There is some fantastic riding in the Chilliwack River Valley, and it's much closer to (your) home.
I think the signs were result of some quad riders trying to keep up to dirt bikes in the tight stuff!
From the Tamihi trails facebook page:
https://www.facebook.com/tamihichipmunk/photos/a.563657850400777.1073741831.554818411284721/563657873734108
"CRAZY QUAD GUY"
This sign you will find on several trails, use extreme caution if you enter a trail with this sign. You will find extreme off camber and narrow trail ahead.
Usually trails marked with this sign have already been proven as a high risk roll over that has ended with severe damage and/or injury. Not all trails are marked and sometimes the signs are removed, please use common sense on all trails in this area"