Evaluating Alternative Operations Strategies to Improve Travel Time ...

SHRP 2 L11: Final Appendices
Determining the Economic Benefits of Improving Travel-Time Reliability
The purpose of this appendix is to present an approach for determining the economic benefits of
improving travel-time reliability under recurring-event scenarios. In the approach, uncertainty
arising from recurring events is converted to a certainty-equivalent measure in order to use
conventional evaluation methods to place a value on the cost of unreliability. The certaintyequivalent
measure is a method that allows us to express the value of reliability in terms of the
increase in the average travel time a person would accept to eliminate uncertainty. For example,
suppose that Mr. A is told to draw a card from a full deck. If he draws a red card he wins $100, and
if he draws a black card he wins nothing. Based on the likelihood of winning or losing, if Mr. A
could be paid $50 to not play the game, then $50 would be his certainty-equivalence. Thus, Mr. A
has placed a dollar value on removing unreliability. Mr. A has foregone the chance at $100, but
has also avoided the equal possibility of receiving no payment. Thus, a variable that can be
characterized by its probability distribution can be converted to a certainty-equivalent measure.
The approach presented for determining the value of reliability associated with recurring is
extended to rare events in Appendix C. Recurring and rare events (and the unreliability produced
by them) differ in the probability distributions that characterize them. Unreliability produced by
recurring events is assumed to display statistical behavior that is best represented by normal
distributions (often, a log-normal distribution). Rare or “extreme” events are better represented by
a class of distributions known as extreme value (EV) distributions. It is often possible to observe
the effect of recurring events on network performance directly by observing the variability in
speeds or other network performance metrics. In the case of unreliability caused by rare events, it
may not be possible to observe the statistical nature of the unreliability in real-world data, because
the number of such incidents in a given region may be so low that coincident performance
measurements are lacking. The rare event approach is briefly described in this appendix and is
presented in full in Appendix C.
Once the certainty-equivalent measure has been computed, valuation can proceed as if the values
involved were deterministic. The value of reliability can be derived for multiple user groups or
market segments by applying a separate value of time that corresponds to each user group along
with the observed average volume for each user group on the roadway segment. The total value of
reliability is then computed as the sum of the reliability values for each user group on the highway
segment.
To illustrate this concept, let us consider a single link of a roadway network that experiences
considerable variation in the average travel time due to recurring congestion. Imagine a commuter
would be willing to spend an additional minute per mile if the uncertainty in travel time were
eliminated by a technical or policy action. This additional minute per mile is the certaintyequivalent
of the unreliability caused by a recurring source of unreliability. The cost of
unreliability to the commuter, converted to the certainty-equivalent measure, can be monetized by
the value that he or she places on each minute of additional delay. Most studies report the value of
time as being between 40 and 50 percent of the wage rate for average trips, and a much higher rate
(around 85 percent of the wage rate) for commuting and intercity trips. The value of time for
freight movement depends on the size of truck and value of the commodity hauled, typically with a
value ranging between $30 and $60 per hour.
Using the above mentioned certainty-equivalent value of 1 minute per mile and assuming that the
average value of time of users of the roadway is $20 per hour (33.3 cents per minute), the roadway
segment is 10 miles long, and it is used by 6000 users per hour during the time of day when
DETERMINING THE ECONOMIC BENEFITS OF IMPROVING TRAVEL-TIME RELIABILITY Page B-2