Evaluating Alternative Operations Strategies to Improve Travel Time ...

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SHRP 2 L11: Final Report help mitigate a reduction in travel-time reliability. The Effects on Sources of Congestion that are described at the bottom of Table 5.5 present the range of future outcomes that may occur. Table 5.5 Alternative Futures Summary Environment & Climate Change Economy Energy Cost & Availability Demographics and Land Use Technology Policy & Institutional Freight Financing Traffic Incidents Alternative 1 Optimistic Scenario Alternative 2 Mediocre Scenario Scenario Drivers Moderate increase in rare Rare weather events insignificant weather events Strong annual GDP growth (3- Moderate annual GDP growth 4%), VMT (2%) (2-3%), VMT (1.5%) Significant development of Development of alternative alternative clean energy clean energy technologies technologies Increasing energy prices Stable energy prices Responding Trends Strong population growth Spreading of cities and settlement of rural areas continue Considerable increase in energy & transportation technologies Increase expectations for reliability GHG emission targets will be met Increase in demand for freight on highways, rail, air, water Extensive application of VMT charges, increasing role of PPPs Increase in incidents offset by safety gains through ITS Moderate population growth Dense development, downtown regentrification & revitalization Increase in energy & transportation technologies Deterioration of transportation infrastructure will decrease reliability GHG emission targets will not be met Modest growth in freight demand. Shifting some demand for freight onto rail and water Broader application of VMT charges, increasing role of PPPs Effects on Sources of Congestion Safety gains through ITS Alternative 3 Pessimistic Scenario Significant increase in rare weather events Slow annual GDP growth (
SHRP 2 L11: Final Report 6. OPERATIONS STRATEGIES AND TREATMENTS TO IMPROVE TRAVEL-TIME RELIABILITY This chapter identifies a list of key strategies and their strengths, weaknesses, threats, and opportunities for improving travel-time reliability under the baseline and three future scenarios developed in Chapter 2. In order to identify the strategies and treatments that are most likely to have the greatest impact, a literature review focused on previous and current work of other SHRP 2 Reliability and Capacity projects including L03, L06, L07, and C05 was conducted. In addition to SHRP2 projects, information from the FHWA, state DOTs, universities, and other countries were reviewed to ensure a broad assessment of strategies and treatments. Innovative technologies that may impact travel-time reliability in the future were also reviewed and presented at the end of this chapter. SOURCES OF CONGESTION/UNRELIABILITY Congestion occurs when the traffic volume on a roadway exceeds the available capacity. However, roadway capacity is not a constant and is influenced by a variety of factors that reduce effective or operational roadway capacity from the “baseline” capacity computed through Highway Capacity Manual procedures. Previous research has identified seven sources of congestion. They are briefly summarized as follows. (1) 1. Physical Bottlenecks: Bottlenecks are sources of congestion that occur on short segments of roadway that exhibit lower capacity than upstream segments of roadway, essentially resulting in unreliable travel. Bottlenecks commonly form either at changes in roadway geometry (e.g., lane drops) or where significant traffic movements reduce effective roadway capacity for a given number of roadway lanes (e.g., merge and weave sections). 2. Traffic Incidents: Traffic incidents are events that disrupt the normal flow of traffic, usually by physical impedance in the travel lanes. Events such as vehicular crashes, breakdowns, and debris in travel lanes are the most common form of incidents. 3. Weather: Environmental conditions can lead to changes in driver behavior that affect traffic flow. Weather events such as fog, snow and heavy rain can negatively impact travel conditions, causing delays and congestion. 4. Work Zones: Construction activities on the roadway can result in physical changes to the highway environment. These changes may include a reduction in the number or width of travel lanes, lane “shifts,” lane diversions, reduction, or elimination of shoulders, and even temporary roadway closures. 5. Traffic Control Device: Intermittent disruption of traffic flow by control devices such as railroad grade crossings and poorly timed signals also contribute to congestion and travel time variability. 6. Fluctuations in Normal Traffic: Variation in day-to-day demand leads to some days with higher traffic volumes than others. 7. Special Events: Special events are a special case of demand fluctuations whereby traffic flow in the vicinity of the event will be radically different from typical patterns. Special events occasionally cause “surges” in traffic demand that overwhelm the system. The FHWA website (2) provides national estimates of the amount of delay caused by each of the sources of congestion noted above. SHRP 2 Project L03 (3) further quantifies these estimates by OPERATIONS STRATEGIES AND TREATMENTS TO IMPROVE TRAVEL-TIME RELIABILITY Page 62
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