In four randomised field experiments, researchers at Columbia University found that lawn signs increased vote share by about 1.7%. They published results of their research online on Christmas Day.
That isn’t going to win an election single-handedly but it could be the difference in a tight race. The advertising didn’t drive turn-out but it did increase awareness of a candidate.
Sasha Issenberg, author of The Victory Lab, looked at lawn signs for Slate magazine in a 2012 article. Issenberg noted studies that showed a correlation between signs and voting. In one study, “households that displayed either an American flag, football insignia, or campaign sign were 2.4 times more likely to have a resident who voted in the elections than houses which had none of the three.”