1 Segment Level Analysis of Travel Time Reliability Meead Saberi K., Portland State University I-5...
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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
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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
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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
0
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.