Model Maintenance Plans
From Traffic Analysis and Microsimulation
Status: Early Draft
Introduction. A Model Maintenance Plan is a formal plan for maintenance and upkeep of a traffic model. Traffic models can rapidly go out of date for many reasons. Some examples include:
- Changes in traffic volumes.
- Changes in traffic patterns resulting from congestion, new development, opening/closure of major employment centers, etc.
- Changes in roadway configuration (completion of new roads, widening of exiting roads, re-striping of existing road space, replacing a signal with a roundabout, etc.).
- Changes in timing plans at signals and ramp meters.
- Effects of increases or decreases in fuel costs, tolls, bus fares, parking fees, etc.
- Changes in schedules and traffic management strategies at special event venues.
- Changes in operational strategies such as freeway service patrols, enforcement of on-street parking restrictions, etc.
Since the value of traffic models declines rapidly if the model is not maintained, the model maintenance plan is an essential part of the model development process. At minimum, the plan should clarify:
- The anticipated ongoing applications for the model.
- Which model forks and scenarios will be maintained.
- How often the model will be updated.
- Which organizational entity will take responsibility for the maintenance.
- The level of resources (staffing or consultant hours) that will de devoted to updates.
- How model maintenance will be funded.
In addition, it may be desirable to identify thresholds that would trigger a major overhaul of the model as opposed to routine updating. An example would be widening of a parallel facility.
When To Develop the Model Maintenance Plan. It is strongly advised that the maintenance plan be completed very early in the modeling process. This is particularly important when the organizational entity that will use the model on an ongoing basis is not the same one that is initally building it. Early coordination helps assure that the model is built in a way that adequately meets the needs of both groups.
Determining the Model Maintenance Level. The level of effort required for model maintenance clearly varies depending on the area that has been modeled. For example an area where there is extensive roadway construction (such as reconstruction of a freeway-to-freeway interchange over a period of several years) may require a higher level of model maintenance than an area where traffic changes very gradually.
The level of effort required for model maintenance also depends on the extent of ongoing need for application of the model. Microsimulation models are often developed to support redesign of a roadway, and in some cases this is the only reason for developing the model. In other instances there will be an ongoing need to apply the model to operational studies, even after the new facility has been completed. Determining an appropriate model maintenance plan often requires a internal agency coordination, since the group responsible for ongoing model applications may be different from the group that developed the model for roadway planning or design.
As a general rule of thumb, an urban model will probably require updating every 1-3 years, and a rural model will probably require updating every 3-5 years.