Travel Time Reliability MeasuresTravel Time Reliability Measures

Shawn TurnerShawn Turner

Texas Transportation InstituteTexas Transportation Institute

NTOC Web Cast: Travel Time ReliabilityNTOC Web Cast: Travel Time Reliability

June 28, 2006June 28, 2006

What is travel time reliability?What is travel time reliability?

• Consistency or dependability in travel Consistency or dependability in travel times measured from:times measured from:– Day-to-day; and/or Day-to-day; and/or – Across different times of dayAcross different times of day

• Not focused on typical delay so much Not focused on typical delay so much as unexpected delayas unexpected delay

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

July travel time

(minutes)

November (minutes)

December

(minutes)

Monday 14 21 22

Tuesday 13 20 19

Wednesday 13 19 13

Thursday 13 19 20

Friday 14 20 24

Average

(range)

13.4

(13 to 14)

19.8

(19 to 21)

19.6

(13 to 24)

Averages don’t tell the full storyAverages don’t tell the full story

Jan. Dec.July

Traveltime

How traffic conditions havebeen communicated

Annual average

Jan. Dec.July

Traveltime

What travelers experience

Travel times varygreatly day-to-day

What theyremember

Communicating the benefits of traffic Communicating the benefits of traffic managementmanagement

When Mn/DOT’s ramp meters were turned off in 2000:

22-percent decline in average travel times

91-percent decline intravel time reliability

Traveltime

Before After

Avg. day

Small improvement inaverage travel times

Larger improvement intravel time reliability

Traveltime

Before After

Worst dayof month

Reliability measuresReliability measures

• Based on estimates or direct Based on estimates or direct measurement of travel timemeasurement of travel time– 90th- or 95th-percentile travel time90th- or 95th-percentile travel time– Buffer indexBuffer index– Planning time indexPlanning time index– Percentage of trips “on time”Percentage of trips “on time”

90th90th-- or 95th or 95th--percentile travel timepercentile travel time

• Definition: Travel time on some of the Definition: Travel time on some of the heaviest traffic daysheaviest traffic days

• Reported as time (minutes)Reported as time (minutes)

• Calculate using database or Calculate using database or spreadsheet functionspreadsheet function

Understanding reliabilityUnderstanding reliability

0

50

100

150

200

250

300

350

10 15 20 25 30 35

Travel time (minutes)

Num

ber

of

trip

s

95th percentile =22.7 minutes

Avg. =15.9 minutes

Free-flow = 11.5 minutes

Buffer

SR 520 EastboundSeattle, 4-7pm weekdays

Buffer indexBuffer index

• Definition: Definition: Extra timeExtra time (or time cushion) that (or time cushion) that travelers should add to their average travel travelers should add to their average travel time to ensure on-time arrivaltime to ensure on-time arrival

• Buffer index (%) = Buffer index (%) =

• Buffer time (minutes) = Buffer time (minutes) =

buffer index × average travel timebuffer index × average travel time

95th95th-- percentile time percentile time ̶ ̶ average time average time

average travel timeaverage travel time

Planning time indexPlanning time index

• Definition: Definition: Total timeTotal time a traveler should allow a traveler should allow to ensure on-time arrivalto 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 timeplanning time index × ideal or free-flow travel time

95th-percentile travel time95th-percentile travel time

Ideal or free-flow travel timeIdeal or free-flow travel time

Buffers and planning timeBuffers and planning time

1.00

1.20

1.40

1.60

1.80

2.00

2.20

3 A 5 A 7 A 9 A 11 A 1 P 3 P 5 P 7 P 9 P 11 P

Time of Weekday

Ind

ex

Va

lue

Travel time index

Planning time index

Buffer

Los Angeles2003 citywide data shown

Percentage of trips “on time”Percentage of trips “on time”

• Definition: The percentage of trips that Definition: The percentage of trips that are completed within a designated are completed within a designated threshold travel time (e.g., average threshold travel time (e.g., average travel time + 20 percent)travel time + 20 percent)

• Same concept as on-time reliability in Same concept as on-time reliability in air travelair travel

Other measuresOther measures

Standard deviationCoefficient of variation

• Okay for internal analyses but difficult to communicate

Additional informationAdditional information

• January 2006 report:January 2006 report:– Travel Time Reliability: Making It There On Travel Time Reliability: Making It There On

Time, All The TimeTime, All The Time– http://www.ops.fhwa.dot.govhttp://www.ops.fhwa.dot.gov

• Contacts:Contacts:– Rich Taylor, FHWA, 202-366-1327Rich Taylor, FHWA, 202-366-1327– Shawn Turner, TTI, 979-845-8829Shawn Turner, TTI, 979-845-8829

QuestionsQuestions