This. Safest speed is the same speed as everyone else.

I was recently driving down a road that had its speed limit dropped from 100km/h to 60km/h. My wife just did 60, and I told her to speed up so she didn't create a hazard.

30 seconds later, a local Commodore whips around us on a solid line on a windy road. It had a closing speed of about 30km/h. Everyone agreed it was quite dangerous, but she refused to accept that I was right.

I'll add a pic. A country road in Victoria, Australia that had no set limit (read: default 100km/h) in 2018.