If it means what I think it means, you take every engineer working on it (and maybe the years of research involved) and add it all up. Say you have a room with about 10 engineers with 10 years of experience per developer, you can claim there's 100 years of developer experience between all of them (maybe the overlaps not unique enough and its more like 30 to 50 years? but in this case I think they're rounding up, and I assume it means thousands of engineers involved in the project) that's how I took it.

My first interview in tech I was asked what the heck I was even doing with the D programming language, followed by the remark that in the next room (where all the devs were) there was at least 100 years of experience between everybody there, and not a single one knew what D was, my manager clearly did, which cracked me up.

It means over 18,600 engineering hours have been spent working on vehicle safety. This is a pretty common metric.