One person dying does not equate to "unsafe roads" in the way that "widespread adoption of motor vehicles" does. People were frequently trampled by horse-drawn carriages prior to cars, but not at anywhere close to the rate after cars became so ubiquitous.

First, yes, but it’s not like people weren’t killed by horses or trolleys either. The death toll was never zero but it went up sharply when everyone was expected to drive much heavier vehicles at higher speeds.