That's the red line in this chart: https://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/US...
The period of most rapid car safety improvements came during a single ten-year period, from 1910 - 1920, where deaths, again, expressed per passenger mile, halved in only ten years.
If the technique wasn't known until the 1920s, the issue of the patent exhibits extreme prescience on the part of both Charles Goodyear (1800-1860) and the US Patent and Trademark Office.
https://en.wikipedia.org/wiki/Charles_Goodyear
Synthetic rubber wasn't invented until WWII, when natural latex supplies were rendered unavailable due to WWII.