I have driven the road probably 30 times in the last 3 years in a variety of vehicles. There is one hill before the dam that is highly problematic for cars. On one trip, we helped a member of the public by hitching a tow strap from our truck to a Mini-van that could not get up the hill on the way back out. I believe the biggest danger is people being able to reach the trailhead in a car by pushing their vehicle, but find themselves unable to get out, especially on wet days. (the difficult hill must be Ascended on the way out).
Furthermore, if people with cars choose to drive the last 4 KM, there are undoubtedly going to be cases of lost mufflers, destroyed oil pans, etc.; possibly even full breakdowns.