[throwaway0a5e]

Safe traffic speed is a tautology since (at least per current civil engineering doctrine) speed limits are based on the Xth percentile (where X is usually somewhere in the 75-90 range).

Speed limits themselves only have a passing correlation to safe traffic speed. Almost nobody actually follows them intentionally at scale (traffic just moves at the maximum safe speed which just sometimes happens to be at or below the posted limit) which is why most municipalities are abandoning the "slap a sign with a low number on it" approach for "traffic calming" road features. Traffic moves at what it considered to be the highest safe speed for the conditions (a bunch of factors too long to list). You're seeing high differences because there's many roads that have speed limits that are unrealistically low for light traffic conditions (often below the designed speed of the road) and traffic density (as opposed to visibly or a sharp curve) was the road condition that was the bottleneck on speed on most busy roads. 'Rona has widened that bottleneck so of course you're seeing bigger differences between the posted limits and the traffic speed on those roads (which is probably all roads if you only ever drive during daytime hours in urban areas).

Edit: kind of got off on a tangent there but the point I should have made is that speed limits are fixed whereas traffic speed is variable based on conditions. As conditions change both slowly over time and day to day/hour to hour the max safe traffic speed changes. With 'Rona chopping congestion across the board it's no surprise that you see traffic speeds much farther from the speed limit on roads where traffic volume was a big factor in the traffic speed.