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9 CHAPTER 2 Literature Review 2.1 Introduction In addition, non-recurring events are also known to con- tribute to delay. The most common of these events are listed This chapter documents previous research and practices below along with the percentage share of each event type (7): regarding freight mobility issues. For each of the three prin- cipal surface transportation modes of freight movement, the Non-fatal crashes (45.5 percent) literature review attempts to capture definitions of mobility Work zones (24.3 percent) constraints, definitions of low-cost improvements, and strate- Breakdowns (12.0 percent) gies to address mobility constraints. Finally, the literature Weather (9.0 percent). review documents examples of low-cost improvements imple- mented to improve freight mobility. An annotated bibliogra- Freight mobility constraints can be caused by physical, oper- phy is also provided in Appendix B. ational, or regulatory factors. Recent National Cooperative Highway Research Program (NCHRP) Project 3-83, "Low- 2.2 Highways/Trucking Cost Improvements for Recurring Freeway Bottlenecks," iden- tified the following as some of the physical or geometric features 2.2.1 Defining the Freight Mobility Problem that contribute to the occurrence of freeway bottlenecks (8): on Highways and Roadways On-ramp sections with no auxiliary lane additions or with A number of factors contribute to constrained freight mobil- short deceleration lanes ity, which, when combined, have significant adverse economic, Weaving sections, particularly out of dropped lanes environmental, safety, and security impacts. One factor is the Lane drops on basic segments, or following off-ramps or growing demand for freight transportation, as reflected by the tunnel sections, where free flow speed may be reduced increasing volume of domestic and international freight that is Horizontal curves, where vehicle paths may cross into the moved on the nation's transportation system. next lane According to USDOT estimates, the volume of goods moved Long upgrades, particularly in the presence of heavy vehicles by truck and rail is projected to increase 98 percent and 88 per- Narrow lanes on older freeways cent, respectively, from 2002 levels by 2035. As a result of Lateral obstructions, which reduce free flow speeds. increasing freight demand, congestion is rising and is expected to increase in the future. This congestion will have a number of negative impacts. For example, producers, shippers, and 2.2.2 Definition of Low-Cost consumers will suffer the higher economic costs of an ineffi- Highway Improvements cient freight transportation system (5). While certain improvements may be considered as low- FHWA (6) categorized freight mobility problems related to cost, there is no general definition of the characteristics of such bottlenecks in the following four constraint types: activities. The Minnesota DOT (9) used the following four cri- teria to identify "short-term, low-cost congestion-reduction Interchange constraints strategies" for specific bottleneck locations: Highway capacity constraints Geometry constraints (i.e., steep grade) 1. Projects were required to have the potential of a 50 percent Intersection-related constraints. reduction in congestion