle is not just encouraged by the lack of subways, but facilitated by highways like Linden Boulevard, the Belt Parkway and even the BQE.
The cars are going somewhere. If you look at the congestion on Sixth avenue in the middle of the day, or Bleecker Street on a Friday night, how do the cars get there? Many of them come on the BQE, on the FDR or on the West Side Highway.
The cars are probably not spending their entire time in the tunnel. If they could get from origin to destination without using any streets, that might not be so bad. But the tunnel (or parkway, or elevated highway) doesn't go everywhere, so the drivers are going to have to use local streets at both ends. When they do, it's a bad scene.
I still remember one time I rented a car and drove down the West Side Highway. Coming off onto 79th Street, I was bewildered and annoyed. What were all these pedestrians doing? And I was supposed to stop for them? I'd just been driving nonstop for a long time! A lot of our most dangerous intersections are highway exits, where drivers are still coming out of highway mode.
So that's why the BQE tunnel proposal sucks, why the Gowanus tunnel proposal sucks, why Westway sucked, why the Big Dig sucks, and why the Alaskan Way tunnel proposal sucks. Car tunnels suck, for at least six reasons. Don't support them, and don't accept that car capacity is sacred.