On 9/11, I have nothing smart to add to the national discussion so I will change topics. Note the picture below. It graphs time trends in the share of workers who commute by bike. There are large differences across liberal cities with respect to biking shares. I'm guessing that the editors of the Times hope that this salient graph will make New Yorkers feel guilty that Portland and San Francisco have made greater progress on embracing the bike while New Yorkers stay on the subway.
In a recent paper, I looked within cities and studied who bikes to work. See Table 5 of this paper. This table focuses on California. Kahn, Matthew E. & Eric A. Morris (2009). Walking the walk: New tests of environmentalism’s association with green travel behavior. Journal of the American Planning Association, 75(4), 389-405. (Refereed)
Those who live closer to the city center and live in "environmentalist communities" are most likely to commute by bike. There is a network externality here. If more people in the community want to commute by bike, the local Mayor has a greater incentive to supply infrastructure to support this choice. As the Mayor invests public funds in improving biking as a commuting mode, more "bike types" will move to this city and more incumbent commuters will choose to commute by bike.
So, this is really a commons issue of reallocating land away from other uses towards being "bike friendly". In China right now, the opposite trend is taking place. Public space that was used by bikes is now being grabbed by cars. Now , can't "we all get along"? This is an open question given that cars and bikes move at different speeds and that the laws of physics are well understood.
As the U.S population ages, will older Boomers in Berkeley travel by bike? Or will solar powered motorcycles be in demand?