This is a case of "if you show up you'll understand". They aren't really public roads, per se. It's around a big industrial oval at.....the port of oakland (tada!). It's several lanes wide (this is not a narrow residential road. Think MacArthur-ish) and there isn't traffic or a sidewalk. Occasionally there will be cars, but it's as close to a closed course as I can imagine without closing roads. If you want to ride slow, stay to the LEFT, as the ride tends to hug the inside (it's all right turns) and if you're on the right side in the gutter (especially on the long, slightly uphill and usually in a crosswind section) when the group motors past at 28, you're ARE GOING TO CAUSE A WRECK.
Yes, it's a public road, and yes, you can legally do whatever you want. However, please don't. Dropped riders usually continue riding around and pick a spot to jump back in. This is usually easier on certain sections than others. At that point you try to get up to speed and get in the group and quickly and smoothly as possible (I've had lots of practice at this part.....)
Like I said, if you show up it'll all make sense. No one's going to yell at you for being there and not being in the pack as long as you are a bit cognizant of the group and stay out of the way. The laps are something like 6-8 minutes (relatively long), so it's not like there is a constant stream of racers whizzing by.