Traffic Bottlenecks: Identification and Solutions

CHAPTER 1. INTRODUCTION

It is evident from currently available literature that no standard definition of congestion is used consistently across the industry; one must differentiate between the terms “congestion” and “bottleneck.” The Federal Highway Administration (FHWA) addresses this directly by stating this tenet: “A bottleneck may cause congestion, but congestion is not always the result of a bottleneck.”⁽⁷⁾ Moreover, supporting the contention that a bottleneck is a subordinate component of congestion (and not vice versa), FHWA has long-opined that bottlenecks comprise a significant portion of the total congestion causes (see figure 1).

Bottlenecks, as the name implies, denote an area of significant breakdown in flow. The question then becomes how significant must this breakdown be to warrant the bottleneck classification as opposed to mere congestion. Although this question is addressed by the HCM and by traffic analysis tools, ideally, this breakdown definition could be customized for local needs when appropriate, such that the guidance sources would simply advocate for specific default definitions.⁽⁶⁾ Subsequently, once the bottleneck locations are identified and prioritized according to agreed-upon metrics, an effective array of bottleneck mitigation strategies should be considered. This report focuses on practical, low-cost methods of bottleneck identification and mitigation.

Bottlenecks historically have been categorized as either recurring (i.e., predictable and routine, as exemplified by that which occurs during commute peak hours) or nonrecurring (i.e., due to random events like incidents, weather, special events, and work zones). Recurring bottlenecks exist at locations that FHWA terms “operationally deficient” but only with the caveat that said locations reveal themselves only when they become overburdened by (typically) peak hour demand; the same location operates fundamentally sound otherwise.⁽⁸⁾ Relief comes chiefly when the overburden falls away. Nonrecurring bottlenecks are random as to location, cause, and duration and typically are only relieved when the event disperses. Another key difference is that the recurring locations may have the opportunity to be corrected by redesign, whereas nonrecurring locations have less design-influenced solutions and more event-response opportunities to improve the condition.

There is a deficiency of cost-effective solutions for identifying and remedying oversaturated locations. To address this deficiency, this report is organized into the following four chapters:

• Chapter 2. Bottleneck Identification.
  
  
• Chapter 3. Bottleneck Mitigation Concepts.
  
  
• Chapter 4. Cost-Effective Solutions for Bottleneck Mitigation.
  
  
• Chapter 5. Innovative Solutions for Bottleneck Mitigation.

In addition, the appendix provides details of all bottleneck playbook solutions.