FHWA estimates that approximately 20 percent of all incidents are secondary crashes (10). Many factors affect whether or not secondary crashes will occur. Recent research (11) indicates that the primary contributing factors to the occurrence of secondary crashes include primary incident type, primary incident duration, and time of day. FHWA defines secondary crashes as the number of unplanned incidents (including crashes, engine stalls, overheating, running out of fuel, etc.) beginning with the time of detection of the primary incident where a crash occurs as a result of the original incident either within the incident scene or within the queue in either direction. To calculate secondary crashes, an agency needs to know the number of total incidents as well as the number of secondary crashes. The percentage of secondary crashes is simply:
((Number of secondary crashes)/(Total number of incidents))∗100
The use of secondary crashes as a TIM performance measure is noticeably lagging on the national front. In part, this is due to the difficulty of needing:
- The direct observation of a queue by responding parties to the secondary crash, and
- The recording of the incident as a secondary crash either on the crash report from law enforcement or in the TMC operator or service patrol logs.
A concern expressed by some metropolitan areas interviewed from the TIM Performance Metric Adoption Campaign was that the FHWA definition is too broad and is subject to interpretation (9). This concern relates to the fact that this definition does not include specific requirements for when (temporal) or where (spatial) the secondary crashes occur with respect to the primary incident. The intent of the FHWA definition, however, is not to be so precise as to preclude the field level judgment from responders on the scene as to if a crash is indeed secondary. This judgment call can also be made by transportation agency operators tracking incidents via CCTVs. Incident responders are encouraged to use their best judgment and to rely upon their experience as to whether a crash is secondary or not.
The difficulty in the application of this definition exists primarily with the assignment of crashes to a secondary status after the fact. Many transportation agencies do not have detailed enough data collection or archival functions on their roadways to determine queues resulting from an incident after the fact. Absent that data, it is common in secondary crash special studies to assume temporal and spatial criteria that relate the event with the primary incident, independent of real-time traffic data and often limited to the same direction of travel. For example, a secondary crash might be defined as any event that occurred within one hour and one mile upstream of a known primary incident. A literature review of temporal and spatial limits performed for an analysis of the interdependency of incident duration and secondary crashes found no agreement on the limits. Spatially, the studies ranged from less than 1 mile to 2 miles, while temporally, the studies ranged from 15 minutes to 2 hours (12). Another study used criteria of 1 mile and clearance time plus 15 minutes. This particular study found a definitive primary-secondary relationship for 35 percent of the incidents studied and determined that the likelihood of a secondary crash increases by 2.8 percent for each additional minute of clearance of the primary incident (4).