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20 appearance problem when used in public discussions. In for its transportation system, land uses, and its quality of life addition, it may be difficult to justify delay being calculated issues and 2) the improvement strategies, programs, and for travel at the PSL. projects that government agencies and private sector interests can implement. Planners can use the targets to identify prob- lem areas and judge which strategy meets both objectives. The 2.7.2 When Would I Use Free-Flow Speed values are desirably the result of a process integrated with in Mobility and Reliability Measure the development of the long-range plan, but they must be Computation? reasonable and realistic since overstatement or understate- Delay and congestion index measure computations can ment could distort the assessment of congestion. be computed relative to FFS. Using FFS for these computa- Urban areas should approach the use of a target TTI with a tions is most appropriate when continuous data sources corridor and system strategy. The target value may be devel- that allow for the computation of the 85th percentile speed oped for every mode or facility as a way to identify individual in the off-peak period are available. The use of a FFS pro- performance levels, but the key application will be as a corri- vides an automated and consistent method for computing dor or system target. Individual facility deficiencies can be delay and index values across different metropolitan areas. addressed through improvements to that mode or route or by The FFS also could be used when the analyst does not have other travel mode improvements, strategies, or policies. For ready access to posted speeds along the corridors included example, the freeway main lanes may not satisfy the target in a mobility and reliability analysis, particularly large value, but if an HOV lane is successful in moving a large num- areawide analyses. ber of people at high speeds, the average TTI, when weighted by person volume, may achieve the target value. Target TTI value can be "adjusted" appropriately irre- 2.7.3 When Would I Use a Posted Speed spective of whether a FFS or a PSL is used in the calculation Limit in Mobility and Reliability of the TTI. For example, if FFS is used, the target TTI value Measure Computation? might be 1.4, whereas the target TTI value might be 1.3 if the PSL also can be used to compute delay and index measures. PSL is used. PSL can be used when continuous data are not available for the mobility or reliability analysis. PSLs are an easy to com- 2.7.6 Summary and Guidance municate threshold, are more stable than FFSs, and do not require value judgments of assessments of goals or targets. FFS is better for matching how people drive given the roadway operating conditions (i.e., "I was traveling 5 mph over the PSL, and I was still being passed"). PSLs are some- 2.7.4 How Can the `Maximum Productivity' times set for public policy reasons, rather than being tied to Concept be Used? actual conditions making comparisons between regions or Maximum productivity (i.e., the combination of relatively comparisons over several years difficult. PSLs could go high vehicle volume and relatively high speed that provides down, reducing the apparent delay, and yet if peak-period the most efficient roadway operation) is a goal for traffic man- speeds declined, which should show more congestion, there agement professionals. It is gaining a useful place in commu- could be less reported delay. nication with nontechnical audiences as the target for agencies These considerations should be evaluated when determin- moves from free-flow travel to reliable service at all times. This ing the appropriate reference (FFS or PSL) in delay and index target reflects the general acceptance of congestion for a few computations for the community and stakeholders involved hours each day in major metropolitan regions. The concept with the analysis. can be used in the same manner that a target condition is implemented by both planners and operators. 2.8 Other Data Elements Several other factors are needed to perform mobility and 2.7.5 How Does the Target Travel-Time reliability analyses, including the following: Index Relate to the Computed Measures? Hourly volumes, expressed in vehicles or persons, may be Target TTI values could be developed with input from very useful for the peak period or 24-hour periods. Many citizens, businesses, decision makers, and transportation roadway and transit analyses focus on the peak hour, but in professionals. The target values represent the crucial link be- most large cities this is not enough information to assess the tween two objectives: 1) the vision that the community has mobility and reliability situation or to analyze alternatives.