Incorporating Reliability Performance Measures into Operations and Planning Modeling Tools: Application Guidelines (2014)

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5Various studies have identified a number of reliability performance measures and pro- vided recommendations on their suitability for different purposes. Lomax et al. (2003) defined three broad categories of reliability performance indicators and discussed a variety of measures based on these concepts: (1) statistical range, (2) buffer time mea- sures, and (3) tardy trip indicators. The authors suggested three specific indicators— percent variation, Misery Index, and Buffer Time Index—as promising measures that provide consistent analytical conclusions. The National Cooperative Highway Research Program (NCHRP) Report 618 (Cambridge Systematics, Inc. et al. 2008) provides guidance on selecting measures for different purposes and types of analyses. The reliability measures recommended by that study include Buffer Index, percent on-time arrival, Planning Time Index, percent variation, and 95th percentile. The SHRP 2 Project L03 (Cambridge Systematics, Inc., et al. 2013) conducted an extensive empirical study and pointed out some shortcomings of the performance metrics recommended by previous studies. For example, the 95th percentile travel time may be too extreme to reflect certain improvements introduced by traffic operations strategies, but the 80th percentile would be useful in such cases. Also, for performance indicators that measure the distance between central and extreme values (e.g., Buffer Index), the median would be a more robust central tendency statistic than the mean because travel time distributions are by nature skewed. Based on such modifications, the study recommended six reliability metrics: Buffer Index, failure/on-time measures, Planning Time Index, 80th percentile Travel Time Index, skew statistic, and Misery Index. Although many previous studies have focused on corridor- or link-level travel time reliability, this project aims to perform a full range of analysis addressing network- level, origin–destination–level (O-D–level), path-level, and segment- or link-level travel 2 TRAVEL TIME RELIABILITY INDICES

6A LEXICON FOR CONVEYING TRAVEL TIME RELIABILITY INFORMATION time reliability using regional planning and operations models. Users need to consider not only the different properties of the reliability measures (as investigated in the stud- ies mentioned in the preceding paragraphs) but also their applicability to an intended analysis level. Table 2.1 presents a list of available reliability measures, categorized on the basis of their applicability to different levels of travel time distributions and associated reliability analysis, namely, network level, O-D level, path/segment level, and link level. For the network level, travel times experienced by vehicles are not directly com- parable because distances traveled by vehicles may be significantly different. In this case, measures that are normalized by the trip distance can be used. Each vehicle’s travel time can be converted into the distance-normalized travel time (i.e., travel time per mile, or TTPM), and various statistics can be extracted from the distribution of TTPMs as presented in Type A measures in Table 2.1. For the O-D level, travel times experienced by vehicles are comparable—although actual trip distances could be dif- ferent depending on the route followed by each vehicle. The O-D–level travel times are not limited to travel times between actual traffic analysis zones (TAZ). Travel time distributions between any two points can be included in this category. Reliability mea- sures that can be used when travel times are comparable include many conventional metrics, such as the mean and standard deviation of travel times, percentiles, and the Buffer Index, as presented in Type B in Table 2.1. For O-D–level analysis, therefore, both Type A and Type B measures can be used. At the path/segment/link level, not only are the travel times for different vehicles comparable but trip distances are also the same. This allows the calculation of the unique free-flow travel time for a given path and, therefore, allows the use of additional measures that require the free-flow travel time. Such measures include Travel Time Index, Planning Time Index, Misery Index, and frequency of congestion as shown in Type C in Table 2.1. Thus, users can use any of Type A, B, and C measures for the path/segment/link-level travel time reli- ability analysis.

7A LEXICON FOR CONVEYING TRAVEL TIME RELIABILITY INFORMATION TABLE 2.1. RELIABILITY MEASURES FOR DIFFERENT ANALYSIS TYPES Analysis Level Network O-D Path/Segment/Link Characteristic Travel times for vehicles Not comparable Comparable Comparable Travel distances for vehicles Different Different Identical Applicable Measures Distance- normalized measures (Type A) • Mean of travel time per mile (TTPM) • Standard deviation of TTPMs • 95th/90th/80th percentile TTPM Measures for comparable travel times (Type B) • Average travel time Standard deviation of travel times • Coefficient of variation Standard deviation of travel times/mean travel time • 95th/90th/80th percentile travel time • Buffer Index (95th percentile travel time – mean travel time)/mean travel time • Skew Index (90th percentile travel time – median travel time)/(median travel time – 10th percentile travel time) • Percent on-time arrival Percentage of travel times < 1.1 × median travel time Measures for the same travel distance (Type C) • Travel Time Index (TTI) Mean travel time/free-flow travel time • Planning Time Index (PTI) 95th percentile travel time/free-flow travel time • Misery Index Mean of the highest 5% of travel times/free- flow travel time • Frequency of congestion Percent of travel times > 2 × free-flow travel time