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Segment Level Analysis of Travel Time

Reliability

Segment Level Analysis of Travel Time

Reliability

Meead Saberi K., Portland State University

I-5 SB, San Diego, CA

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ObjectivesObjectives

How can we measure travel time reliability?

How does reliability vary across freeway segments?

Identifying the locations and times in which travel time reliability drops

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Study AreaStudy Area

I-5 Northbound

Portland, OR

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MethodologyMethodology

Breaking the overall I-5 NB into shorter segments: 22 segments from milepost 286.1 to 307.9

5-minute aggregated travel times for February and March 2007 from PORTAL

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Definitions of ReliabilityDefinitions of Reliability

The consistency or dependability in travel times from day to day (1)

The variability between the expected travel time and the actual travel time (2)

The variability and predictability of travel times (3)

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What is TT Reliability?What is TT Reliability?

0

2

4

6

8

10

12

2/1/2007 2/12/2007 2/23/2007 3/7/2007 3/18/2007 3/30/2007

Time (January and February 2007)

Tra

vel

Tim

e

Average Travel Time

Departure time: 17:00

7

0

2

4

6

8

10

12

2/1/2007 2/12/2007 2/23/2007 3/7/2007 3/18/2007 3/30/2007

Time (January and February 2007)

Tra

vel

Tim

e

Average Travel Time

What is TT Reliability?What is TT Reliability?

Departure time: 17:00

8

0

2

4

6

8

10

12

2/1/2007 2/12/2007 2/23/2007 3/7/2007 3/18/2007 3/30/2007

Time (January and February 2007)

Tra

vel

Tim

e

Average Travel Time

What travelers remember ...

What is TT Reliability?What is TT Reliability?

Departure time: 17:00

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Measures of Reliability Measures of Reliability 90th or 95th Percentile Travel Time Coefficient of Variation: Std/Mean

Travel Time Index: Mean/FFTT

Buffer Index: (%95th - Mean)/Mean

Planning Time Index: %95th/FFTT

Congestion Frequency Lognormal Distribution On-Time Travel: Γ(1.1*Mean)

Misery Index: Γ-¹ (%85th)/Mean - 1

(1)

(1)

(1)

(1)

(1)

(1)

(4)

(5)

(5)

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Coefficient of VariationCoefficient of Variation

Coeffi

cient o

f V

aria

tion

Time

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Travel Time IndexTravel Time IndexTra

vel T

ime In

dex

Time

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Buffer Time IndexBuffer Time IndexB

uff

er T

ime In

dex

Time

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Planning Time IndexPlanning Time IndexPla

nn

ing T

ime In

dex

Time

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Congestion FrequencyCongestion FrequencyC

ongestio

n

Frequency

TimeCongested speed threshold: 30

mph

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Congestion FrequencyCongestion FrequencyC

ongestio

n

Frequency

Time

Sensitivity AnalysisSensitivity Analysis

Congested speed threshold: 35 mph

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Congestion FrequencyCongestion FrequencyC

ongestio

n

Frequency

Time

Sensitivity AnalysisSensitivity Analysis

Congested speed threshold: 40 mph

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Congestion FrequencyCongestion FrequencyC

ongestio

n

Frequency

Time

Sensitivity AnalysisSensitivity Analysis

Congested speed threshold: 45 mph

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Lognormal DistributionLognormal Distribution

y = 0.5091Ln(x) - 0.1326

R2 = 0.9801

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8 9 10 11

Travel Time (minute)

Lo

gn

orm

al P

rob

abili

ty

Departure time: 5 pm

Average TT = 4:05 min

90%th TT = 7:36 min

Milepost 305.12

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Lognormal DistributionLognormal Distribution

Departure time: 8 am

Average TT = 1:08 min

90%th TT = 1:11 min

Milepost 305.12

y = 11.525Ln(x) - 1.0531

R2 = 0.9735

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0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.09 1.11 1.13 1.15 1.17 1.19 1.21 1.23

Travel Time (minute)

Lo

gn

orm

al P

rob

abili

ty

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ConclusionsConclusions

Travel time reliability substantially varies across freeway segments.

Arbitrary thresholds affect the reliability estimates.

Using the segment travel time reliability measures in regional transportation planning and operations is beneficial for evaluating and prioritizing roadway segments (corridor-level analysis is not sufficient).

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Next StepsNext Steps

Studying the relationship between the freeway corridor reliability and the reliability of its segments (a probability-based approach).

Studying the relationship between travel time reliability and the variability of flow and speed

How does reliability vary across lanes? Which reliability measure will actually help

managing our freeways? Developing a model to estimate more reliable

travel times for a corridor based on the historical segment travel times, flow and speed variability in each segment.

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AcknowledgmentAcknowledgment

Prof. Robert L. Bertini Priya Chavan PORTAL

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Questions?Questions?Questions?Questions?

Thanks

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ReferencesReferences

1. FHWA. Travel Time Reliability: Making It There On Time, All the Time.

2. Lomax, T., S. Turner, M. Hallenbeck, C. Boon, R. Margiotta, and A. O’Brien. Traffic Congestion and Travel Reliability: How Bad Is the Situation and What Is Being Done About it? FHWA.

3. Tu, H., Ir. J. W. C., van Lint, H. J., van Zuylen. Travel Time Reliability Model on Freeways. 87th annual meeting of the Transportation Research Board, Washington D.C., 2008.

4. Emam, E. B., H. Al-Deek. Using Real-Life Dual-Loop Detector Data to Develop New Methodology for Estimating Freeway Travel Time Reliability. Transportation Research Record, No. 1959, Washington D.C., 2006, pp. 140-150.

5. NCHRP. Cost-Effective Performance Measures for Travel Time Delay, Variation, and Reliability. Report No. 618.