Here are some useless street signs:
No Hazardous Cargo - First of all, I doubt whether even the drivers of trucks with hazardous cargo pay attention to these signs. Chances are, if you see one and your cargo is hazardous, you're going the wrong way. Also, how hazardous is this stuff? Shouldn't it be properly stored so it's not going to spill everywhere if the truck tips over anyway?
Bridge May Ice In Cold Weather - Retarded. There's one of these signs in front of every bridge in America. If you haven't figured out after seeing one sign and being on one icy bridge that the bridge may ice, you're not going to figure it out. When I come to a bridge that doesn't have one of these signs, I don't think "Sweet, I don't have to worry about ice on this bridge!" Instead, I think "Sweet, somebody stole one of those stupid bridge ice signs!"
Slow Deaf Child - OK, so he's deaf. Is he also blind? The first thing I was taught was to look both ways when crossing the street. Listening had little to do with it.
Winding Road - What are you supposed to do when you see this sign? Slam on the brakes and turn around for fear of the frighteningly curvy road ahead? No. You run over the sign, then proceed to take those curves at no less than 60 MPH to show them who's boss.
Truck Rollover Warning - Useless. Trucks should already know that if they turn too quickly, they're gonna flip. Morons.
Then there are the signs that are either impossibly hard to interpret or just plain stupid. Such signs do nothing but leave the typical American driver shaken and confused.
(some images: street signs usa)