Understanding Bottlenecks

Traffic bottlenecks are specific physical locations on roadways that routinely and predictably experience congestion because traffic volumes exceed highway capacity. Surge demand higher than can be accommodated by base capacity brings about bottleneck congestion. Bottlenecks are characterized by queues upstream and freely flowing traffic downstream.

Bottlenecks may be compared to a storm pipe that can carry only so much water – during floods the excess water just backs up behind it, much the same as traffic at bottleneck locations. However, the situation is even worse for traffic. Once the traffic flow breaks down to stop-and-go conditions, capacity is actually reduced – fewer cars can get through the bottleneck because of the extra turbulence.

Figure 2. Types of Freeway Bottlenecks

How bad congestion becomes at a bottleneck is related to its physical design. Some bottlenecks were originally constructed many years ago using designs that are now considered to be antiquated. Others that have been built to extremely high design specifications are simply overwhelmed by traffic. Whatever the root cause, operational conflicts can occur at:

• A "lane drop," where one or more traffic lanes are lost. These typically appear at bridge crossings and in work zones. The latter, however, is a nonrecurring event and is usually remedied when the work zone is removed.
• A "weaving area," where traffic must merge across one or more lanes to access entry or exit ramps.
• "Freeway on-ramps" are merging areas where traffic from local streets can join a freeway.
• "Freeway-to-freeway interchanges" are special cases of on ramps where flow from one freeway is directed to another. These are typically the most severe form of physical bottlenecks because of the high traffic volumes involved.
• "Abrupt changes in highway alignment" occur at sharp curves and hills and cause drivers to slow down either because of safety concerns or because their vehicles cannot maintain speed on upgrades. Another example of this type of bottleneck is in work zones where lanes may be redirected or "shifted" during construction.
• "Intended interruptions to traffic flow" are literally "traffic disruptions on purpose" that are sometimes necessary in order to manage system flow. Traffic signals, freeway ramp meters, and tollbooths are all examples of this type of capacity loss.

FHWA Survey Suggests Opportunities for Lower Cost Solutions Respondents to the FHWA Division Office survey said that 71 percent of bottlenecks were on freeways. Further, 42 percent were interchange related and 36 percent had no specific improvement or plan underway. Full interchange or freeway reconstructions were the most commonly perceived solutions. However, at least a portion of those might qualify for low-cost response actions that can be implemented in the short-term to improve traffic flow. Recent discussions with several state partners have reinforced their position. Specifically, high-ranking state DOT officials and mid-level staffers engaged in day-to-day operations have all opined that there are "tremendous" and "significant" benefits to pursuing low-cost operational improvements. These benefits range from the direct (reductions in delay, increases in traffic throughput) to the indirect (public confidence and agency image-boosting effects).