Long time lurker. First time poster!
First off as..as far as using street signs AKA bandit signs; they are highly illegal in most places in the US. There is even an organization dedicated to stopping their use: h**p://causs.org.
Any ways If you are going to do this; hire someone to set them out after 5pm Friday and pick them up Sunday night. Code enforcement officers don't work weekends!.
Focus placing signs at high traffic areas, traffic bottlenecks, places where people congregate (shopping malls, movie theaters, retail stores, etc).
Use property appraiser records to find homeowners who own houses near busy streets. Offer, to pay a nominal monthly fee to hang your sign on their property.
Approach pizza delivery places and other businesses that regularly give out coupons in your area. Offer, to print coupons with yours and their message, on the coupons, in exchange for them distributing them to their customers.
Get flyers made and placed at communal mailboxes.
Have small, gaudy, florescent stickers made and placed at gas station pumps, restaurant drive throughs.
Research, whom comes in contact with your target demographic. Then approach them and set up a profit sharing arrangement for every lead they send your way
Any more ideas?