Traffic Bottlenecks: Identification and Solutions

CHAPTER 3. BOTTLENECK MITIGATION CONCEPTS

Definition of Bottlenecks

For at least 15 years, FHWA has stated that 40 percent of all congestion nationwide is attributable to bottlenecks, while another 5 percent is attributable to poor signal timing, which is also a tangible problem and not a random event.⁽⁵⁾ These two causes together are said to define recurring congestion and account for 45 percent of congestion. While this number has been oft-quoted in FHWA literature and widely used by others, it is purportedly based primarily on a congestion study undertaken mostly for, or using data mostly from, the I-95 Corridor Coalition. (See references 24, 35, and 36–39.) Whether or not recurring congestion is 45 percent of all congestion is less the issue than having confidence that recurring causes were correctly identified and aggregated because it is also safe to say that said study did not break out, identify, or otherwise stratify the indicators.⁽⁵⁾ In short, recurring congestion just exists, and all other congestion is considered nonrecurring (e.g., special events, bad weather, work zones, and traffic incidents) (see figure 1). Regardless, it is fair to say that no confirmation or additional study has been made since. There is little consensus on a bottleneck definition in the literature other than to say that bottlenecks, by whatever cause, result in the formation of queues upstream of the bottleneck and free-flowing traffic downstream. According to a theory proposed by Kerner, traffic is categorized into three phases: free-flow, synchronized flow, and wide-moving jams.⁽⁴⁰⁾ Free-flow is an unrestricted condition. Synchronized flow entails localized congestion. Kerner’s synchronized traffic is fixed at a downstream front, which is a defacto bottleneck. Wide-moving jams are more or less systemic congestion, as the downstream traffic velocity moves, even as traffic may propagate upstream through bottlenecks.

The Merriam-Webster Dictionary defines a bottleneck as: (1) a narrow route or (2) a point of traffic congestion.⁽⁴¹⁾ However, a road does not necessarily have to narrow for a bottleneck to exist (e.g., bottlenecks caused by a weaving condition, sun glare, rubbernecking, or a vertical climb). A layman’s understanding of a bottleneck might be too many cars trying to use a highway at the same time. A bottleneck is distinct from congestion because it occurs at a specific location and not pervasively along the entire corridor. The term “traffic bottleneck” infers a localized congestion problem, not a systemic congestion problem. A physical bottleneck cause (e.g., a lane drop or other operationally deficient design on the roadway) only manifests itself when traffic demand overwhelms the available roadway capacity. Otherwise, the design operates safely (otherwise it would have drawn attention on that merit) and unobtrusively, and no one is delayed. The takeaway here is that in the case of a recurring bottleneck location, it takes both a deficiency in the design as well as traffic overburden for the delay to occur, whereas a nonrecurring event is the sole causal event that causes traffic.

Attributions of Bottlenecks

Many papers have referenced the definition used by Daganzo, which attempts to explain a bottleneck’s signature; namely, a bottleneck is considered to be “active” if traffic is detected to be “queued upstream of the location and unqueued downstream.”(pg. 106)⁽⁴²⁾ Geroliminis et al. provides another useful bottleneck definition, as follows: “A phenomenon where the full performance level (capacity) of an entire system cannot be realized due to an abnormality at a single component of the system. The performance at one location thus brings down the performance of the entire system.”(pg. 5)⁽⁴³⁾ In supporting Geroliminis’ contention, it is also FHWA’s Office of Operation’s observation that locally subordinate locations are often indeed the defacto “congestion” that is unfairly attributed to the entire facility (e.g., the highway is congested when in fact only the vicinity of eastbound (EB) Exit 12 is the problem.).⁽⁷⁾ This is akin to one bad apple unjustly tainting the entire crop.

Severe congestion can be caused at locations where a physical reduction in roadway width (e.g., lane blockage or lane drop) occurs. This cause may be called a “physical bottleneck.” Physical bottlenecks have been the focus of transportation improvements for many years for the simple reason that they are tangible and correctable by redesign in contrast to nonphysical causes of slowdown like rubbernecking, weather, or weaving. Regardless, motorists only care about the result, not the reason behind the bottleneck. Some of these locations are notorious to the extent that they have acquired colorful nicknames by the local motorists, such as the following:

• “Spaghetti Bowl” in Las Vegas, NV.
  
