I figure that every time I have a nasty glare-down with one of these people I toss a little ill-will into the world (unless they're just that oblivious). So it is important to smile, wave, bow, or otherwise express my gratitude for the people who do take the time to realize there are other people around. They probably don't need my appreciation as much, but hamming it up never hurt anyone.
By the way, this isn't the freeway I'm talking about. This is a parkway, paths around the lake and other places where you'd hope drivers were more aware of their surroundings than their destinations. I have a silly, passive-aggressive fantasy about placing one of those accident memorials on the street by a recreational path. I'd do it up, with flowers and a big, fat cross. I would post a sign with a broken heart and the plea, "Please Slow Down". Manipulative? Yes, but I have to believe that making people think would do more good than harm - even if it just gets them wondering what a manslaughter charge would do to their insurance.