Apparently, lawn sign don't make a bit of difference.
Lawn signs are a way for campaigns to make a little cash, and for those who plan on voting to support and for them to get names for their rolls...
In a national election, I really don’t think anyone in modern times is likely to have changed their decision based on a yard sign, unless it was simply they desired to follow the herd because most of their neighbors are visibly backing a candidate.. but an individual yard sign in and of itself is about allowing the supporter to show support, not about influencing others... at least in a national race.
Local race, name recognition needs yard signs could swing things.