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17 Total Delay Actual FFS orPSL Vehicle Volume VehicleOccupanc cy 1hour = Travel Time - Travel Time (Eq. 2.7) (persons - hours) (vehicles) (persons /vehicle) (minutes) (minutes) 60minutes Congested Travel Congested Vehicle Volume = SegmentLength (vehicles) (Eq. 2.8) (vehicle - miles) (miles) m Actual Travel FFS or PSL Vehicle Vehicle Time i - Travel Timei Volumei Occupancy i Percent of i =1 ( minutes ) ( minutes ) ( vehicles ) ( persons/vehicle ) Each congested segment Congested = 100 Vehicl e Vehicle (Eq. 2.9) Travel n Actual Travel Ratei Length i Volumei Occupancy i i =1 ( minutes per mile ) ( miles ) ( vehicles ) ( persons/vehicle ) All segments also is difficult to combine route or trip travel times into a of road, hours per 1,000 miles traveled, or hours per 1,000 subarea or citywide average. travelers might be more meaningful to agencies at the corri- Several other statistical measures of variability have been dor level, but the public may not understand these measures suggested to quantify travel-time reliability, such as standard since it is difficult to relate to key travel decisions or travel deviation and coefficient of variation. These are discouraged experience. as performance measures, as they are not readily understood Congested travel is a measure that captures the extent of by nontechnical audiences nor easily related to everyday congestion. It estimates the extent of the system affected by commuting experiences. The 90th or 95th travel time, or the congestion. Equation 2.8 illustrates the computation of indexes such as the BI or Planning Time Index, are recom- congested travel in vehicle-miles as the product of the con- mended as simpler ways to express the variability of expected gested segment length and the vehicle volume summed across travel time in a way that travelers can relate more directly to all congested segments. their travel expectations or experience. The percent of congested travel is an extension of the con- gested travel measure. It also measures the extent of congestion. When speed and occupancy data are available for each roadway 2.5 Area Measures segment, this measure can be computed. It is computed as the The mobility and reliability measures described in the pre- ratio of the congested segment person-hours of travel to the total vious section mainly relate to the individual traveler making person-hours of travel. Equation 2.9 shows the computation. a particular trip. The measures described in this section are Congested roadway is another measure of the extent of area measures where the area may be a corridor or region. congestion. It is the sum of the mileage of roadways that op- These measures may be better suited to large scale system erate under free-flow or posted speed limit conditions. This planning analysis. is shown in Equation 2.10. The total delay (in person- or vehicle-hours) for a transit or roadway segment is the sum of time lost due to congestion. Congested Roadway Congested Segment (Eq. 2.10) = Delay can be expressed as a value relative to free-flow travel or ( miles ) Lengths ( miles ) relative to the posted speed limit. Total delay in a corridor or Accessibility is a measure that often accompanies mobility an urban area is calculated as the sum of individual segment measures. It quantifies the extent that different opportunities delays. This quantity is used to estimate the impact of im- can be realized. These might be accessibility to jobs, a transit provements on transportation systems. The values can be used station, or other land use or trip attractor of interest. Acces- to illustrate the effect of major improvements to one portion sibility is satisfied if the travel time to perform the desired ac- of a corridor that affects several other elements of the corridor. tivity is less than or equal to the target travel time as indicted The quantity is particularly useful in economic or benefit/cost in Equation 2.11. analyses that use information about the magnitude of the mo- bility improvement for cost-effectiveness decisions. Objective Fulfillment Accessibility Opportunities ( e.g., jobs ) , Where (Eq. 2.11) Equation 2.7 shows the computation of delay in person- (opportunities )= Travel Time Target Travel Time hours. In addition, using a delay measure of hours per mile