travel time reliability...travel time index (tti) travel time reliability indices buffer time index...
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Farid Rahman | Transportation Engineer
June, 2020
Travel Time ReliabilityA Performance Measure in Selecting the Preferred Alternative
Presented at
ITE Western District 2020 Annual Meeting
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Introductions
Project Description
Background
Project
Alternative Selection
Travel Time Reliability
Data
Data Sources
Methodology
Travel Time Forecasting
Results
Reliability Matrices
B/C Analysis
Conclusion
Question/CommentsHighway or Parking Lot?
Agenda for Today
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Defining Travel Time Reliability
• How consistent is the travel time
through the facility?
• How much extra time should be
allocated to reach a destination on
time?
• Simplest measure: nth percentile travel
times (80th, 95th )
• Travel Time Indices: Buffer Time Index
(BTI), Travel Time Index (TTI), Planning
Time Index (PTI), Etc.
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Project Background
• Installing a 12’ wide Part Time
Travel Lane (PTTL) on shoulder
along US 101 SB in Pismo Beach
(~6.5 miles).
• 11 – Southbound On/Off Ramps
• Peak Hour existing count 3,400
veh/hr (SB ADT ~32,000)
• FREQ12 Version 3.02 macro-
simulation modeling software used
to simulate freeway operations
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Intiated
January 2015
(SLOCOG and
Caltrans)
Expected to be
completed early
2021 TBD
Completed
December 2015
TBD
PSR-PDS PSR-PDS PA/EDPS&E/
ROWConstruction
Project Timeline
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• No Build
• Alt 1 - Part Time Shoulder Travel Lane (PTTL) Only
• Alt 2 – PTTL+ Truck Climbing Lane Extension
• Alt 3 – PTTL (Little Longer) + Truck Climbing Lane converted to all-purpose lane
Project Alternatives
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Typical Alternative Selection Used Alternative Selction
• Operational Analysis
• Level of Service
• Improvements to serve future traffic
• Benefit to Cost Analysis
• Savings from Delay
• Costs and benefits with a reduction of
collisions
• Environmental Impact
• Vehicle Miles Traveled
• Right of Way
• Operational Analysis
• Level of Service
• Improvements to serve future traffic
• Travel Time Reliability Matrices
• Benefit to Cost Analysis
• Savings from Vehicle Delay
• Costs and Benefits with a reduction of
collisions
• Savings due to Buffer time reduction
• Environmental Impact
• Vehicle Miles Traveled
• Right of Way
Alternative Selection Method
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Benefit of Using Reliability in Alternative Selection
• Focuses on User Experience
– Users are more tolerant to predictable delays
• Reflects the impact of an Active Traffic Management (ATM) program by
focusing on improvements to mitigate the worst travel experience.
• Buffer time savings supplements Benefit in B/C analysis to make alternatives
more competitive.
• Quantifiable impact on fully-saturated freeways.
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Reliability Measures focuses on the worst user experience rather than
focusing on the average user experience.
Worst Travel Experience
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“ the consistency or dependability in travel times, as
measured from day-to-day and/or across different
times of the day.”
Cambridge, & TTI. (2006, January 1). Travel Time Reliability: Making It There On Time, AllThe Time. Retrieved June 22, 2020, from https://ops.fhwa.dot.gov/publications/tt_reliability/
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Travel Time Index (TTI)
Travel Time Reliability Indices
Buffer Time Index (BTI)
• How much time you should
allocate to make sure you
arrive on time.
Note: For this study free flow travel time is used as the average travel time for the corridor. Free
flow travel time is derived from the 85th percentile speed between Midnight to 3 AM.
• Travel time experienced by
the motorist during peak
travel time.
Buffer Time Index – 80th Percentile
(BTI80)
• FHWA Transportation
Performance Management
Rule (82 FR 5970, 01/2017)
defines Level of Travel Time
Reliability using the 80th
percentile travel time.
https://www.fhwa.dot.gov/tpm/rule/pm3/reliability.pdf
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Data Needs
Probe based travel time data
• Directional
• 5, 10 or 15 min interval
• Peak period to match operation analysis
• Seasonal variation could be considered
• Weekday vs Weekend should be separated
• Free flow travel time 12 AM – 3 AM
Example of Variability in BTI (not US 101)
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Results Existing Conditions
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Travel Time Reliability Forecasting
• University of Florida Travel Time Reliability
Model (HCS 7)
• Need to build additional model
• SHRP II Travel Time Reliability Model by FHWA
• Simplified Approach
• Percent change in TTI as projected by FREQ
between existing condition and future scenarios
• Percent change in TTI used to grow Buffer Time
• Assume non-congestion related incidents,
construction activities and weather in future years
will emulate current condition.
• Similar NCHRP 255 Ratio Method for forecasting
future model volume
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Simplified Approach Discussion
• Availability of Model Outputs for future
scenarios
• Alternative: Use model generated average
travel time, and existing free flow speed to
calculate TTI
• Alternative: Nth percentile travel time can be
calculated from simulated travel times through
the network.
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Results Travel Time Reliability
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Calculating Buffer Time Savings
Calculate Buffer time
per vehicle
Calculate Yearly
Buffer time savings
for all weekdays
(208) and weekends
(157)
Calculate total
monetary savings
due to decrease
buffer time for life
cycle of the project
Calculate Daily
buffer time savings
for all vehicles
Calculate monetary
value of societal
hourly cost
(Weighted Caltrans
average $15.46)
Step 1 Step 2 Step 3 Step 4 Step 5
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Alternative Recommendation by B/C
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Alternative Recommendation by Individual Criteria