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14 performance can be recorded based on its interaction with control types) and the characteristics of the vehicles and driv- other vehicles and traffic controls. These performance statis- ers using the roadway (e.g., the proportion of trucks or heavy tics can be summarized in many ways, including commonly vehicles, daily and hourly variations in use, familiarity of the used performance measures, such as travel time and delays, typical drivers with the roadway). With knowledge of the char- travel speeds, and queue lengths. Also, some microsimulation acteristics of a roadway section and the vehicles (and drivers) models produce a visual display of the simulated roadway using the roadway, it is possible to calculate its capacity--the operations, which can be helpful when evaluating operations "maximum hourly rate" of vehicles flowing past a point. or presenting results. However, it is not possible or desirable for a roadway to Of the three methods for analyzing airport roadway condi- operate at its capacity for sustained periods, because any minor tions, microsimulation methods are the most complex and disruption will cause congestion, which results in delays or require the most effort and skill on the part of the user, but they lengthy queues and undesirable levels of safety and driver com- also produce the most detailed and reliable results. The use fort. Thus, roadway capacity, while stated in terms of "base" of microsimulation methods is suggested when macroscopic vehicles (e.g., passenger car equivalents) per hour, is sometimes methods do not yield reasonable results, do not provide suffi- computed for only the peak 15-minute flow rate within that cient detail, or when the conditions being analyzed are outside hour. In addition, roadway operations are characterized in the ranges addressed by macroscopic methods. terms of level of service and "service flow rate"--the maximum Additional information regarding the application of these flow rate that can be accommodated while maintaining a des- three analysis methods is presented in subsequent chapters ignated level of service. Similar to capacity (maximum hourly of this Guide. vehicle flow rates), service flow rates vary according to the characteristics of a roadway section and the vehicles using the roadway. Overview of Capacity and Level-of-Service Concepts Level-of-Service Concept The concepts of capacity and level of service, as presented in the 2000 HCM, are fundamental to analyses of roadway and Level of service is a qualitative measure of roadway (or other transportation facilities and well understood by traffic other transportation facility) operations. Six levels of service engineers and transportation planning professionals. This sec- are defined in the 2000 HCM, with LOS A representing the tion is intended to provide an overview of these concepts for highest (or best) level of service and LOS F representing the users not familiar with the 2000 HCM. lowest (or worst) level of service. The 2000 HCM defines level of service as follows: Capacity Concept . . . a quality measure describing operational conditions within a traffic stream, generally in terms of such service measures as The capacity of a rectangle or a box can be defined easily by speed and travel time, freedom to maneuver, traffic interrup- its size (i.e., its area or volume) because the maximum amount tions, and comfort and convenience. the object can accommodate is fixed. This is not true with objects that serve as "processors," such as roadways, ticket Levels of service are defined in terms of parameters that can counters, or runways. The capacity of a roadway, for example, be perceived by the users of a transportation facility and that can depends not only on its size (e.g., the number of lanes and be measured and predicted. On roadways, each level of service other geometric design aspects), but also on the characteristics corresponds to a specific maximum flow rate (i.e., the upper of the vehicles using the roadway (e.g., their size, performance, limit of the performance measure threshold (or flow rate)) for spacing, speed, and many other operating characteristics). If all that level of service. The parameters or measures of effectiveness the vehicles on a roadway were identical in size, distance defining each level of service are (1) the density of the traffic apart, speed, driver characteristics, and other characteristics, flow (passenger cars per mile per travel lane) for a freeway or then the capacity of the roadway (number of vehicles travers- other unsignalized multilane roadway and (2) delay (seconds ing a point or section during a unit of time) would be expected per vehicle) for signalized and unsignalized intersections. to be substantially higher than the capacity of the same road- way if it were serving a mix of vehicle sizes, speeds, and driver Commonly Used Level-of-Service characteristics. Definitions for Airport Terminal Accordingly, the capacity of a roadway--even roadways Area Roadways with the same number of lanes--varies based both on the char- acteristics of the roadway (e.g., lane and shoulder widths, ver- As noted, the 2000 HCM defines six levels of service, as pre- tical grades, intersection and driveway spacing, and traffic sented below. (These definitions were taken from the 2000
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15 HCM, but have been modified slightly for the benefit of airport LOS E represents operations at or near capacity. Opera- planners and others not familiar with the HCM.) tions at this level are volatile because there are virtually no LOS A represents operations where free-flow speeds prevail. usable gaps in the traffic stream. Vehicles are closely spaced, The ability of each driver to maneuver within the traffic stream, leaving little room to maneuver (or allow for lane changes or change lanes, merge, or weave is almost completely unimpeded weaving) within the traffic stream. Any disruption of the traf- by other vehicles because of low traffic densities. The effects of fic stream, such as vehicles entering from a ramp or a vehicle transient blockages or incidents (e.g., an accident, vehicle break- changing lanes, can disrupt upstream traffic flows. At capac- down, or other event that impedes the flow of traffic) are easily ity, the traffic stream has no ability to absorb even the most absorbed at this level of service. minor disruptions, and any incident can be expected to pro- LOS B represents conditions in which free-flow speeds are duce a serious breakdown with extensive queuing. Maneu- maintained. The ability of each driver to maneuver within the verability within the traffic stream is extremely limited and traffic stream, change lanes, or weave is only slightly restricted the level of physical and psychological comfort afforded the by the presence of other vehicles. The general physical and psy- driver is poor. chological comfort of drivers is still high. The effects of minor LOS F represents breakdowns in vehicular flow. Such con- incidents and point breakdowns (e.g., a breakdown in traffic ditions generally exist within queues forming behind bottle- flow where traffic enters, leaves, or crosses a roadway) are still neck points. Bottlenecks occur as a result of (1) traffic accidents easily absorbed. or incidents, (2) typical traffic congestion areas, such as lane LOS C represents traffic flow with speeds at or near the free- drops, weaving segments, or merges, (3) parking maneu- flow speeds of the roadway. Freedom to maneuver within the vers, or (4) traffic conditions when the projected hourly flow traffic stream is noticeably restricted (by the presence of other exceeds the estimated capacity of the roadway segment. vehicles) and lane changes may require more care and vigilance on the part of the driver because of high traffic densities. Minor Acceptable Levels of Service blockages or incidents may still be absorbed, but the local dete- for Terminal Area Roadways rioration in service will be substantial. Queues may be expected to form behind any significant blockage. On airport roadways, As noted, levels of service are typically used to determine if LOS C is generally considered to be the minimum "acceptable" a roadway can properly accommodate existing or future traf- level of service because of the lack of alternative travel paths fic operations or compare alternative improvement options. and the significant negative consequences of travel delays. On regional freeways and arterials and in densely developed LOS D represents the level at which speeds begin to decline urban areas, LOS D is often considered acceptable because slightly with increasing flows, and density (on freeways and motorists traveling on regional roadway networks can select other roadways with uninterrupted flows) begins to increase alternative travel paths should their preferred path be con- somewhat more quickly. Freedom to maneuver within the gested. However, on airport roadways, where only a single path traffic stream is more noticeably limited (because of the lack of is available (and the cost of delay to the traveler is great), LOS gaps between successive vehicles), and the driver experiences C is typically considered to be the minimum acceptable level of reduced physical and psychological comfort. Even minor service because of the lack of alternative travel paths and the blockages or incidents can be expected to quickly create queues significant negative consequences resulting from travel delays because the traffic stream has little space to absorb disruptions. (e.g., passengers missing their flights).