For a class I am taking I read the National Resource Defense Council report on Stormwater Strategies: Community Responses to Pollution Runoff.  In that report I happily stumbled upon the following paragraphs (emphasis mine):

Transport imperviousness generally exceeds rooftop imperviousness in urban areas of the United States.5Cumulative figures show that, worldwide, at least one third of all developed urban land is devoted to roads, parking lots, and other motor vehicle infrastructure. In the urban United States, the automobile consumes close to half the land area of cities; in Los Angeles the figure approaches two thirds.”6 The city of Olympia, Washington, also found that transport imperviousness constituted approximately two-thirds of total imperviousness in several residential and commercial areas.7 This distinction is important because rainfall on transportation surfaces drains directly to a stream or stormwater collection system that discharges to a waterbody usually without treatment, whereas some roofs drain into seepage pits or other infiltration devices. Research has also found a strong relationship between curb density and overall imperviousness in residential areas suggesting that roads lead to the creation of other impervious surfaces.8

The creation of additional impervious cover also reduces vegetation, which magnifies the effect of the reduced infiltration. Trees, shrubs, meadows, and wetlands, like most soil, intercept and store significant amounts of precipitation. Vegetation is also important in reducing the erosional forces of rain and runoff. In one study, conversion of forest to impervious cover resulted in an estimated 29 percent increase in runoff during a peak storm event.9

Urban life will always have impervious surfaces, it’s the nature of human settlement.  We cannot possibly achieve runoff totals that mimic life before urban development.  However, that does not mean we cannot plan for the future or current establishments to cut back on the total amount of roads, parking lots, driveways, garages and other automobile related structures.  While railroad tracks exist on firmly packed land and are therefore impervious as well, they also are not the same as asphalt in terms of the type of imperviousness.  Moreover, light rail can exist in green spaces, as in the above picture.

However, the most important part of rail technology is it takes up less space than roads.  The number of people that can travel on a skinny railroad track can mimic the number of people on a busy multi-lane highway.  As I always say, roads are not about to and nor should they disappear.  However, decreasing the number of roads and other auto-dependent land uses would be a boon to the environment.

Runoff is a danger for a number of reason: for the pollutants it carries, for the erosion that occurs, for the way it prevents water from getting back to aquifiers, ground water, and other elements of the watershed.  Decreasing our impervious surface area by relying on rail more and our roads less would be a boon to our cities not just for ecological and economic reasons, but also because it would open up more space for the city to either grow in density or for public space to be available to be used.  Imagine your busy roads now being parks instead!