The posted speed is usually an upper bound (though in some cases only a presumptive one, not a conclusive one, the way the law is actually written.)
> You could get a ticket for going slower if you are impeding traffic trying to safely go the posted speed.
You can get a ticket for impeding traffic, but usually going slower than the limit is not sufficient for that. (And may not even be necessary.)
> Are there roads in the US that specifically allow a range of speeds (beyond the variation seen because human drivers don't follow the law)?
Every highway (which is much broader than freeways) in California that doesn't have an expressly posted minimum speed limit, plus every one that does have an expressly posted minimum speed limit that isn't identical to the posted maximum limit. (California Vehicle Code Sec. 22400 allows a minimum speed limit to be established and posted -- which is still, where established, a soft limit -- and provides a violation for impeding traffic with or without such a limit, but does not set the posted maximum speed limit as a minimum speed limit, soft or otherwise, where no minimum speed limit is posted.)