I went approximately 6 years ago on the self-driving tour, and agree with others, the road back then was horrible -- filled with potholes, rutted, dips, etc...
I drove a SUV, and couldn't imagine a sedan navigating well on that unpaved, dirt road.
Then early last year (2022), I saw here on TripAdvisor others posted that the road was "better" and they had no problem/issue driving.
If you go back and look at older messages/posts around early 2022 (or maybe late 2021) -- if I remember correctly, there should be others who share their experience.
Anyhow, I went last spring, but this time, in a lowered car, and like others said, the road was much better.
As someone else mentioned, the most challenging part was going up the dirt road during the incline.
I don't know if the road has degraded since then, but overall, I think most sedans can safely navigate on the dirt, unpaved gravel road. YMMV.