Assessing the Traffic Impacts of Freeway Incidents and Driver Information BY DAN GARRISON AND FRED MANNERING

T raffic congestion is a problem facing virtually every major metropolitan area in theUnited States. Because of prospects of continued traffic growth and relatively little money available for the expansion of highway networks, effective management of traffic flow is a vital concern. Within this context, perhaps one of the least understood and most disruptive phenomenon facing traffic flow management is traffic congestion resulting from incidents (which include both vehicle accidents and disablements). According to a recent study of the nation’s 37 largest urban areas, 60 percent of all urban freeway congestion is caused by incidents. The congestion from these incidents causes an estimated loss to users in excess of $5 billion in additional fuel consumption and motorist delay.’ Given the magnitude of this problem, the profession needs to develop an understanding of incident-induced traffic congestion so that appropriate management strategies can be formulated. Three questions are fundamental to this understanding: