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Travel Time Reliability Measures. Shawn Turner Texas Transportation Institute NTOC Web Cast: Travel Time Reliability June 28, 2006. What is travel time reliability?. Consistency or dependability in travel times measured from: Day-to-day; and/or Across different times of day

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travel time reliability measures

Travel Time Reliability Measures

Shawn Turner

Texas Transportation Institute

NTOC Web Cast: Travel Time Reliability

June 28, 2006

what is travel time reliability
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
averages don t tell the full story
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.

communicating the benefits of traffic management
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

reliability measures
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”
90th or 95th percentile travel time
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
understanding reliability
Understanding reliability

SR 520 Eastbound

Seattle, 4-7pm weekdays

buffer index
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

planning time index
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

buffers and planning time
Buffers and planning time

Los Angeles

2003 citywide data shown

percentage of trips on time
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
other measures
Other measures

Standard deviation

Coefficient of variation

  • Okay for internal analyses
  • but difficult to communicate
additional information
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
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