I agree, the clearer the better.
Be sure to think about how the average Joe will interpret the message.
As an example of what NOT to do:
In 2001 there was an atricle in the local paper with a headline like "Offroaders Tear Through the Colville", describing the damage done by people driving off roads over Memorial Day weekend. There were some examples of obviously bad behavior, like mud bogging in the middle of a meadow.
But they also showed a picture of tracks right beside a sign and claimed that showed that people were ignoring the sign and intentionally tearing things up.
When I went to look at the area, it was real clear to me what had happened. The signs said something like "Vehicle damage to vegetation is a violation of CFR...". These signs were posted in several places along the roads in the area, which has several large meadows popular for camping. The sign in the picture was right next to a track that had obviously been used for quite some time to access one of the meadows. So people interpreted the sign as meaning they should not drive over trees, bushes, etc. Therefore, they should use the existing track to avoid damaging vegetation. Unfortunately, that particular track crossed a low area between the road and the meadow that had become quite muddy due to wet weather before and/or during the holiday weekend.
So, people trying to do the right thing (or at least not do the wrong thing) caused issues that got them lumped in with those who did not care.
I'm not sure how you get the complete, correct message across in that situation, without trying to write a novel on a small sign.