Free on-line mapping s/w with route constraints?

I am taking a road trip from Baltimore to Memphis, and want to see what the time tradeoffs are among several different routes.

I seem to remember that used to allow you to constrain the route (e.g., specify taking I-77 vs. I-79). But it doesn’t appear to currently. Am I missing something? And if not, is there anything else available online? Thanks,

Have you tried the Triple A site? You don’t have to have AAA in order to use the mapping service. I believe you can specify roads to avoid.