For what it is worth, here is my journal for trying to follow the Lincoln Highway across Iowa and stopping at Cedar Rapids:
http://www.mvermeulen.org/lincoln/index.php
The Lincoln Highway in parts turns into US 30 and in parts are smaller parallel roads. You can see comments like this one for July 8th - "Today some delightful cycling along the old Lincoln Highway. Also some rather obnoxious riding along US30." Not all of US-30 was awful but gravel shoulders and busy bits had me happy to follow Lincoln Highway signs when they pointed off the busiest parts.
Prior to departure I had read a fair amount about the Lincoln Highway. That was a while ago, but still remember (a) there was a competition between Lincoln Highway and early routes such as the Coast-to-Coast Highway, US-6 if I recall (b) there was a Lincoln Highway association that was promoting early paving of roads. Eventually that faded away as governments assumed more of that role - but then re-surged as a nostalgia organization (c) they often spliced together existing routes. In some places such as say the Platte River in Nebraska routes often converged to a more obvious routing e.g. follow the Platte. However, in Iowa it wasn't as obvious and hence there were four or five different parallel routes all crossing the state.