Given any road and any conditions, and barring any speed regulation, you get something akin to a bell curve distribution of speeds, which essentially defines "reasonable and prudent" (which, of course, isn't the same for everyone, yet I would still argue for minimum speeds on limited-access roads, not to mention "keep right except to pass").
If you set a regulated speed equal to the peak of that curve, nothing changes.
If you set it lower than that speed, the curve shifts down, but not as much as the lowered regulated speed, because drivers realize it's artificial (not to mention they get bored and start texting their friends, etc., which obviously introduces its own problems). And the more you shift the regulated speed below the natural peak, the greater the discrepancy between the two, because drivers realize it's bullshit. Thus generating less respect for speed laws in general.
If, on the other hand, you set a higher regulated speed, the bell curve will shift slightly higher than the natural peak, but only because that official sanction is an unnatural enticement for a few drivers. A dare, if you will, and so better not to post it at all. Such drivers would still be liable for any irresponsible damage they caused, and would likely have caused regardless of finger-wagging, anyway. But at some point, don't we all have to grow up and shoulder responsibility for our actions? OK, shouldn't we be expected to? Should we be driving if we aren't? Shouldn't we be encouraged to, rather than depending on some asserted protective government cocoon to restrict us? One that, believe it or not, isn't really always there, if ever? Think "jogger meets nature" in the LA hills.
Most drivers, however, faced with artificially high limits, will simply drive at the speed at which they were previously already comfortable. Higher speeds scared them before, and gentle coaxing won't change that.
Therefore, the, "We can't post higher than 65 because they're doing 75 already," argument is pure delusional nanny-state crap, since the interstate highway system was even designed for a "reasonable and prudent" 85mph on '50s-era suspensions and brakes in the first place.