Analysis of incidents and crashes occurring along urban arterial roadways suggests that as many as 15 percent of the crashes occurring along these roadways may have been, in part, caused by an earlier incident. To conduct the analysis, data covering reported vehicular crashes, fires, disablements, traffic enforcement, and other incidents affecting roadways were collected from seven contiguous urban municipalities in the Northern Chicago, Illinois, metropolitan region. Crashes represented 35 percent of the incidents; traffic enforcement, 30 percent; and disabled vehicles, 27 percent. Other events representing the remaining 8 percent. To associate an initial event with a subsequent crash, any crash that occurred on roadways affected by the primary event, within the duration of the primary event plus 15 min. and within 1600 m of that event (based on use of geographic information systems software) was assumed to have been related. Analysis of 1,796 incidents in the data base identified 97 secondary crashes related to 81 initial incidents. A random sample of all events in the data base showed that only 3.6 percent of the crashes instead of 15 percent might be expected to fall within the required temporal and spatial limits in relation to an earlier event. This finding suggested that the selected events may not have been random occurrences. Finally, analysis of the secondary crashes showed that 1 of every 10 may have been related to an earlier crash; 1 of every 11 to a disabled vehicle, and up to 1 of every 40 to a police stop for a major traffic violation. What must be studied in greater detail is the degree of relationship between the initial event and the subsequent crash. This can only be done through better crash reporting, in which officers clearly indicate any associated events that may have been contributing factors.