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: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 delayNot focused on typical delay so much as unexpected delay

3. Hypothetical commute (free-flow travel time = 12 minutes) July travel time (minutes) November (minutes) December (minutes) Monday142122 Tuesday132019 Wednesday131913 Thursday131920 Friday142024 Average (range) 13.4 (13 to 14) 19.8 (19 to 21) 19.6 (13 to 24)

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

5. Communicating the benefits of traffic management 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 Travel time Before After Avg. day Small improvement in average travel times Larger improvement in travel time reliability Travel time Before After Worst day of month

6. Reliability measures Based on estimates or direct measurement of travel timeBased 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 daysDefinition: Travel time on some of the heaviest traffic days Reported as time (minutes)Reported as time (minutes) Calculate using database or spreadsheet functionCalculate 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 arrivalDefinition: Extra time (or time cushion) that travelers should add to their average travel time to ensure on-time arrival Buffer index (%) =Buffer index (%) = Buffer time (minutes) =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 arrivalDefinition: Total time a traveler should allow to ensure on-time arrival Planning time index =Planning time index = Results in index value >= 1Results in index value >= 1 Planning time (minutes) =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)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 travelSame 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:January 2006 report: –Travel Time Reliability: Making It There On Time, All The Time –http://www.ops.fhwa.dot.gov Contacts:Contacts: –Rich Taylor, FHWA, 202-366-1327 –Shawn Turner, TTI, 979-845-8829

15. Questions