There is a section on I-81 South just after Blacksburg, VA and before Wytheville that always seem to have a lot of state troopers or county sheriffs. They park on on ramps which are very well hidden as you drive south. Definitely take it easy in that section. I've seen over 5-6 different police in that section on any given day.
After Wytheville it usually didn't seem as bad.
Similarly, on I-81 South just south of Roanoke, there are several dedicated congested areas with lower speed limits. These are patrolled heavily as is around Harrisonburg.