Speeding enforcement in the United States is fairly loose, with usually roughly a 10mph buffer. (The exception being various "speed traps", atypically mostly rural towns that rely on ticket revenue for an outsized portion of their town budget.)

I think this adds to the problem with left lane issues; even if the "true" speed limit is 80mph, if the "posted" speed limit is 70mph, some people will do no more than 70mph no matter where they are.