There are only two ways to increase the throughput of vehicles on a highway—either construct more freeway lanes, or accommodate a shortened average spacing between individual vehicles (while maintaining speed). Providing a rational means for close-following is an attempt to accomplish the latter. The maximum throughput for a typical freeway lane is about 2000 vehicles/hour, and is roughly independent   of the attributes of any particular freeway. The number is remarkably   inelastic varying little over the past twenty years in spite of great improvements in the design of freeways and in the handling characteristics of automobiles (see Browand, Zabat & Tokumaru, 1997, for a recent example). Surprisingly, the average vehicle spacing at maximum throughput is approximately 35 meters. Shortening this headway to the order of a car length (five meters, typically) while maintaining a high speed would provide a significant improvement in   throughput. Practical estimates that take into account  safety particularly   in wet weather suggest that a factor of between two and three improvement in throughput may be a reasonable goal (Kanaris, Ioannou & Ho, 1997).

The present wind tunnel experiment describes 6-component force and moment data measured for both the cab and the trailer of a simplified model truck. Forces and moments are presented in coefficient form. The cab is sufficiently smooth that no flow separation occurs at zero yaw. The trailer has rounded forward vertical edges and sharp upper and lower edges. Both cab and trailer have wheels. The test matrix includes variation of the cab-trailer gap, and the yaw angle between the model plane of symmetry and the axis of the wind tunnel. The yaw angle is meant to account for the presence of an over-the-road side-wind. Cab-trailer gap separation is varied from 0-1.55√A, where A is the frontal cross-sectional area of the trailer. Yaw angle is varied from 0-16 degrees.