Interstate 66 is a 75-mile highway extending from I-81 at Strasburg in the Shenandoah Valley of Virginia to Washington, D.C. The first 65 miles stretching eastward from I-81 were constructed routinely and with few problems. The final 10-mile segment in the Washington suburbs of Northern Virginia, however, has been surrounded by considerable controversy in the public arena because of its impacts on the environment and on commuters. This section was opened to traffic in late 1982 as a 4-lane, limited access, parkway-type facility from which heavy-duty trucks are excluded at all times. Further, peak period and direction usage are restricted to HOVs, emergency vehicles, and vehicles bound to and from Dulles Airport. Finally, to maintain safe and efficient traffic flows on the facility, a comprehensive computer-controlled traffic management system (TMS) was implemented in June 1985. The TMS was also installed on an existing segment of I-395 that contains reversible HOV lanes. The Virginia Department of Highways and Transportation initiated a two-phase study to investigate and evaluate the operation of the HOV section on I-66 and the TMS on both I-66 and I-395. This report presents the results of Phase 1 of the study, which focuses on I-66. Specifically, the study is an evaluation of the operating characteristics of I-66, of the impacts of I-66 in the region, and of the local response to I-66. A Phase II report by the Department will evaluate the TMS. Although the facility is currently operating on a congressionally mandated demonstration basis at a HOV-3 level, it is important to note that this report evaluates the initial HOV-4 level in operation from 6:30 to 9:00 A.M. eastbound and 3:30 to 6:30 P.M. westbound. A separate evaluation of the HOV-3 level of operation was performed by a consultant.