Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
11 This chapter introduces a variety of treatments that may be applicable to reducing nonrecurrent congestion on freeways. The treatments presented in this guide are pri- marily geared toward freeway facilities. Some are more appropriate for urban areas, and others tend to address nonrecurrent congestion issues more commonly found in rural areas. The breadth of treatments presented in this guide should ensure that it can serve as a resource to managers of any freeway system. Table 2.1 presents the specific design treatments that are addressed in this guide. The treatments are categorized into two groups: ⢠Nonrecurrent congestion design treatments. This category includes treatments implemented through physical changes in the highway or roadside that are used primarily to reduce nonrecurrent congestion. ⢠Secondary treatments. This category includes two kinds of treatments: those that are operational in nature, but may require geometric infrastructure to implement; and those that are design treatments, but are primarily used for recurrent conges- tion (that also have potential application to nonrecurrent congestion situations). For any of the general causes of nonrecurrent congestion discussed in this guide, there are several design treatments that may offer benefits. In addition, many of the treatments discussed in this guide can offer benefits in more than one nonrecurrent congestion scenario. The design treatments that should be considered for alleviating nonrecurrent congestion depend on a combination of the cause of congestion, the spe- cific benefit that is desired, and the existing characteristics of the freeway segment. Fig- ures 2.1 and 2.2 present decision trees based on the causes of nonrecurrent congestion and the specific means by which a treatment could reduce nonrecurrent congestion to help identify the most appropriate treatments to consider in a comparison of benefits and costs. 2 SELECTING DESIGN TREATMENTS TO ADDRESS NONRECURRENT CONGESTION
12 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION TABLE 2.1. DESIGN TREATMENTS Nonrecurrent Congestion Design Treatments Secondary Treatments Medians Emergency crossovers Movable traffic barriers Gated median barrier Movable cable median barrier Extra-height median barrier Mountable/traversable medians Shoulders Accessible shoulders Drivable shoulders Alternating shoulders Portable incident screens Vehicle turnouts Bus turnouts Crash Investigation Sites Crash investigation sites Right-of-Way Edge Emergency access between interchanges Arterials and Ramps Ramp widening Ramp closure Ramp terminal traffic control Ramp turn restrictions Detours Improvements to detour routes Truck Incident Design Considerations Runaway truck ramp Construction Reduced construction duration Improved worksite access and circulation AnimalâVehicle Collision Design Considerations Wildlife fencing, overpasses, and underpasses Weather Snow fences Blowing sand mitigation Anti-icing systems Lane Types and Uses Contraflow lanes for emergency evacuation Contraflow lanes for work zones High-occupancy vehicle and high-occupancy toll lanes Dual facilities Reversible lanes Work-zone express lanes Traffic Signals and Traffic Control Traffic signal preemption Queue-jump and bypass lanes Traffic signal improvements Signal timing systems Ramp metering and flow signals Temporary traffic signals Variable speed limits and speed limit reduction Technology Electronic toll collection Overheight vehicle detection and warning systems Emergency Response Notification Reference location signs Roadside call boxes Weather Fog detection systems Road weather information systems Flood warning systems Wind warning systems The decision trees are provided to give the user a starting point for using the SHRP 2 Project L07 treatment evaluation tool. Figure 2.1 includes treatments cat- egorized as nonrecurrent congestion design treatments, and Figure 2.2 includes treat- ments categorized as secondary treatments. Following the decision tree to the final branch that most accurately describes the cause of nonrecurrent congestion and the desired method of reducing that nonrecurrent congestion on the freeway segment of interest will provide a short list of the treatments that are typically well suited for that scenario. However, other treatments not listed may also provide benefits and can be included in the comparison if the user chooses to consider them. In addition, some
13 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION of the treatments listed may not be appropriate for certain freeway segments. For example, alternating shoulders would not be a candidate treatment on a segment that already has full left and right shoulders available. There were several possible ways to organize the treatments, as there are various frameworks for categorizing the causes of nonrecurrent congestion and the types of benefits provided by the treatments. The framework used here was chosen because it allows users to first answer the most basic question (what is the major cause of non- recurrent congestion?), and then guides them through considering the elements that contribute to that cause of congestion so that areas where improvement is desired can be identified. In some cases, users may have certain treatments identified for consideration already, in which case, this treatment selection process is not needed. Users can cer- tainly use the SHRP 2 Project L07 Treatment Analysis Tool without completing this process if they have already identified a treatment or treatments they would like to evaluate. CHOOSING TREATMENTS FOR CONSIDERATION To use the decision tree shown in Figure 2.1 for identifying treatments to be compared, the user should begin by identifying the primary cause of nonrecurrent congestion on the freeway segment of interest. Although six causes of nonrecurrent congestion are discussed in this guide, this tree begins with the four most common causes relevant to freeway segments: ⢠Incidents ⢠Weather ⢠Work zones ⢠Special events Most freeway segments experience nonrecurrent congestion from more than one of these causes, but often agencies have identified one specific area on which to focus improvement efforts. In any case, the user can follow more than one decision path in the tree and choose to compare treatments from different lists. Once the primary cause of congestion is identified, the decision tree then divides on the basis of the primary mechanism by which the cause of congestion could be minimized. For some mechanisms, the categories are further divided into more specific benefits. Once the decision tree has been followed to the final branch, there may be several appropriate treatments listed, which can be further narrowed according to the specific characteristics of the freeway segment of interest. For other branches, only one or two treatments may be listed. In this case, the user may choose to follow additional branches that could be provided by other treatments.
14 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Fi g u re 2 .1 . N on re cu rr en t co ng es tio n de si gn t re at m en ts .
15 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Fi g u re 2 .2 . Se co nd ar y tr ea tm en ts .
16 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION EVALUATING AND COMPARING TREATMENTS Ideally, users will identify two to five treatments from the decision tree shown in Figure 2.1 to evaluate and compare in the SHRP 2 Project L07 treatment Analysis Tool. The tool is applied to a specific segment of roadway on which nonrecurrent congestion occurs and compares the benefits and costs of the selected treatments as applied to that particular segment of roadway. The tool asks the user specific questions about the characteristics of the freeway segment on which the treatments are being considered for application, as well as questions about the intended implementation of the treatment. These questions may include information about geometrics, policies, or other factors that could influence the effectiveness of the treatment. These inputs allow the tool to estimate a change in the travel time reliability of that particular roadway segment that would be realized as a result of the various treatmentsâ installation. With user-provided input on the cost of implementation, the tool will provide a benefitâcost ratio for each treatment being considered and allow comparison of this ratio among several treatments. It is recognized that often treatments have costs and benefits beyond those that can be accounted for in an analysis tool, often related to political will, community support, environmental considerations, and others. Users are encouraged to use the benefitâcost ratio provided by the Analysis Tool as one variable in the decision-making process that must be balanced with the more subjective costs and benefits associated with a given treatment.
