IMO, the Lincoln Highway and Great River Road are more worthy than some of the things we currently include (not that I endorse removing anything, I'm on the "include as much as possible" train). Both are signed extremely well, sometimes better than state routes. In the case of the LH, it's often signed better than Old 40 and Old 50 in California/ Nevada and are functionally equivalent to historic US routes.
Anything is signed better than Old 40/Old 50 in Nevada (because they are not signed at all), but the LH is patchily signed in the state. There's a few old route markers dotted about, and
one sign, but nothing forming something that could form something resembling a signed route.
As for California, the Lincoln Highway is better signed where it is signed, but Old 40 is signed more often where the two run in the same corridor. LH vs Old 50 is the opposite - Old 50 is only signed on the short bit in Folsom, and so has a high signs:length ratio that the LH (which is pretty well signed where it is signed) can't match!
(signage per following the routes on GMSV - there may be new signs, or ones I've failed to see).
Best to know your Lincoln Highways (yes, plural) then.
https://www.lincolnhighwayassoc.org/map/
That map is very useful, but is very fussy. There's not much signing of multiple routes - California, Iowa and Indiana are the only ones who bother. Some states have it well signed, others barely at all.
check the system you have exactly 25% clinched to see what I found that was signedLewis and Clark Trail too, there's a good online map for it (https://experience.arcgis.com/experience/87266b6613aa443cb437ef26c2077fff/) and it's really well signed in my experience.
Very well signed in the Dakotas and down the Missouri towards St Louis. It's like another Great River Road. It's OK signed in the St Louis area and the Pacific Northwest. It's not signed at all, AFAICS, in Montana.