Highway 401, through the Greater Toronto Area, is one of the busiest stretches of expressway in North America, with more than 400,000 vehicles per day. In addition to serving commuting flows across the Greater Toronto Area, Highway 401 serves as the primary corridor for passenger and commercial traffic between Quebec and Windsor and points beyond. In 2003, the Ministry of Transportation of Ontario (MTO) investigated the possibility of closing the eastbound Highway 401 express lanes, between Kipling Avenue and Jane Street, for two weekends in 2004 to rehabilitate the road base. Such a closure would have previously been considered too risky in terms of the impact on traffic flow. However, the alternative construction schedule would involve approximately 70 overnight closures and an additional cost of $1.5 million. A micro-simulation model (VISSIM) was used to assist in the assessment of traffic impacts and the development of a traffic management strategy to keep traffic moving on the collector lanes through the work-zone area. Through micro-simulation, it was demonstrated that the proposed closure was feasible, assuming a combination of a comprehensive public communications plan, selected on and off-ramp closures, an incident management plan, and reduction in traffic levels of 20 to 25% through diversion. Simulated conditions, during peak weekend traffic periods with the closure in place, were projected to be equivalent to or better than typical weekday morning peakperiod conditions. The closure was implemented on the third and fourth weekends of August, 2004. Observed traffic conditions were essentially as predicted, with operation falling in between typical weekend and weekday peak conditions. The projected level of diversion was achieved, traffic was kept moving, and only minor incidents, quickly cleared, were recorded. The construction work was completed on time and met all standards and specifications.