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18 Misery Index seeks to measure the length of delay of only the between desired conditions and the conditions being ana- worst trips. The metric is computed by subtracting the average lyzed. Variability refers to both regular and irregular changes travel rate from the upper 10 percent (or 15 or 20 percent) of in the other three components, and is a distinguishing com- travel rates. This yields the time difference (as a proportion or ponent of reliability measures versus mobility measures. If percent) between the average trip and the slowest 10 percent of enough is known about the variation in these other three trips. It is computed according to the following formula: components, for example, knowing the statistical distribu- tion of travel times on a given facility, then reliability meas- Mean(Top20%Times) ures can be calculated that indicate, for example, the likeli- MI = -1 (Eq. 2.12) hood of arriving on time, the incremental amount of time MeanTime required to be on time 95 percent of the time, etc. where MI = Misery Index; 2.6 Basic Data Elements Mean(Top20%Times) = The mean of the highest 20 per- cent of measured travel times; This section describes the basic data elements used to de- and fine the mobility and reliability measures described previ- MeanTime = The computed mean of the ously. The units are noted for typical urban analyses. measured travel time. Travel time (in minutes) is the time required to traverse a seg- For example, if the mean travel time of the slowest 20 percent ment or complete a trip. Times may be measured directly using of trips in a corridor is 90 minutes and the mean travel time of field studies or archived data from traffic management centers, all trips in the same corridor is 60 minutes, the Misery Index is or can be estimated using empirical relationships with traffic vol- calculated as (90/60) 1, or 1.5 1.0 = 0.5 (i.e., the slowest trips ume and roadway characteristics, computerized transportation are 50 percent longer than the average trip). network models, or the projected effects of improvements. Exhibit 2.5 summarizes key characteristics of the primary Segment or trip length (in miles) is the distance associated mobility and reliability measures described in this section. with the travel time. Length can be measured directly with a The "components of congestion" have been defined as vehicle odometer or scaled from accurate maps but is typically duration, extent, intensity, and variability or variation (2). an established item in a transit or roadway inventory database. Duration is the length of time during which congestion Average speed (in miles per hour) for a segment can be affects the system or facility. Extent can describe either the used to calculate travel rate or travel times if field data are not geographic distribution of congestion, or the number of readily available. people/vehicles/freight-tons affected by congestion. Intensity Average travel rate (in minutes per mile) is the rate a is the severity of the congestion, preferably from the traveler's segment is traversed or a trip is completed (Equation 2.13). perspective, and is frequently expressed as the difference Travel rates may be determined directly using travel-time Congestion Performance Measure Component Addressed Geographic Area Addressed Delay per Traveler Intensity Region, Subarea, Section, Corridor Travel-Time Index Intensity Region, Subarea, Section, Corridor Buffer Index Intensity, Variability Region, Subarea, Section, Corridor Planning Time Index, Percent Variation Intensity, Variability Region, Subarea, Section, Corridor Percent On-Time Arrival Variability Facility, Corridor, System Total Delay Intensity Region, Subarea, Section, Corridor Congested Travel Extent, Intensity Region, Subarea Percent of Congested Travel Duration, Extent, Intensity Region, Subarea Congested Roadway Extent, Intensity Region, Subarea Misery Index Intensity, Variability Region, Subarea, Corridor Accessibility Extent, Intensity Region, Subarea Exhibit 2.5. Key characteristics of mobility and reliability measures.