“I really want to pay more taxes”
Said no one ever.
“Voluntary” = We hope this will shut all of you up while we put the infrastructure in place. Then next year there will be a financial crisis and we will “temporarily” make the per-mile tax mandatory.
Forever.
A lot of rich democrats, like Warren Buffett, say that, but then they hire $500 an hour accountants and lawyers to make sure they don't.