If you get a mirror, be sure to actually use it. I come up behind bikes with mirrors on cycling courses, and sometimes have trouble getting around because these riders don't hear me (pretty much all of them are wearing earbuds), and NEVER look in their mirrors. I use my ears and my neck, which, fortunately, still work pretty well. I can tell by the sound when a vehicle is coming up behind me, if it is going faster or slower, and how far to the side it is (the closer a car gets to the side of the the road, the more debris its tires hit). Get in the habit of turning your head to look behind you while riding. It is cheaper and easier than installing a mirror, and works remarkably well.