"...those big red R's and L's on fence post and telephone pole..."

A curious passage from Sinclair Lewis' novel Free Air:

She rarely lost her way.  She was guided by the friendly trail signs -- those big red R's and L's on fence post and telephone pole, magically telling the way from the Mississippi to the Pacific. (p.69)

The story involves a young woman's drive from Minnesota to Seattle in 1919.  I'm puzzled by the reference in the text to those letters on fence posts (which aren't mentioned again and are not important to the plot, but piqued my curiosity).

At first I thought the reference was to geology survey markers, like the "brass cap" above that I photographed in the woods in northern Minnesota, but those markers are driven into the ground; sometimes signs are posted on trees noting the location of nearby markers, but they don't fit Lewis' description.

Then I thought perhaps the reference was to railroad signage, which could be mounted on nearby telephone poles, but he also mentions fence posts.  And how to these letters "tell the way" across the western states?

I'm at a loss.  There's some historical trivia here which may or may not still exist.  I'll throw this out for readers, to see if anyone has seen (or knows of) such signs.

Addendum - A hat tip to charles for suggesting the Lincoln Highway:

In 1914, Effie Gladding wrote Across the Continent by the Lincoln Highway about her travel adventures on the road with her husband Thomas. Subsequently, Gladding wrote the foreword to the Lincoln Highway Association's first road guide, directing it to women motorists. Her 1914 book was the first full-size hardback book to discuss transcontinental travel, as well as the first to mention the Lincoln Highway:

We were now to traverse the Lincoln Highway and were to be guided by the red, white, and blue marks: sometimes painted on telephone poles; sometimes put up by way of advertisement over garage doors or swinging on hotel signboards; sometimes painted on little stakes, like croquet goals, scattered along over the great spaces of the desert. We learned to love the red, white, and blue, and the familiar big L which told us that we were on the right road.

That should take care of the "L." Still working on the "R", and I'll see if I can find some pix.

Found one.  From Nebraska, presumably restored, dated 1920: