This study sought to evaluate the relationships between vehicles on shoulder, congestion, and crashes. Three pertinent datasets on interstates were obtained and then integrated using a spatiotemporal approach. The analysis showed that about 36% of the crashes had vehicle on shoulder present in their vicinity, defined as 0.25 miles upstream and downstream of a crash site and 30 minutes before crash occurrence. The percentage increased considerably as spatial/temporal window expanded. In addition, congestion was found to be associated with about 25% of the crashes. The presence of both vehicle on shoulder and congestion was found for 11.7% of the crashes, signifying a high correlation between them and crashes. Based on crash narrative review, 1.8% of all crashes directly involved vehicles on shoulder and 23% of the carshes cited congestion as a contributor. However, there’s little indication in the crash narratives on how vehicles on shoulder contributed to crashes, beyond their direct involvement, or how they contributed to congestion which may led to crashes. Only 6 out of the 512 crashes flagged for review through the keyword search process specified a vehicle on shoulder as a contributor to congestion and subsequent crashes. While a small fraction of crashes were attributed to vehicles on shoulder, these crashes tended to be more severe than average interstate crashes.

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