Oftentimes, the posted speed limit has nothing whatsoever to do with the speed of traffic.
On US-3 from Nashua down into Massachusetts, the speed limit is 55 miles per hour, but the prevailing speed of free-flowing traffic on this road, which is 10 lanes across in some places, is essentially identical to that on I-93 which has a speed limit of 65mph.
And I routinely go 80 MPH with no issue whatsoever.