Actually, if you use Large Diaphragm Condenser Mics (the standard VO mics) you speak pretty much normal volume, most of the time. It's not like you scream into them, unless you once in a while get a job where you have to do that. In terms of people hearing your voice, I doubt people will hear it in the parking lot. I mean, do they hear your normal speaking voice in the parking lot? I think that's just typical beginner performer's paranoia.

Instead, you want to get some foam around you, or around the corner where you record anyway due to reflective reasons. Typical VO mics (large condensers) are fairly sensitive, even cheap ones. If you talk into one of those without any sound treatment, it'll sound like you're talking in a bathroom because the mics pick up all the sound reflections which you normally don't hear. Hence, you need to get some foam or blankets around you in any case, to cut down on those reflections. Look up sound absorption on youtube, some VO people talk about how they used blankets in the beginning for that. While that is not sound proofing (difference between sound absorption and sound proofing) it will dampen your speaking voice somewhat coming out of there. Chances are people don't hear your speaking voice that much anyway and if you get blankets around your recording space (to cut down on reflections) they will hear even less.