Speed limit is 65. You drive 60 because that's legal and better gas mileage Someone comes barreling along at 75, clips you, you both end up in the ditch, an hour later cops finally show up, an hour after that tow truck hauls you out of the ditch, you arrive two and a half hours late.
Versus, you drive 70 because 5 mph over the limit is reasonable and no cop is gonna bother busting you for 5 mph ticket assuming dry high vis conditions. Someone overtakes you at 75 because hey, same really applies to 10mph over. Plenty of time for him to react and pass you. No accident. You arrive 5 minutes early because you went 5 mph over the limit... 2 hours 35 minutes faster than you would have if you'd been a rolling road hazard.
Traffic does not, in general, obey the posted limit, it obeys the "this is reasonable for conditions" limit.
Here's an oddity, traveling 5 miles under the "reasonable" limit is just as likely to get you in an accident as 5 miles over it, the only difference is the "over" accident will be a little more dangerous. But traveling the reasonable limit reduces chance of an accident the most.
When I go 80 in a 65 where everyone else is going 75 (the PA turnpike for example) I'm actually less likely to be in an accident than someone going 65. I should still slow my ass down to 75 and be even safer... but racecar.