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Reliability in Simulation and Planning Models (L04)

Visit FHWA's new SHRP2 Reliability page for the latest resources on this SHRP2 Reliability Solution.


The SHRP2 Reliability focus area identified seven sources of nonrecurring congestion: incidents, weather, work zones, special events, traffic control devices not working properly, unusual fluctuations in demand, and bottlenecks that can exacerbate these sources of unreliability. These nonrecurring sources of congestion can affect supply, demand, or both. These supply and demand factors influence the travel time for origin–destination pairs across the network and, in turn, the distribution of travel time from which various reliability measures can be derived.


The Reliability in Simulation and Planning Models (L04) SHRP2 Solution is designed to assist State DOTs, MPOs, and other transportation agencies in moving reliability into their business practices. This SHRP2 product offers application guidelines for incorporating reliability into micro- and/or meso-simulation models that identify key steps for integrating demand and network models. This project adds a distinguishing classification that recognizes systematic variation factors and those that vary randomly (loosely corresponds to recurrent and non-recurrent congestion) for both travel demand and network supply. The variability in system performance that is at the center of interest has both systematic causes, which can be modeled and predicted, as well as causes that can only be modeled as random variables and which occur according to some probabilistic mechanism. The new methods are useful for project evaluation and many types of planning. Three main components provide the capability to produce reliability performance measures as output from operational planning and simulation models; (1) the Scenario Manager, which captures exogenous i.e. outside unreliability sources such as special events, adverse weather, work zone and travel demand variation; (2) reliability-integrated simulation tools that model sources of unreliability endogenously i.e. within the model, including user heterogeneity (diversity), flow breakdown, accidents and so forth; and (3) a Vehicle Trajectory Processor, which extracts reliability information from the simulation output, namely vehicle trajectories.

The Scenario Manager is a preprocessor that produces randomized input into simulation models, and the Trajectory Processor is a post-processor that portrays the variability in travel-time and reliability performance metrics from origins to destinations and for segments and links. such tools are integrated into an agency’s business processes to improve its capability to analyze and improve travel time reliability.


The Guidelines for Incorporating Reliability Performance Measures into Travel Models (L04) solution has closed an important gap in the underlying conceptual foundations of travel modeling and traffic simulation, and can provide a practical means of generating realistic reliability measures using network simulation models in a variety of application contexts.

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