Chances are you encounter them all the time. You also encounter murders, psychopaths, and people who are armed to the teeth. Depending on how far you commute and how many cars you see, that's what determines how often the encounters are.

I ride over glass constantly, mostly from beer bottles. I don't know how the percentages break out, but I know it comes from passengers, transients, people hauling unsecured garbage, and a couple drunken fools. I have noticed that my commute home on Friday nights is consistently crazy in comparison with other times.

Having said that, I encounter people not paying attention because they're sleepy, messing with the radio, yakking on phones, disciplining kids, or are simply clueless much more often. They're the much bigger threat.