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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
