after they got used to 85. They drive 75-80 now, so 85 still seems safe. After driving 85 MPH for a while, they'd try 90 and realize that with newer cars 90 still feels safe (assuming good weather, light, etc)
I don't know where I (or people generally) would ultimately settle if there were no speed limit on the freeway, but it would be a lot higher than the current speed limits.