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21 A range of improvements, including demand management, actual travel rate during the off-peak is higher than the target advanced traveler information systems, and HOV lanes, value. The BI and delay measures also could be useful in the have an effect on other hours in the peak period. off-peak period in locations that may be experiencing some Daily volume variation is the variability in person or vehi- congestion in the off-peak. cle volume from day to day. These data are particularly im- portant in analyses that examine mobility and reliability 2.9.2 Daily Analysis levels on particularly heavy volume days (e.g., Fridays or days before holidays) or days/time periods with different Analysis using daily averages is often less useful with the travel patterns (e.g., special events or weekends). TTI and BI. Using 24-hour speeds for computing the TTI Incident information includes the number and duration of is not meaningful because the measure is meant to com- crashes and vehicle breakdowns that occur on roadway pare peak and off-peak travel conditions. Likewise, the BI segments and transit routes. This information is used in is intended to be a measure of reliability during a peak analyses of the variation in mobility and reliability. The re- period. Daily values "wash out" the effect of congestion in liability of transportation systems is a particular concern in peak periods with the longer off-peak periods. Total delay analyses of incident management programs, value pricing is more meaningful as a daily congestion measure. Though projects, and freight movement studies. the total delay in person- or vehicle-hours is less meaning- Weather information can explain a significant amount of ful to an individual driver, it is a good measure for analyz- the variation in travel conditions. Snow, ice, fog, and rain ing trends from year to year. Daily delay is used in this can be noted in a database used for mobility and reliabil- manner in the FHWA-sponsored Mobility Monitoring ity analyses. Program (MMP). Road work information includes construction and main- tenance activities and their location. This includes the lo- 2.9.3 Seasonal Analysis cation, number of lanes affected, and time period. Peak direction hourly travel demand and volume are two Investigating variations in mobility and reliability over the measures of person or vehicle travel used in system analy- seasons of the year also may be of interest. Many areas have ses. The two may be the same for uncongested corridors. unique peaking characteristics due to seasonal events (e.g., Demand is higher than volume in congested corridors, academic calendars, sporting events, and tourism). These ac- however, and the "excess" volume travels on the main tivities can alter the length and extent of the peak period. All route in hours adjacent to the peak hour and on alternate of the measures discussed in this chapter can be used in a routes. Improvements to primary routes or travel modes mobility or reliability analysis that compares peak or off-peak may result in higher traffic volumes in the peak hour that period measure changes by month of year. can be predicted if demand is estimated. 2.9.4 Urban or Rural Analysis 2.9 Time Periods for Analysis The preceding discussion has assumed an urban mobility Selecting the appropriate time period is an important or reliability analysis. Rural locations also can be the subject part of building the data collection plan and analysis frame- of mobility and reliability analyses. For example, there might work. Considerations include the nature of the problem(s) be an interest in freight movements in rural areas. Special to be addressed through the analysis, the geography of the events and tourism activities also are situations that may study area, and the presence of any special seasonal events generate interest in a rural analysis. or conditions that could dramatically alter data or interpre- As mentioned previously, continuous data sources provide tation of results. speed (travel time), volume, and classification information in some urban areas. Point-to-point travel-time information also is of interest for rural freight operations. As with travel 2.9.1 Peak and Off-Peak Period Analysis conditions on an urban congestion map, such point-to-point Peak period is the time period most often used for urban travel-time information would allow insight into rural freight mobility and reliability analyses. Off-peak periods may be of operations. Transponders could be used to provide the interest to study the extent of peak spreading at one area com- continuous information. The University of Washington is in- pared to another area. The TTI is computed relative to the vestigating such applications in rural areas in the state of Wash- FFS or PSL. If the analyst is investigating the TTI of an off- ington. Of the primary measures discussed in this chapter, TTI peak period that is beginning to experience congestion, the and delay measures could be used for this rural application. TTI could be used to illustrate the increased congestion if the The TTI could be used to compare current travel rates to a