  
• “Malfunction Junction” in Knoxville, TN (since remedied).
  
  
• “Hillside Strangler” in Chicago, IL.
  
  
• “Mixmaster” in Dallas, TX.

NCHRP Report 3-83, Low-Cost Improvements for Recurring Freeway Bottlenecks, emphasizes that hidden bottlenecks occur when downstream demands are metered by upstream bottlenecks.⁽⁴⁴⁾ When hidden downstream bottlenecks are activated by improvements to upstream bottlenecks, it potentially causes negative system-wide impacts. This makes the case for using predictive tools and real-time data to drive bottleneck solutions even more compelling.

The degree of congestion at a bottleneck is related to its physical design. Some bottlenecks exist on roadways constructed many years ago using designs that were appropriate at the time but are now considered antiquated. Others have been built to sufficiently high design specifications but are simply overwhelmed by traffic. In the FHWA report, Recurring Traffic Bottlenecks: A Primer Focus on Low-Cost Operational Improvements, FHWA identified the worst physical bottlenecks in the country and examined the potential benefits that improving them could have for travel times, safety, emissions, and fuel consumption.⁽²⁾ Ultimately, many definitions and attributes for bottlenecks arose from the literature. However, Margiotta and Spiller were able to summarize the different attributions of bottlenecks (see table 3).

Bottleneck Identification Framework

Bottlenecks can have various and interrelated causes. This section focuses on identifying geometric or operational challenges associated with a bottleneck location and tailoring a solution or set of solutions to alleviate congestion. A given bottleneck location can have one or more causes. One cause can have multiple solutions, and one solution may address multiple causes, which is discussed further in the next section. Figure 29 shows a decision tree to help transportation agencies identify potential causes of an active bottleneck. The decision tree is divided into geometric and operational challenges. Further subdivisions within the two categories delve into exactly what aspects of geometry or operations may be causing bottleneck conditions.

The appendix contains a detailed description of each potential bottleneck cause in figure 29 and identifies potential solutions. Note that the numbers in the figure correlate to the numbers provided in the appendix. Each subcategory has a description or definition, a list of theoretical or empirical effects of that potential cause, and identification of existing and proposed solutions, where possible.

1. Try to classify the bottleneck according to the seven categories listed in figure 29.
   
   
2. Browse the available bottleneck causes listed under the chosen category.
   
   
3. Refer to the appendix to obtain additional information on associated mitigation strategy (or strategies) for the chosen bottleneck cause.
   
   
4. Consider the applicability of several mitigation strategies given in chapters 4 and 5 whose details are embedded within those sections.

For example, suppose a bottleneck is periodically identified on a section of freeway with none of the typical physical characteristics that tend to cause bottlenecks (e.g., lane drops, nearby ramps, horizontal or vertical curvature, etc.). If further study determines that this bottleneck tends to form during periods of poor weather, this could potentially be classified under the “Geometric—Roadway Specific,” “Operational Challenges—Agency Related,” and “Operational Challenges—Driver Related” categories listed in figure 29. Regarding the “Geometric—Roadway Specific” bottleneck cause sun glare, which is a weather-related issue, one suggestion from the appendix is the installation of redundant traffic control devices. Regarding “responses to weather” under “Operational Challenges—Agency Related,” the appendix lists options including education campaigns, reflective lighting, roadside lighting, roadside shelters for pulling over, signage (e.g., lights on when raining), salting or other material to increase traction, and restrictions during weather events. Regarding “unsafe vehicle conditions for weather conditions present” under “Operational Challenges—Driver Related,” the appendix discusses vehicle inspection requirements and electronic enforcement. Regarding chapters 4 and 5, none of these solutions specifically target weather-induced, nonrecurrent bottlenecks. Therefore, the framework and playbook provided in figure 29 and the appendix produce a useful set of considerations and/or options in this case.

Figure 29. Flowchart. Framework to identify causes of bottlenecks.