Where I live (Texas) the de facto norm really does appear to be that you're entitled to add 10 mph (~15 kph) to the posted speed limit, and that's roughly what

Where I live (Texas) the de facto norm really does appear to be that you're entitled to add 10 mph (~15 kph) to the posted speed limit, and that's roughly what the police enforce. People get really angry and start writing their politicians if police start frequently writing tickets for less than that, so they usually don't. Some police out in West Texas used to be notorious for setting up speed traps on the highways and enforcing the speed limit strictly (as a source of revenue), so the state legislature actually cracked down on them because people were so angry.