I know the US interstate system required the building of new roads starting in the 1950s, but how were other major routes created prior to this?

For example, the original Lee Highway ran from New York to California, starting in the 1920s. Were roads built for this purpose, or did they simply designate existing roads as part of the new National Auto Trail? Likewise for Lincoln Highway and others.

I'm particularly curious about how this was done on the east coast, where roads were already long established, versus in the west, where there were perhaps fewer roads.

(This question originated from a discussion in /r/nova so any special info about Northern Virginia would be awesome if available!)