More information. There are two installations, one on Thomas Rd and the 101, and another on Roosevelt and 5th St. From the Scottsdale Museum of Contemporary Art's website:

Created especially for the Scottsdale Museum of Contemporary Art's 10th Anniversary exhibition, Seriously Funny, Nina Katchadourian's installation Monument to the Unelected takes the form of signs resembling those typically displayed on street corners in the Phoenix Metro area during political campaigns. However, Katchadourian's signs pop up anachronistically in February when no election is taking place. The 56 signs on two different sites display the names of all the candidates who have ever lost an American presidential election.

It is a fascinating public installation. Photos of the Roosevelt Rd and 5th St monument.