Every night, through a gap between the towers across the street, I see a red flow of tail lights streaming away across the Brooklyn Bridge. I think about all the gasoline those cars are burning, and the plume of fumes coming up off the bridge, caught in the wind, fouling the air above the river. Despite the disruption caused by all that traffic, the bridge carries far fewer people these days than it was designed to. A friend pointed out a paper (pdf warning) that describes the engineering challenges of maintaining the Brooklyn, Manhattan, Williamsburg and Queensboro Bridges. This paper includes an important graphic (on p. 4) that shows changing transportation modes side by side with the number of trips made over the bridges each day. Here's a detail for the Brooklyn Bridge:

Despite a wondrous century of technological marvels, the Brooklyn Bridge has declined substantially in efficiency. In 1907, 426,000 trips were made each day across the bridge via two tracks of streetcar lines, two tracks of elevated subway trains, on foot, and on two lanes for vehicular traffic — in those days horses, carriages and horseless carriages. By 1989, only 178,000 trips were made across the bridge, a 42 percent decline in usefulness since 1907, even though there were now six modern lanes carrying speedy, powerful cars. The statistics and layout diagrams for the other bridges tell similar stories. The Williamsburg Bridge, which once had six tracks for trolleys and subways and four lanes for vehicles, carried 505,000 daily trips in 1924. By 1989, with only two tracks remaining for the J, M and Z trains, the bridge's space devoted to cars had doubled but its usefulness had fallen by half to just 240,000 trips per day.

Even though vehicular traffic has increased, people moving across the bridges has decreased. How did this happen? After World War II people began anew with the thought that the automobile would be the ultimate form of human transportation, the be-all and end-all of personal, private mobility. The trolleys and elevated subways were destroyed and every square inch that could be was put into service for the movement and storage of the automobile. The city paved over the streetcar lines and demolished as many elevated subway tracks as it could. Only a fraction of the many elevated lines remain as part of today's subway system. Here is a partial map of the Brooklyn-Manhattan Transit Corporation's system in 1924.

Note the elevated lines over lower Myrtle, Lexington, Fifth and Third Avenue. These are all gone, as is rail service over the Brooklyn Bridge. It was once possible to hop on a train at Park Row and travel to northern Bedford-Stuyvesant without a transfer. That's impossible now, but there are still many neighborhoods where elevated subways travel. The demolition of the trolleys was total. Nothing remains of the dense grid of streetcar lines that fed the Brooklyn Bridge for decades.

When the streetcar lines and the elevated subways were demolished, the people were decanted out into the green spaces surrounding the city, and the suburbs of the modern sprawling metropolis were born. Meanwhile, the city had to deal with a host of urban ills, including abandonment, crime, poverty, arson and urban decay. Thankfully, this has been changing recently.

The East River Briges have stood through all of it, but today serve at half capacity, like gifts handed down to us from prior generations that we're not really sure how to use. Some people call the years after World War II the start of the Cheap Oil Bonanza, and believe that the end this bonanza may now be beginning. If cheap oil becomes a thing of the past, there may decades that lie ahead when something like trolleys or elevated subways return to the Brooklyn Bridge.

- Managing the East River Bridges in New York City [Federal Highway Administration]
- Hubbert peak theory [Wikipedia]