Evaluating Alternative Operations Strategies to Improve Travel Time ...

SHRP 2 L11: Final Appendices
Figure B.5 - Diurnal Speed and Speed Variability
The Solution
The variability in the a.m. peak period in the northbound direction on the facility generates costs
that are borne by the users of the facility. The cost of the travel-time variability can be converted to
a certainty-equivalent value, which tells us the additional time motorists are willing to spend on the
facility if only the variability in travel times could be eliminated. This certainty-equivalent value of
unreliability can then be converted to real dollar values by applying the user’s value of time.
Data Needs
• High-resolution speed data
• Volume data by vehicle type
Cautions
• Applicable to data for which travel-time variation can be suitably characterized by the lognormal
distribution
Procedure
• The steps involved in the calculation are outlined below:
Step 1 – Characterize the Recurring Congestion Problem
A. Obtain speed data for each 5-minute interval (or other appropriate interval).
B. Calculate the average speed and the standard deviation of the log of speed.
C. Construct log-normal distribution using the log mean and log standard deviation of the
speed to confirm the speed data are log-normally distributed.
Step 1 – Characterize the Recurring Congestion Problem
DETERMINING THE ECONOMIC BENEFITS OF IMPROVING TRAVEL-TIME RELIABILITY Page B-41