I agree with Crankin. Not much you can do other than a polite request and maybe a few words on etiquette. Anything else is likely to be counterproductive. As to being too close to the trail's edge and fearing a crash, have had to leave the pavement and ride onto the shoulder of a road more than once without crashing. Can be done, at least sometimes. Knowing what to do and having the right bike helps. On my fat bike and my 29er plus, I actually deliberately ride off the pavement and onto the shoulder at times, just for fun. Pretty risky to do on a road bike with 25mm tires, though. Be careful.