Never mind all of us saying how bad it is to drop litter. What about some constructive suggestions to solve the problem.
Some have said they pick it up, which is great, but sadly wont deter the **** that leave it.
Perhaps the centers can put up a big picture right at the start of the trail, it could show the trail with litter on it and tell everyone that leaving litter is stupid. Make it a big sign with a clear picture and a simple clear slogan – make it **** obvious and brutal, the message may get through.
Offer a parking refund for anyone who takes a bag round the trail with them and brings it back full of trail litter.
We did a similar thing when I was a dive guide in Egypt – we cleaned up some of the shite from reefs in the Red Sea using volunteers help.