Major highways bypassed by non-toll freeways are almost always most fit for primary classification. For travelling between DC and Gainesville, I 66 is the best route. I 66 is a (usually) free-flowing high-capacity freeway whereas US 29/50 are very congested and have lots of traffic lights, acting more like business routes. The only case it would be better to take the Lee Highway over 66 is if the latter has heavy traffic. 

There is a part of US 50 that is bypassed by I 66 between Merrifield and Arlington tagged as since the Custis Memorial Parkway section of 66 is a toll road, making US 50 act as a shunpike. This would be one of the only rare cases for a bypassed highway like this to warrant trunk classification.