Here's an example from one trail system. This used to be beautiful narrow singletrack. I think it originally was one of the ruts that now has grass in it. Every time there is even a bit of a rut people start riding on either edge of it. Rocks start poking out, and they ride a bit wider to avoid them. Then a bit wider, and a bit wider more. This is 100% bikers. There are very few hikers and zero horses here. Unfortunately in places like this we have no features like rocks, logs or brush to help block sections off.
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