I can't speak to the reality of any given evacuation route or scenario, but in general if the plan is mass evacuation, unless you plan on joining the mass, don't go near the route. The plan would be to block access, and to actively remove non-evacuees who are getting in the way. Meaning, if the plan is for evacuation by bus, private cars and trucks would be blocked from access - or shuffled to a parking area, where buses pick them up, and move folks to the evacuation site. Best planning option I know of, to get to the evacuation site, you would probably first go to a rally site, such as a nearby church or school, get documented, and get on a bus. That way the site planners have an idea how many evacuees are in their pipeline, headed for the site. I'm not familiar with the Frederick Fair Grounds, but if there's a freeway near it, I would expect all lanes to be commandeered for movement of evacuees towards the evacuation site, much like highways headed away from hurricane tracks before a landfall.

Ask around, your local or regional EM may have a published plan, if DC area plans aren't publicized owing to fears over targetting of evacuees by terrorists.

That's generalized planning, real world events can muck it all up of course - too few evacuators, too many evacuees, bad geography, not enough enforcement of the evacuation plan, not enough buses, too much or too bad weather. But the goal is not to create a death march to reach the evacuation site, which should offer food, shelter and medical help to those who need it.