Similar to travel demand forecasting, a fundamental requirement for establishing transportation analysis that meets environmental regulatory conditions as well as provides a high level of confidence in the analysis results is to follow state of the practice or best practice methodology. For traffic operations analysis, methodology from the latest version of the Highway Capacity Manual (HCM) prepared by the Transportation Research Board should be used except where agency standards specifically require alternative procedures. An important aspect of the HCM methodology is that the methodology prescribed in the manual generally applies to conditions without congestion as noted below:
“…the Highway Capacity Manual (HCM) methods are generally not appropriate (unless the analyst performs a special intervention) for the evaluation of…queues that are building over both time and space ….”
page 9‐1 of the 2000 HCM
When congestion does occur, the HCM offers the following type of advice. “Certain freeway traffic conditions cannot easily be analyzed by the methodology. Multiple overlapping bottlenecks are an example. Therefore, other tools may be more appropriate for specific applications beyond the capabilities of the methodology. Refer to Part V of this manual for a discussion of simulation and other models.”
page 22‐1 of the 2000 HCM
The use of simulation models has become a fundamental part of arterial, highway, and freeway analysis due to the level of congestion and the need to understand how local and State roadways operates as part of integrated network. The operation of the network depends on traffic flows that are largely metered during peak period conditions by bottlenecks or other “valves” in the network (i.e., ramp meters). Since simulation tools can simultaneously evaluate the vehicle interactions across a complete network (including the interaction of multiple modes), they are able to provide a more complete understanding of traffic operating conditions during peak congested periods and what may happen when a specific bottleneck is modified or eliminated. The use of simulation models should be considered whenever congested conditions occur or when a project could significantly influence the operation of the study area network. An example of this latter case would include study areas involving multiple modes such as an interchange located near a light rail transit stop where trains, bicyclists, and pedestrians all interact with the street network and influence traffic control operations and vehicle flows.