Most places (in the US) utilize the 85th percentile rule - the speed at which 85% of traffic naturally travels = arbitrary and stupid.
You just defined a rule, i.e. the exact opposite of arbitrary. The speed you described is based on something physical. Also the reason the speed limit has an upper percentile is because two elements affect road safety: Total speed, and speed variance. The 85% rule exists to reduce upper speed variance improving road safety. It was set based on a statistical analysis of accidents, and the only "arbitrary" component of this is how many citizens are considered expendable to keep cars moving quickly, and even that wasn't completely arbitrary as that point was chosen to be a point below where the accident rate increases at a significantly higher rate.
Also the US government no longer recommends the use of the 85% rule and the guidance to states is to set speed limits based on local context and road design.
If you are distracted by speed limit signs to the point where you can't focus on driving, hand your license in. Especially in the USA where the skyline is made up of nothing but billboards and adverts.