We generally offer a 5% discount on all residential if they allow us to keep our signs in thier yard for 1 month. This is already a fixed price in the total anyway. Alot of our signs are put up all over the place. Trees, Post, Other Ad boards (Wooden) etc..... Each one of my guys keeps 5 in their trucks and puts them in various places when we see a good location as well as when time will permit. Be careful though, your state highways might have laws against doing this. We have very strick laws here in Maryland and if you post one to thier signs, they take it down and trash it. They can also fine you for littering.
So bottom line, place were you deem fit but do not take away from the surrounding area. Place in high visibility areas such as downtown, busy cities, busy towns etc.... Intersections galore but be tasteful. There are signs everywhere. Signs that have been there 20 years, post to those at the bottom. New shopping centers usually have (NEW PAD SITES AVALIBLE) post to those underneath. There are many areas you can place these to drive business. Just be creative and not messy.
Oh and one more thing. We did something 2 years ago that you and your company should do ( Adopt a Road) This is free advertisment paid for by your state roads dept and the only thing ya have to do is pick up trash every 3 months. This we dont mind because the area is right down the road from the office. When we do this we also place like 20 signs on both sides of the road while we are picking up trash and remove them when we are done for the day. But the State sign stays there all year round.