Evaluating Alternative Operations Strategies to Improve Travel Time ...

SHRP 2 L11: Final Report
• Disruption Management: measures related to how an agency responds to disruptions in
normal roadway operations (“output” measures).
• Information Dissemination: measures related to how well an agency informs highway
users about current and expected travel conditions in order to improve their ability to
manage their lives and businesses.
Within each of these three categories, a variety of measures are needed to understand the
performance of the roadway and agency. A good performance monitoring system will produce
and use many performance measures. This report only describes the limited number of measures
that could be reported for public presentation, since they are particularly useful in meeting
stakeholder needs. It is expected that agencies that are actively managing their resources will
produce and internally use a large number of additional measures to examine specific
performance issues related to equipment and staff utilization and performance, as well as the
performance of facilities with specific functions (e.g., HOT or other managed lanes). The
performance measures suggested within each of the categories listed above are discussed in the
following subsections. The last subsection presents a summary of the most-useful performance
measures.
Roadway Performance Measures
Performance measures relate to the physical performance of a roadway, considering travel
demand and a variety of factors both within and outside of the roadway agency’s control. Within
physical roadway performance measures, the primary focus is on the fluctuation of travel time
across the year given the demand that occurs. Tracking demand and travel times on a continuous
basis, helps provide a comprehensive picture of the quality of service along a particular facility.
The following five measures are aimed at characterizing roadway performance:
• The mean travel time along defined segments of the roadway system at specified times of
day, days of the week, and times of year;
• The 80 th percentile travel time of defined segments of the roadway system at specified
times of day, days of the week, and times of year;
• The 95 th -percentile travel time of defined segments of the roadway system at specified
times of day, days of the week, and times of year;
• The percentage of time and/or trips that each of those defined segments of the roadway
system operate at lower than a reporting standard adopted by the roadway agency; and
• The traffic volume on defined segments of the roadway system at specified time of day,
day of the week, and time of year.
For reporting purposes, the 95 th percentile travel times can be reported as Buffer Time, Planning
Time, Buffer Time Indices, or Planning Time Indices, depending on the specific question being
answered. The 80 th percentile travel times can also be presented in similar formats. The first
three measures characterize the variability of travel occurring on the roadway system. Three
different aspects of that travel (mean condition, 80 th percentile, and 95 th percentile) are tracked to
describe the variability experienced on the roadway segment under study. The need for three
statistics rather than one relate to the fact that the importance of travel-time reliability changes
(for both freight movers and passenger travel) from trip to trip, depending on the purpose of that
trip. For some trips, arriving on time (prior to some deadline) is extremely important, and
therefore a traveler might plan with the 95 th percentile travel time in mind. For other trips, ontime
arrival is less important. A weekend trip to the mall is an example of a trip for which
information about the mean travel time is adequate. In many commercial situations where
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