…at Los Penasquitos Canyon Preserve (and anywhere else for that matter, I guess): if you do not comprehend what these signs mean, please get off that bike.
Every single time I hike in the canyon, there’s at least one (group of) mountain biker(s) riding on the single track trails. I know they’re more fun, but they’re not for you, that’s what these signs mean.
If you ignore them, you’re giving the entire group of people you represent (willingly or not) a bad name. If you want to ride narrow single track trails, buy your own piece of land, where you make the rules – and create and maintain your own set of trails. On public lands though, please abide by what rules have been set out and agreed upon so that all visitors may enjoy the experience*. Thanks.
*) which translates to not having to jump off into the Poison-oak because some jerk on a bike shouts “on your right!!!” from behind.