I used to live in Winchester, VA and have ridden down I81 many times to Deals Gap. Recently I rode I81 as part of my first SS1K effort from Southern Maryland to Little Rock, AR. In general, I81 is fast moving. There is a fair share of shipping trucks mixed with local and holiday car traffic. Because of the truckers, I haven't experienced a high percentage of left-lane hangers lasting long. If the weather is rainy, you can experience an occasional hazy/foggy situation in the lower dips, but usually conditions have to be just right for that to happen. In general, be cautious, but I don't consider I81 dangerous. Hope that helps.
Jersey