Using whose definition of safe? Traffic engineers country-wide are already doing this, but they use "safe" as presently defined and people are still dying and getting injured. "Highest speed at which it is ever safe" would lead to 5mph on all urban roads as it's the highest you would want to travek is a party of school kids were walking along the road, or the dustbin lorry was ahead of you Precisely, it’s a fantastic idea, but maximum safe speed is a matter of opinion and of course affected by many constantly changing variables. In my opinion I don’t believe an agreement would ever be reached on the matter.