Its bizarre how we handle speed limits as a country.
The speed limit is supposed to be an upper limit, not a lower limit. But enforcement is haphazard so people treat it as a lower limit like you do, knowing that most cops won't pull someone over for going 5 over, even if they do, you're playing the odds that it won't be you who gets pulled over, and then that you'll be able to talk your way out of it or beat it in court.
At no point in the average person's calculus does the risk of death or serious accident even play a part - it's the ticket.
And the state has zero motivation to change this because they literally build their budget around a certain number of tickets and the cost of enforcing them. We pay the state to hire officers, buy cars and radar and sometimes even aircraft, build court rooms and hire judges and mail tickets and process fines and report to car insurance companies so we can have the speed limits we refuse to obey.
And we just kind of accept it because we know if there were no speed limits, more people would drive recklessly at ridiculous speeds around other people who aren't skilled enough to share the road with them.
The lawmakers are not unaware of this, and know that if they raise the speed limit from 65 to 70 then people will go 75-80 instead of 70-75. So their decisions are based around the data for people going 75 vs 80, not 65 vs 70, because they know people will speed.
And the cops know that all of this is true so most of them treat the speed limit of 65 as 70-75 but when you're driving, you don't know which because you don't know if there's any cop ahead, let alone what their personal interpretation of the speed limit is.
So you're in this weird prisoner's dilemma where you really should treat the speed limit as an upper limit if everyone else did too but it's so clear that they won't that you are wasting time if you don't speed. But then because everyone is forced into speeding, it's maybe even more dangerous to other cars if you actually obey the speed limit, because a cluster of cars going 70 catching up to a guy going 65 is going to mean a lot of braking and lane changing and that's the most dangerous thing on a highway.
So the right answer is really flow of traffic, and 5 to 10 over the limit unless you see a cop, see brake lights ahead, or traffic conditions suck.
With three exceptions.
One, if you are in the middle of nowhere and there's no cars nearby you should always assume there's a cop because you don't have the protection of other cars to keep you from being the one who gets the ticket if you do get caught on radar. Wait for someone speeding way more than you to pass you, then follow them about a half mile back and slightly slower so they're the ones who get tagged.
Two, if there's a change in speed limit, like a smaller highway that goes through a town center and the speed limit drops from 50 to 30, there will always be a cop waiting. Same thing with work zones where they double fines. Just not worth the risk to shave ten seconds off your trip.
And three if there's people at risk- a residential neighborhood, school zone, work zone with actual work being done, a cop outside their car, a big truck with an oversized load, a possibly drunk driver, a student driver, people who seem uncomfortable driving in the rain even though its only rain... whatever... just ####### slow down.
Man I am really procrastinating going for this run