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13 demand. These methods, however, are less than satisfactory for estimating the operating performance of a given roadway or for refining a given design. If information on the actual per- formance of a given facility or how to refine a particular design is desired, then macroscopic methods (described below) should be used. Macroscopic Methods Macroscopic methods are used to consider the flows of vehi- cle streams, rather than the flows or operations of individual vehicles. The HCM is an example of a set of macroscopic meth- ods for evaluating roadway operations. As such, these methods Source: LeighFisher. approximate the interactions between individual vehicles, the behavior of individual drivers, and detailed characteristics of Figure 2-9. Supplemental curbside at Salt Lake City the roadways (or other facilities). Adjustment factors, typically International Airport. developed through empirical observations or microsimulation methods, often are used to account for atypical vehicles or The analytical procedures described in this Guide are most driver characteristics, traffic flow constraints, or other opera- relevant for airports with traditional curb spaces because of tional characteristics. These methods produce results that are the differing dwell times and through-lane operations that considered acceptable, more accurate than quick-estimation occur with other configurations. methods, and can be used with less training and experience than microsimulation methods. Macroscopic methods can provide reliable estimates of the Overview of Analytical Framework steady-state performance of a roadway averaged over a given Hierarchy analysis period. They are best for determining the refinements Subsequent chapters of this Guide present alternative meth- to a proposed design (or existing facility) that would elimi- ods for analyzing airport roadways, weaving sections, and nate capacity and congestion problems. These methods are less curbside areas, recognizing the unique characteristics of these satisfactory for quantifying facility operations under heavy facilities. The alternative analysis methods or hierarchy differ congestion conditions. in terms of (1) the level of effort or time needed to conduct the Macroscopic methods are generally unsatisfactory for com- analysis, (2) the expected level of accuracy or reliability of the paring alternative improvements that reduce but do not elim- results, and (3) the necessary level of user skill or experience. inate congestion. Under heavily congested conditions (hourly The three methods--quick-estimation methods, macroscopic demand exceeding capacity), queuing vehicles from one part methods, and microsimulation methods--are described in the of the roadway affect both upstream and downstream opera- following paragraphs. tions in a manner that cannot be estimated easily using macro- scopic methods. Macroscopic methods also cannot be used for unusual facility types or situations for which they were not Quick-Estimation Methods designed. In those situations, microsimulation methods must Quick-estimation methods, as the name suggests, can be be used. used simply and rapidly to produce preliminary analyses of roadway operations (or other facilities). They generally con- Microsimulation Methods sist of look-up tables, simple formulas based on regression analysis of databases, or rules of thumb, and are based on Microsimulation methods consist of the use of sophisti- broad assumptions about the characteristics of the facility cated computer programs to simulate the operation of indi- being analyzed. As such, they provide a first test of the ability vidual vehicles on simulated roadway networks. Each vehicle of a roadway or other facility to properly accommodate the is assigned characteristics, such as a destination, perfor- estimated requirements (existing or future) or the adequacy mance capabilities, and driver behavior. Each roadway net- of a potential improvement measure. work is defined using characteristics such as number, length, Quick-estimation methods are ideal for quickly sizing a and width of lanes; operating speeds; traffic controls; and facility. The analyst can easily check which of many possible pedestrian activity. As each imaginary vehicle travels through roadway design options is sufficient to serve the forecast the computerized roadway network, various aspects of its