I actually find that, given that I only use a single bud in my left ear (away from the road), I can hear my surroundings MUCH better if I use a bud that seals out as much sound as possible. If it doesn't, the wind noise in my ear causes me to crank up the volume to almost painful levels just so I can barely hear the reader. This is very distracting.
With a well sealed bud in the right ear, I can have the volume very low and still hear. I have used this method for years and I can hear cars at least a quarter mile away with one bud in.