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19 field studies, or can be estimated using transit schedules or expansions, operating improvements, program enhance- empirical relationships (e.g., Bureau of Public Roads ments, or policy implementations. The facility/mode target formula) between traffic volume and roadway characteris- travel rates can be used for evaluation, but improvement tics (e.g., capacity). strategies and amounts should be based on corridor-level decisions. Travel Time Travel Rate ( minutes ) 60 (minutes = = (Eq. 2.13) 2.7 Definition and Discussion per mile) Segment Length Average Speed of Speed Terms ( miles ) ( mph ) This section provides definitions of primary speed meas- Person volume is the number of people traversing the ures and guidance on their use in mobility and reliability segment being studied. The person volume can be collected analyses. for each travel mode or estimated using average vehicle FFS is the average speed that can be accommodated under occupancy rates for different types or classes of vehicles. relatively low traffic volumes (i.e., no vehicle interactions) Freight volume is the amount of goods moved on a on a uniform roadway segment under prevailing roadway transport segment or system. It can be measured in units of and traffic conditions. It can be calculated or estimated in a ton-miles if the data are available, or it can be described more number of ways, with a common approach being to use the simply from truck percentages in the traffic stream. Freight 85th percentile speed in the off-peak period. The off-peak volume may be particularly important in analyses dealing periods can be defined by time period (e.g., overnight = with travel-time reliability due to the sensitive nature of 10:00 p.m. to 6:00 a.m., or midday = 9:00 a.m. to 4:00 p.m.) "just-in-time" manufacturing processes and goods delivery or vehicle volume. Vehicle headways of 5 seconds or more services. could be used to define FFS operating conditions (i.e., traffic Person-miles of travel is the magnitude of travel on a sec- volumes of approximately 700 vehicles per hour per lane tion of the transportation system or on several elements of [vphpl]). Ideally, a continuous data source (e.g., ITS, the system. It is a particularly useful measure in corridor Weigh-in-motion [WIM], Automatic Traffic Recorder and areawide analyses where total travel demand is used in [ATR], etc.) could be used to identify the FFS using at least calculations. Equation 2.14 indicates it is the product of one year of valid data. distance and person volume. Person volume can be esti- PSL is the posted speed of the roadway. For specific facilities mated as the product of vehicle volume and average vehi- or sections thereof this value is obtained by field data collec- cle occupancy. tion. Posted speed is a typical roadway inventory data element; Person-miles Person Volume Distance therefore, posted speeds can be obtained from such roadway = ( persons ) ( miles ) (Eq. 2.14) of Travel ( PMT ) inventories, particularly for a system-level analysis that includes numerous facilities. Target travel time (in minutes) is the time that indicates a Target speed is the speed associated with the target TTI. system or mode is operating according to locally determined The target speed can be computed given the target TTI and performance goals. It focuses on the door-to-door trip time the free-flow travel rate or the PSL travel rate. from origin to destination. The target travel time can be differentiated by the purpose of the travel, the expectation for 2.7.1 Threshold Speed Values each mode within the transportation system, the type of activity, and the time of day. It should be influenced by com- Many analyses begin with the question, "What should we munity input, particularly on the issue of the balance between compare to?" The issue usually can be framed as a choice transportation quality, economic activity, land use patterns, between using a desirable condition or using an achievable and environmental issues. condition given the funding, approval, and other con- Target travel rate (in minutes per mile) is the maximum rate straints. (slowest speed) a segment is traversed or a trip is completed It should be noted that PSLs are included in most without experiencing an unsatisfactory level of mobility. The roadway inventory files and should be readily available for target travel rate is based on factors similar to the target travel analytical procedures. Computerized analysis procedures time. This is similar to the process used by many states and should be modified so that a negative delay value is not in- cities where a target level of service (LOS) is used to determine cluded in the calculations. If estimated FFS are used in the the need for additional transportation improvements. calculation of delay, the speed data collected from field In practice, there also will be a need for a corridor average studies may include values with very fast speeds (above the travel rate value. This would be used as the target for facility FFS). FFSs higher than the PSL may present an illegal