Farid Rahman | Transportation Engineer

June, 2020

Travel Time ReliabilityA Performance Measure in Selecting the Preferred Alternative

Presented at

ITE Western District 2020 Annual Meeting

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

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.

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

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

• 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

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

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.

Reliability Measures focuses on the worst user experience rather than

focusing on the average user experience.

Worst Travel Experience

“ 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/

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

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)

Results Existing Conditions

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

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.

Results Travel Time Reliability

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

Alternative Recommendation by B/C

Alternative Recommendation by Individual Criteria

Questions? Comments?

Faridur.Rahman@GHD.com

T: +1 805 858 3127