I couldn't say it any better than jobob did.

I'd add that I don't know what difference it makes to an automobile driver whether she's doing hill repeats or just passing through. Unless the motorist is ALSO doing hill repeats, they'll encounter her exactly once.

As far as it being a private road ... that's a tough one. Our group does a couple of loops through gated communities. Although I don't join them for it, they do sprint intervals there one day a week. I would hate to see the relationship get confrontational. Because the low traffic density means they tend to be the safest place to ride.