Traffic safety specialists assume that "secondary crashes" result from incidents which increase traffic congestion. Yet, the literature presents few references to frequency of occurrence of these crashes, and no references to contributing factors. Without knowing the extent of the problem, recommendations for resolution are not practical. This article explores temporal and spatial analysis of incidents on urban arterial roadways to help identify those crashes, which were likely to be secondary. It found more than 15% of the crashes reported by police may have fit the definition; that is, occurred partly or entirely as the result of an earlier event. Such crashes appear to result more from external distractions to the driving task rather than commonly held beliefs that causation results mainly from internal distractions and errors in driver perceptions. Issues of defining such crashes are addressed through the use of geographic information systems along with temporal assumptions. This article shows when secondary crashes are likely to occur and what classes of incidents were most likely to be related. In general, one third of all crashes classed as secondary occurred within 200 meters of the initial event and most within the time required to handle that event. Additionally, crashes represented approximately 60% of the primary incidents. Disabled vehicles and police citing motorists for traffic violations represented most of the remaining causes. Finally, the article identifies potential countermeasures which reduce the likelihood of external distractions affecting the driver and leading to the secondary crash. (A)
Abstract