Transit Lines/Networks

Grid Systems

The rectangular grid system, with streets of uniform width regularly spaced and laid out parallel or at right angles to each other, is one of the oldest city [Page 1455]layouts known. There is archaeological evidence that it was used in ancient Egypt, in the Babylonian empire, in China, and in ancient Mexico City, among other places.

Many American cities were also laid out in a grid pattern, or the grid pattern was imposed at some point in the city's development. For instance, in New York City, the Commissioners Plan of 1811 imposed a grid pattern on the island of Manhattan north of Houston Street, while the older streets south of Houston retained a much more irregular and random pattern. When public transportation routes began operation in cities constructed on the grid layout, they naturally followed those patterns as well.

Rectangular grid patterns have many advantages. For one, they offer good transportation service across the grid if riders are allowed to transfer between lines, because no point in the grid lies too far from either a north-south or east-west line. This means that no point in the grid is inherently privileged over any other when it comes to accessibility by public transit, and this is particularly beneficial if an area has multiple key areas of activity.

Grids also work best if the lines are anchored by major destinations at their endpoints, because that means there will be lots of people riding to the end of the line. Grids are also intuitively easy to understand, although they have the disadvantage that they work best in relatively flat regions. San Francisco is a famous example of a city that laid out its roads on a grid system without considering the underlying topography; the result was many roads running up and down extremely steep hills, a reality that motivated the development of the city's signature cable car system.

Another type of grid is the polar or radial grid, which centers on a single area (for example, the downtown area of a city) with transit lines laid out in concentric rings around this central area; straight lines cross one another throughout the central area, creating the appearance of a spider web. With this system, areas near the city center are close to one or more transit lines, while areas farther away from the center have a higher probability of being farther from any transit line.

This system has advantages if the central point is truly primary in the travel patterns of the majority of transit users in the area, if areas farther away from the center are less likely to be visited by people using public transportation, and if it is assumed that alternate transportation (for example, an automobile) is an available option for those living away from the city center. This layout is less intuitive than a rectangular grid but increasingly familiar because many U.S. urban areas (for example, Washington, D.C., and greater Boston) have roadway patterns with this design, in which a beltway or ring road surrounds the city or metropolitan area and a second system of roads leads to and from the city center.

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