Evaluating Alternative Operations Strategies to Improve Travel Time ...

SHRP 2 L11: Final Appendices
MPOs and other agencies to evaluate network improvements because they are pure mathematical
constructs rather than simulation models, per se. They can be considered to be sketch models as
opposed to regional-model implementations.
In this regard, these mathematical models may not be fully faithful to real-world networks, but
offer a way to evaluate generic policies in abstract network representations. Examples of these
models include Clark and Watling (2005), Kaparias et al. (2008), Iida (1999), and Bell and Iida
(2003). Unlike many regional models implemented for project evaluation, these mathematical
models may capture system reliability issues in a way that can guide policy. Depending upon the
context, these models can be used to evaluate either reliability issues associated with recurring
events or with rare events. The impacts on reliability can be measured on a few links or network
wide.
None of these three methods directly measures impacts beyond those that occur on the network
itself. Broader economic impacts must be represented, if appropriate, by other tools that link
transportation infrastructure to regional economic viability.
Quantifying the Value of Time
The dollar value of reliability is determined by multiplying the certainty-equivalent penalty
(measured in minutes per mile) by the value of time. The value of reliability can be derived for
multiple roadway user groups by applying a separate value of time that corresponds to each user
groups, 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 roadway segment.
A literature review on the value of time was conducted to determine the value of time for
passenger travelers and for freight movers. The literature that concerns the value of time has been a
productive topic for research. The more-useful measures of time value have been developed from
mode-choice or path-choice (route-choice) studies based on household/shipper surveys or from
studies of traveler behavior on facilities such as high-occupancy toll (HOT) lanes.
Though the value of travel time may be referred to as an average value, the value of travel time is
recognized to take on a range of values that depend on user income and other demographic
characteristics (in the case of passenger travel) or on operations and shipment characteristics (in
the case of freight travel). The value of time may also depend on factors such as the trip purpose,
time of day, or travel conditions.
Value of Time Methodology and Data
Value of time modeling and estimation dates back to at least the 1960s, where early empirical
research focused on traveler mode choice to determine the value of time by looking at the marginal
rate of substitution between travel time and cost across mode alternatives. In addition to mode
choice, route choice and, to a lesser extent, housing choice models have also been developed to
help practitioners understand traveler behavior and decisions with respect to travel time, travel
cost, and residence and work location (Small 1992).
When revealed preference data has been difficult to obtain, the alternative for researchers has been
to conduct stated-preference surveys. Stated preference surveys ask respondents to make travel
decisions for hypothetical scenarios in which the alternatives in each scenario have different travel
times, travel costs, or other trip attributes. Brownstone and Small (2005) note the difficulty in
creating realistic scenarios and accurately presenting the scenarios such that the key variables of
DETERMINING THE ECONOMIC BENEFITS OF IMPROVING TRAVEL-TIME RELIABILITY Page B-20