This is the era of the cell phone. Why would you ask a passerby to do that which you can do easier and better yourself? I am assuming you are in a coverage area, which is actually pretty likely if you are on a major highway. If you are in an isolated area, a drive by is more likely to spontaneously inquire if you need assistance without any sign being displayed.

My experience has been that getting outside assistance has not been a significant problem. I, for one, regularly stop, or, at least, slow down, survey the situation, and make eye contact and/or inquiries.