Mystery: Tombstone Highway

Originally uploaded by afiler

I came across this photo of a sign in my hometown county, Kittson County, that I wasn't aware of before. All my research has proven fruitless so far.

Can any reader enlighten me as to what the significance is to the sign? What is/was 'Tombstone Highway'? Why was it called that?

12/15/2015 UPDATE:  I recently found out the answer to this mystery was, ironically, contained within the historical essays archived on the Red River Valley website, co-administered by me and Kris Baldwin Ohmann.