I think the signs are a type of thing that grow on people. It is just like marketing. When I used to use email blasts for another job I was told that the bulk of your sales would be on the 6-8 email. Less then 1% respond to an email blast the first two times they read them.
I imagine the numbers are close to the same with signage. The more someone sees your trucks around town the more your reputation can be built that you are working and not going anywhere for awhile. I agree with "Jasonlawncare" that you may not get a phone call just because you have a sign but for the cost and effort it can not hurt.
In my area it is a must as i do get people that walk up to me while working to see if i can give them quotes to either cutting or treating their lawns. To my defense we have a lot of outsiders and a lot of people from NY / PA that have summer places and are always looking fro someone to do the job right and on time. So a little different from the year round community.