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Abstract
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Transportation practice aspires to be "rational" in its approach to decision making. That is, it relies heavily on the use of performance measures and project evaluation criteria, derived from empirical data, to examine whether specific transportation investments may advance transportation-related goals and objectives.
A common misconception is that, because the transportation performance measures are derived from "objective" data, the decisions that emerge through the use of these measures are likewise "objective," or at least free from professional bias.
Yet this is not so. Knowing that a four-lane roadway carries 50,000 vehicles per day, or that a particular intersection operates at level-of-service "F" (Fail) during the peak hour, is no more a call to a specific course of action than is knowing that there is a 30 percent chance of rain. All three are simply descriptive statements. It is only when this information is filtered through our values and expectations, as reflected in our measures of transportation system performance, that they gain importance.
In this article, we use the example of level-of-service to detail how norms, values, and preferences are embedded in the data we use for transportation decision making. We proceed to detail how these values influence, in a profound way, our understanding of transportation problems and the types of solutions that are "acceptable" to resolve them. We conclude by detailing four principles of which transportation professionals should be aware when developing and applying measures of transportation system performance.
In the United States, the most commonly used metric of transportation performance--and in many jurisdictions, the only metric--is level-of- service (LOS). This calculation strives to measure the convenience of driving a personal automobile. For urban streets, LOS is based on the number of seconds motorists are delayed at intersections, as well as reductions in free-flow speed that may occur as a result of the presence of other motorists. LOS is typically reported for either the most congested hour or most congested 15 minutes of the day.
While LOS is rooted in quantitative, volume-to-capacity analysis, it is reported in a qualitative, A-to-F letter scale, like a school report card. Intersection LOS is rarely reported as, for example, "84 seconds of delay," but instead as "LOS F." This terminology implies that the intersection has failed, even if it has only exceeded its capacity for 15 minutes of the day. Asserting that a roadway has failed is not a descriptive statement; it is a call to action.
To illustrate the value-laden nature of this measure, we have restated three of its implicit assumptions as explicit "guiding principles" that could be included as part of a community's long-range plan. While these principles would undoubtedly result is a great deal of public debate if presented to the public for consider- ation, in practice they are given little or no consideration because they are layered as implicit assumptions in the use of LOS as a performance measure.
While most transportation plans include a goal stating the importance of moving people and goods, LOS, the primary performance measure used in support of this goal, says nothing about moving either. The appropriate measure of the movement
of people is the number of people that are transported through a spot or along a corridor, such as persons per hour per lane. Instead of providing us with information on people, LOS tells us only about automobile delay and vehicle speeds.
Under this measure, the people onboard a full 50-person bus are each valued at one-fiftieth the worth of a person driving alone in a car. The needs of bicyclists and pedestrians are not measured at all. Any design changes that create delay or discomfort pedestrians, such as alteration in signal timings, are seen as entirely positive from an LOS perspective if they reduce vehicle delay or increase speed on urban streets. Transit, walking, and cycling are treated as being worthwhile only insofar as they lead to reductions to motorist delay, regardless of however efficient they may be at moving people.
This the other principles, various assumptions and other issues are discussed throughout the article.
Language: en |