We examine the effect of the introduction of ridehailing in U.S. cities on fatal traffic accidents. The arrival of ridehailing is associated with an increase of approximately 3% in the number of fatalities and fatal accidents, for both vehicle occupants and pedestrians. The effects persist when controlling for proxies for smartphone adoption patterns. Consistent with ridehailing increasing congestion and road usage, we find that introduction is associated with an increase in arterial vehicle miles traveled, excess gas consumption, and annual hours of delay in traffic. On the extensive margin, ridehailing’s arrival is also associated with an increase in new car registrations. These effects are higher in cities with prior higher use of public transportation and carpools, consistent with a substitution effect, and in larger cities. These effects persist over time. Back-of-the-envelope estimates of the annual cost in human lives range from $5.33B to $13.24B.