A recent study showed that, on average, lawn signs increase vote share by 1.7 percentage points.
According to the authors, "It appears that [yard] signs typically have a modest effect on advertising candidates' vote shares — an effect that is probably greater than zero but unlikely to be large enough to alter the outcome of a contest that would otherwise be decided by more than a few percentage points," the researchers wrote.
Whether 1.7% is material, I suppose, depends a lot on how close an election is expected to be, as well as how much budget there is. At $2-$5 per yard sign, it can be considerably more expensive than other forms of advertising.
But as John Thomas of C&E writes, "The simple truth is lawn signs are only effective at two things: driving consultants mad and making candidates and supporters feel good. They don’t win elections.