The spring clean up is a favorite in our neighborhood. This year we filled 3.5 trucks. The site we use is on the city street next to a park. at times when it gets busy cars are lined up waiting to unload. This becomes problematic when other cars queue up behind them, not realizing they don't need to wait.
Some sign on a tripod saying, "Clean-up ahead, pass with care". We also have problems with people coming around the curve too fast so the basic "slow" would be helpful. In our case some arrow sign would be good as well, since we need to change the usual traffic flow.
For the last few years we have been supplied with traffic cones, which have helped tremendously.