On December 18th, 2017, the U.S. Department of Transportation implemented a controversial mandate requiring the vast majority of truck drivers to record their working hours using electronic logging devices (ELDs). ELDs are harder to manipulate than traditional handwritten paper logbooks, and thus make it more difficult for drivers to violate restrictions on their working hours without getting caught by government inspectors. Because the work-hour restrictions (known as “hours-of-service” (HOS) regulations) are designed to reduce driver fatigue, the ultimate goal of the mandate was to reduce accidents on roads and highways. We combine detailed data from millions of driver inspections and all federally-recordable crashes from January 1st, 2017 through September 1st, 2018 to assess the effectiveness of the mandate. Using a difference-in-differences strategy, we show the mandate clearly achieved its first-order effect: drivers increased their compliance with HOS regulations, with drivers for small carriers most affected because many large carriers had already adopted ELDs and violated HOS regulations infrequently prior to the mandate. However, there is no evidence to suggest that the number of accidents decreased. Our results show that accident counts for small carriers did not fall relative to large carriers, and may have increased. Further, drivers for small carriers appear to have increased their frequency of unsafe driving (e.g., speeding) in response to the productivity losses caused by the mandate, which could explain why accidents did not decrease. We discuss implications for policymakers.