California Partners for Advanced Transportation Technology

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).