Pre-1973, before the federal government got involved, some states had speed limit signs that read "Reasonable & Prudent" or similar phrases on the freeways. Which basically meant you only got stopped if the cops thought you were going too fast for conditions (weather/traffic/etc). Cars travelling over 100 were common on those interstates.
Then there was 13 years of the 55 speed limit, which was widely ignored, both by drivers and cops. But it did get average traffic to slow down to 70 on freeways. That sort of set current practice where everyone goes at least 10 miles over the posted limit on freeways. So long as you're not going faster than the rest of the traffic, you're not likely to get stopped.