Over time, the truck driver, the motor carrier firm, and eventually the shipper will react to these changes by adjusting their travel behavior and changing their use of the highway system. They may reallocate their trips—shifting the hour or day that a trip is made, changing the route, and changing the destination; or they may make new trips, take longer trips, and shift freight between truck and rail or truck and air. By altering the total truck miles of travel and its allocation between congested and uncongested roads, changes in highway capacity can increase or decrease congestion, engine emissions, and energy consumption.

The general impact of changes in highway capacity on truck traffic is examined in the first section of this appendix. The author argues that, in the short term, changes in highway capacity are not likely to result in significant changes in truck travel. The three major reasons for this argument are the marginal nature of most changes in highway capacity today, the moderate exposure of trucks to severe congestion, and the overriding influence of low freight transportation costs.

Reviewed in the second section is the fragmentary evidence on the specific responses of motor carriers to changes in highway capacity, the primary ones being changes in the time of travel, route, and mode.

Structural changes in the economy, freight logistics, and trucking that may make truck travel more sensitive to changes in highway capacity in the future are discussed in the third section. These trends include a shift toward longer and more time-sensitive supply chains and distribution networks that leave trucks exposed to congestion and a countervailing shift toward the use of information technology to improve the productivity and flexibility of freight transportation.

In the fourth section research findings are reviewed on the relationship between truck accidents and congestion, which suggest that reducing peak-period congestion may reduce the frequency of common accidents, but will have little effect on the frequency of major truck accidents, which tend to occur during uncongested off-peak periods.

The state of truck travel modeling and the data available to transportation planners and engineers to analyze trucking issues are reviewed in the fifth section.

In the final section the author's conclusions are summarized and the implications for highway capacity planning, air quality, and energy use are discussed.

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