Travel Time Reliability Measures - PowerPoint PPT Presentation

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Travel Time Reliability Measures

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  1. Travel Time Reliability Measures Shawn Turner Texas Transportation Institute NTOC Web Cast: Travel Time Reliability June 28, 2006

  2. What is travel time reliability? • Consistency or dependability in travel times measured from: • Day-to-day; and/or • Across different times of day • Not focused on typical delay so much as unexpected delay

  3. Hypothetical commute(free-flow travel time = 12 minutes)

  4. Averages don’t tell the full story How traffic conditions have been communicated Annual average Travel time Jan. July Dec. What travelers experience What they remember Travel time Travel times vary greatly day-to-day Jan. July Dec.

  5. Communicating the benefits of traffic management Small improvement in average travel times Larger improvement in travel time reliability Travel time Travel time Worst day of month Avg. day Before Before After After When Mn/DOT’s ramp meters were turned off in 2000: 22-percent decline in average travel times 91-percent decline in travel time reliability

  6. Reliability measures • Based on estimates or direct measurement of travel time • 90th- or 95th-percentile travel time • Buffer index • Planning time index • Percentage of trips “on time”

  7. 90th- or 95th-percentile travel time • Definition: Travel time on some of the heaviest traffic days • Reported as time (minutes) • Calculate using database or spreadsheet function

  8. Understanding reliability SR 520 Eastbound Seattle, 4-7pm weekdays

  9. Buffer index • Definition: Extra time (or time cushion) that travelers should add to their average travel time to ensure on-time arrival • Buffer index (%) = • Buffer time (minutes) = buffer index × average travel time 95th- percentile time ̶ average time average travel time

  10. Planning time index • Definition: Total time a traveler should allow to ensure on-time arrival • Planning time index = • Results in index value >= 1 • Planning time (minutes) = planning time index × ideal or free-flow travel time 95th-percentile travel time Ideal or free-flow travel time

  11. Buffers and planning time Los Angeles 2003 citywide data shown

  12. Percentage of trips “on time” • Definition: The percentage of trips that are completed within a designated threshold travel time (e.g., average travel time + 20 percent) • Same concept as on-time reliability in air travel

  13. Other measures Standard deviation Coefficient of variation • Okay for internal analyses • but difficult to communicate

  14. Additional information • January 2006 report: • Travel Time Reliability: Making It There On Time, All The Time • http://www.ops.fhwa.dot.gov • Contacts: • Rich Taylor, FHWA, 202-366-1327 • Shawn Turner, TTI, 979-845-8829

  15. Questions