Evaluating Alternative Operations Strategies to Improve Travel Time ...

SHRP 2 L11: Final Appendices
There are three alternate approaches to addressing the aggregation of reliability considerations on
the network level. Each of them has advantages and disadvantages.
Direct Measurement of Unreliability at the Network Level
This method involves measuring unreliability separately for each roadway segment in the roadway
network. If this can be accomplished, then the network-level effects can be aggregated from the
individual link effects. Alternatively, unreliability could be measured at the network level. For
example, one might measure speed volatility using network-wide VMT and VHT data. The mean
and standard deviation of the log mean and standard deviation of the ratio of regional VHT and
VMT could be computed by vehicle class and by trip type from daily observations over some
period of time. The certainty-equivalent value of unreliability then can be calculated directly using
the European put option approach described earlier in this appendix.
The certainty equivalent delay associated with each category of travel could then be monetized by
applying the appropriate value of time. Strategies or policies that mitigate unreliable travel could
then be evaluated using conventional benefit-cost techniques, so long as the strategy or policy can
be characterized by a change in the reliability performance measures.
Measurement using Region-Specific Travel Models
Simulation of the effect of strategies or policies that are intended to improve reliability at the
network level can be measured using modeling techniques for the regional network that are
sensitive to the impact of these strategies (such as dynamic tolling) that affect system reliability. In
this approach, a model of the regional network is used to examine the effect of any changes in
reliability that occur on the affected segments or that occur on the network as a whole. Certaintyequivalent
option models can be used in conjunction with microsimulation, dynamic traffic
assignment, or other model platforms that measure how unreliability is affected by a pricing-, or
operational-, or capacity-improvement policy.
This approach has the potential to be both comprehensive and respectful of regional network
idiosyncrasies. The Puget Sound Regional Council's four-step travel demand model has
incorporated link-level augmentations to allow measurement of unreliability effects. Although it is
applied only to freeway links (using a representative stochastic rendering for all links), it is a
convenient way to automatically consider the benefits of improving reliability when evaluating
various strategies and policies (not just policies designed to address unreliability issues).
This regional-model approach also has the potential to help extrapolate events that occur on only a
few links in the network to determine the impact on the network as a whole. Xie and Levinson
(2008) used this general method to evaluate the effect of the failure of the I-35W bridge in the
Minneapolis region. In this case, total delay was addressed, but reliability was not.
The accuracy of this method is limited only by the capabilities of the model. Obviously, a model
that simulates the dynamic behavior of traffic (and the volatility of travel speeds throughout the
network) provides a better way to represent unreliability with and without the application of
various strategies or policies. The evaluation exercise then proceeds to capture the performance
data (link by link or in the aggregate across the network) and apply the valuation methods
described above to the reliability metrics.
Mathematical Representation of Network Reliability
A third approach is to employ mathematical models of network reliability to simulate the impact of
a strategy or policy on network performance. These models differ from regional models used by
DETERMINING THE ECONOMIC BENEFITS OF IMPROVING TRAVEL-TIME RELIABILITY Page B-19