If you look at the statistics of what causes car accidents, everything from drunk or impaired drivers to people who are tired and fall asleep or people fooling around with their phones, if you remove a significant percentage of those people thru FSD, it will make the roads significantly safter.
If it just keeps a car in its own lane, it could prevent a lot of fatal accidents.
As you stated, sleeping, passed out, drunk, medical emergency, etc., are the cause of so many horrific head-on crashes.
Fewer accidents will also make the roads faster.
I’d expect vehicles to start talking to each other. This will enable them to drive closer together at higher speeds. Faster merges.
On the other hand, self driving cars enable the driver to do other things during driving time, increasing the number of cars are on the road. Faster transit will also increase the number of cars.