consequence, the importance of managing existing capacity for optimal performance is rapidly increasing.

Optimal performance for many users (travelers and freight haulers) is a totally free-flowing system. However, it is unlikely that congestion can be entirely eliminated, or even significantly reduced. In fact, recent FHWA surveys show that drivers have accepted a certain amount of recurrent travel delay and have made lifestyle adjustments to accommodate the reality of congestion (FHWA 2001b). Moreover, shippers and carriers of goods account for expected congestion in their scheduling and logistics algorithms. The real problem for users is unexpected delay, the time they do not plan to spend on the highway—in short, the unreliability of the system (Loudon and Layden 2000; Golob and Regan 2001). The closer to capacity a system operates, the more severely it will react to disruptions, and therefore the greater will be the impact of its unreliability on users. For freight carriers, unreliability is a two-edged sword: as a result of the emergence of just-in-time delivery, carriers are penalized for being too early as well as too late. Reliability of the highway system is important for transit vehicles as well, so they can adhere to the schedules their riders expect. And when emergency evacuation is required, reliability is required to move people to safety predictably and consistently on very short notice.

There are many sources of unreliability in highway travel time. Perhaps most obvious are nonrecurring incidents such as crashes, broken-down vehicles, road debris, and spills. In addition, the need to renew significant portions of the highway system means an increasing number of work zones, with corresponding impacts on reliability. Special events such as parades, sporting events, and large conventions can significantly increase travel times as well if drivers are not aware of them in advance and are not given the opportunity to adjust their travel time or route. Precipitation, sun glare, and the occasional lost tourist can also have dramatic effects on the flow of traffic. Efforts to improve system reliability may address all these issues and more.

Improving travel time reliability is primarily a matter of system operations, but the discipline of highway operations is still an emerging field.² While