Transportation: Traditional Approaches
Although cities are commonly viewed as the source of a disproportionate share of greenhouse gases (GHG), in fact, city dwellers create far fewer emissions than suburban and rural households on a per capita basis. Emissions per acre within cities are high in part because of the high number of housing units per acre and in part because of traffic, but each urban household actually emits fewer emissions.
In contrast, private vehicle travel and the preponderance of detached single-family homes in suburban communities create emissions per household at a rate two to three times that of the densest parts of the city. Figure 1 vividly demonstrates this fact for the Metro-Atlanta area. Similar maps of other metropolitan areas are available through the Center for Neighborhood Technology (www.htaindex.cnt.org).
Municipal transportation systems are primarily defined by the following elements:
Sidewalks: The most basic transportation system is the sidewalk, which gives residents access by foot to nearby amenities. Many cities still struggle to provide adequate sidewalks in their communities due to limited budgets.
Streets: Streets are considered the backbone of the transportation system. They enable delivery of goods, as well as travel by residents and visitors. Streets are generally classified according to the volume of vehicular traffic for which they were designed. Common classifications include:
- Arterials facilitate relatively longer trip lengths at moderate to high speeds with limited access to adjacent properties. They are the high traffic volume corridors serving major urban centers.
- Collectors collect and distribute significant amounts of traffic between arterials, minor collectors and local streets at moderate to low operating speeds.
- Local Streets provide direct access to abutting properties. They have relatively low traffic volumes, operating speeds, and minimal through traffic.
Transit: Two basic types of transit are bus and rail. However, new, hybrid forms, like bus rapid transit, can combine the best of both systems. Mid-sized and large cities offer bus service routinely. Only the largest cities have a full rail system, although many cities now have at least one rail route. Streetcars are often the preferred option for new urban rail systems.
Bicycle Routes: City bicycle travel generally takes place on city streets along with other traffic, often in designated bike lanes. Many cities also offer off-road paths.
Private Fleets: Taxicab companies are the most common type of private fleets. However, other specialized fleets are being offered by private companies and non-profit organizations, including carsharing, bike sharing, pedi-cab companies and tourist trolleys.
Public Fleets: Most cities maintain a fleet of vehicles. Some, like emergency vehicles or sanitation vehicles, are highly specialized. Others are simply cars used for city business.
Greenhouse gases are a part of the Earth's atmosphere and are both naturally occurring and the result of human chemical processes. The most common greenhouse gases are carbon dioxide, water vapor, methane, nitrous oxide, ozone, and chlorofluourocarbons. These gases trap heat and thus contribute to the warming of the planet. See also CFCS and GREENHOUSE EFFECT.
