Office of Operations
21st Century Operations Using 21st Century Technologies
Table of Contents

Traffic Analysis Toolbox Volume X:
Localized Bottleneck Congestion Analysis
Focusing on What Analysis Tools Are Available, Necessary and
Productive for Localized Congestion Remediation

3.0 What to Analyze?

This may seem an unnecessary question; i.e., the obvious answer would seem to be "the problem!" However, the purpose of this section is to remind agencies that secondary and tertiary impacts may result.

The transportation industry has dozens of infrastructural and operational strategies for mitigating congestion, including priced-tolling, high-occupancy lanes, telecommuting, public transit, and driver incentives like car-sharing and parking strategies, to name but just a few. At the highest levels, models and simulations offer the chance to test out congestion mitigation strategies without expensive construction or pilot projects. Using the appropriate simulation tools, planners can estimate the future conditions of a specific site with and without the mitigation strategies.

But a microsimulation study may be "overkill"; or not. For any analysis to be effective, it must consider the entire area affected by the bottleneck. For example, changing the signal timing at a frequently congested intersection may eliminate bottlenecks at the site, but if this improvement causes impacts to the neighboring intersections, this may not be a wise strategy. A large enough area must be considered to ensure that the analysis can account for all of the contributing and resulting factors. The questions below can help gauge the geographic/spatial extent of the analysis.

3.1 What Does One Mean By "Localized?"

For a bottleneck to be localized (per the definitions outlined in section 2.1) the factors causing that bottleneck ideally should not influence upon, or be influenced by, any other part of the transportation system; however, in a practical sense, the planner should consider any impact to the closest up- or downstream entity. If your bottleneck is deemed to be "the entire corridor" or something greater, then it is not intended to be covered by this guidance. For a much more detailed discourse on this subject, please refer to FHWA's web site on this subject at https://ops.fhwa.dot.gov/bn/index.htm and download or request the document "Recurring Traffic Bottlenecks: A Primer." FHWA publication number FHWA-HOP-09-037.

3.2 Is the Study Area Large Enough to be Meaningful?

One must fully consider the size of the study area in determining the scope of analysis that is necessary.

For example, consider an apparently isolated congested intersection. Suppose that modeling it as a roundabout, or simulating impacts of signal timing adjustments, or restriping, eliminates or greatly reduces the occurrence of bottlenecks. The planner knows this because he or she observes the simulated traffic and no longer sees bottlenecks congesting the study area. The planner then prepares to recommend a mitigation technique, but is unable to quantify the effects of these strategies. The travel time for a car through the roundabout, even under the worst conditions, is one minute or less. How can the planner make a meaningful case for one strategy over another if the results of each are only negligibly different? In this instance, localized Highway Capacity Manual (HCM) analysis may be more cost effective than a microsimulation product.

However, suppose that in resolving the bottleneck at this intersection it threatens to cause new bottlenecks to form downstream. If the original analysis is localized, its impacts would only be observed at the simulated intersection. If the study area is expanded, the opportunity to improve the entire system would be plausible (discussed below), and the measurable differences between compared strategies is likely to be starker. This is where simulation analysis is likely to make a meaningful contribution.

3.3 What Elements Need to be Analyzed?

Understanding the breadth of the analysis is critical to assessing what level of analysis is justified.

  • What are the impacted limits of the study?
  • Are upstream and downstream facilities impacted?
  • What alternatives can be considered?
  • How many hours of congestion are present? What is the optimal outcome?
  • How will the public be impacted?
    • Will they accept the temporary inconvenience of work zone?
    • Will they accept the changes in routine; i.e., new routes?
    • Are businesses on board?
  • What degree of precision do the decision-makers require?