Major vehicle crashes account for about half of all congestion-related traffic delays. The reported NIJ-funded study found that using UAS can reduce the amount of time needed to clear a crash scene, which reduces both the length of traffic delays and the length of time police officers are at risk of injury while managing a vehicle crash scene. In addition, the study compared the accuracy of measurements made using different crash reconstruction methods. It determined that UAS slightly reduces some measurement errors. Several law enforcement agencies across the country are currently using UAS in crash investigations, primarily to take aerial photographs in complementing other methods of accident reconstruction. A recent study compared the use of UAS with established methods used in crash-scene investigations. Using a mock crash scene, the researchers found that UAS used in support of traditional reconstruction methods that use a manual or robotic "total station," reduced the time to clear the scene by 35-45 minutes, and the time that officers were at risk in the roadway was reduced approximately 28 minutes. When UAS alone was used in crash reconstruction, it took, on average, 1 hour less than data collection by a robotic station and 2 hours less than data collected by a manual or robotic "total station." Factors that may hinder efficiency in the use of UAS are noted, such as weather conditions, low light, and Federal Aviation Administration regulations on UAS use.