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CONVERTIBLE ROADWAYS AND LANES SUMMARY It is no secret that the level of traffic congestion on the nationâs roadways continues to in- crease. Numerous reports and studies have shown that the steady growth in traffic volume has resulted in ever-growing societal and environmental costs associated with delay, pollution, and driver frustration. Although a seemingly endless stream of methods have been proposed to ad- dress these problems, including the construction of new roads, use of advanced transportation management systems, and increasing use of transit and travel demand management programs, each method comes with significant monetary costs and, perhaps more importantly, a signifi- cant shift in the way personal transportation is funded and used. The development of new roadways and the conversion of existing ones that can be adapted for different uses at different times of the day has been one method used by transportation agencies to more cost-effectively accommodate the constantly changing needs of highway travel. Convertible roadways encompass a variety of techniques that afford an agency an added measure of flexibility in how and when it responds to the needs for more on-street park- ing, additional intersection turning lane capacity, and short-term corridor capacity. Originally, the intent of this report was to synthesize information related to convertible lanes. However, owing to the lack of readily accessible information within that topic, the scope of the synthesis was narrowed to primarily cover reversible roadways and lanes, one of the largest and more well-documented subsets of convertible facilities. Reversible traffic operations are widely regarded as one of the most cost-effective methods to increase the capacity of an existing roadway. The principle of reversible roadways is to con- figure the lanes of a roadway to match available capacity to the traffic demand. These road- ways are particularly effective because they take advantage of the unused capacity in the mi- nor-flow direction lanes to increase the capacity in the major-flow direction, thereby eliminating the need to construct additional lanes. They are most effective when highly unbal- anced directional flows are present, such as those that occur during daily peak-period travel times, before and after large events, and during emergency evacuations. The relatively simple concept of reversible flow roadways belies their actual complexity and complicated operational requirements. They can require considerable effort to plan and design, and they often require special control and management strategies to keep traffic mov- ing safely and efficiently. Surprisingly, despite the long history and widespread use of reversi- ble roadways worldwide, there have been few quantitative evaluations and research studies conducted on their performance. There are also a limited number of published guidelines and standards related to their planning, design, operation, control, management, and enforcement. Therefore, most reversible lane systems have been developed and managed based primarily on experience, professional judgment, and empirical observation. The limited availability of standardized and formalized practices has resulted in considerable variation within the prac- tices, philosophies, and policies associated with their use. Furthermore, many of the actual costs and benefits of reversible lane systems remain largely unexplored, which may in turn mean insufficient understanding of their operational and safety effects.
2 This synthesis was undertaken to address the need to increase the level of understanding of reversible roadways and to improve the knowledge of their characteristics and operational requirements, costs, and benefits so that reversible operations can be more effectively im- plemented in the future. The synthesis documents the historical development of reversible lanes, applications for various needs, lessons learned from previous implementation, costs and benefits associated with their use, and various techniques and successful practices that have been developed. The report was based on previous research and evaluation studies, a survey of known and potential users of reversible lanes, and informal interviews with repre- sentatives of highway agencies that currently use them. Four main findings of reversible lane and roadway use were revealed. The first was how common is its use. Reversible lane operations in one form or another are currently (or have been) used in nearly every large city and many small- to medium-size cities in the United States. Second, there was a general agreement on the conditions that warrant reversible opera- tions and the basic requirements for their effective use. The conditions that warrant reversible operation include volumes at or near capacity, predictable patterns of high demand and/or congestion, limited right-of-way (or ability to acquire it) to construct additional lanes, ratios of primary directional volume to secondary directional volume of approximately 2:1 or greater, and inadequate capacity or mobility on adjacent parallel streets. Third, there was a wide variety in the design, control, and management methods used to plan and operate reversible roadways. Of the more than 20 locations reviewed as part of the study, no two were alike in their key design, control, or management features. This variabil- ity was likely related to a lack of established standards and guidelines for their use. The fourth finding of the synthesis was the extent to which the benefits and costs of re- versible roadway operations are not well understood. Although a handful of comprehensive studies exist in the literature, they do not provide a basis for determining the applicability and effectiveness of reversible lane use in most locations. One significant gap in knowledge exists in the understanding, measurement, and assessment of the fundamental characteristics of reversible traffic streams. Currently, some of the most basic characteristics of reversible lanes, including their capacity, remain unknown. This lack of understanding has led to an in- ability to accurately assess the benefits (or drawbacks) of reversible lane alternatives and may contribute to the widespread reluctance to implement new reversible facilities and dis- continue many of those in use today.
