Modeling Approach for AMS Phase 1

For AMS Phase 1, the Connected Corridors team selected the macroscopic modeling approach (specifically, the Cell Transmission Model or CTM). It was chosen for the following reasons:

  • Simulation and development of control strategies: The CTM model can simulate existing control and management strategies as outlined in the Concept of Operations document for the I-210 corridor. Existing strategies include ramp metering strategies (fixed time, traffic-responsive demand-capacity, ALINEA, SWARM), incident management strategies (diversion), and signal control (progression of fixed time signals along arterials).  The model can be also used in the development of new strategies (model-based control or MBC). 
  • Simplicity in model development and calibration: Model inputs include link geometrics (number of lanes, length) which can be automatically imported from digital maps.  The calibration of the model parameters (fundamental diagram) can be accomplished from the data provided by the loop detectors located at each cell.  Furthermore, algorithms are available for data checking and verification, and imputation of missing data.
  • Empirical observation: It is important to recognize that the model parameters (e.g., free-flow speed, capacity) can be readily observed in the field, as opposed to microscopic models where model capacity is based on  driver-vehicle characteristics that cannot be observed.  Multiple model runs have to be performed for microscopic models to come up with a set of parameters that result in observed capacities.  This is a complicated process taking into consideration that microscopic models are stochastic, i.e., multiple repetitions are required for each model run.
  • Model execution: The model can be simulated quickly, which makes it a preferred tool in a decision support system framework as envisioned in the I-210 test corridor. Several scenarios and interventions can be simulated in real time and the best intervention can be recommended to the operator.

The CTM model consists of homogeneous road segments (cells or links) and nodes (location of on- or off-ramps or changes in the link characteristics).  Traffic (in terms of flow/unit of time) moves across links subject to the demand, capacity of the cell, and available space at the next cell.  Model parameters include free-flow speed, capacity, jam density, and congestion speed, i.e., the basic parameters of the fundamental diagram of traffic flow. For a more detailed description of the CTM model, see Cell Transmission Model (CTM) Framework.

Model limitations

It should be noted that a macro model like CTM cannot model in detail certain strategies for the freeway and arterial. These include dynamic mobility applications (DMA) that are based on vehicle connectivity (cooperative adaptive cruise control, speed harmonization, queue warning), adaptive signal control on arterials, and alternative intersection designs.  Most of these treatments require the detail and realism of microscopic simulation models.   Furthermore, the CTM model cannot directly model traveler information services to individual vehicles to multiple destinations.  This is typically modeled through mesoscopic models.