If taxes were voluntary, they wouldn't be taxes anymore, but rather payments for services. No one would have to elect anybody - not more so than we need to elect officials who decide which car everyone should buy - people would just start paying to service providers they like (includes police, fire department, education, law and yes, roads). As soon as this service provider stops doing a good job, people would stop paying. That would be a real accountability tied to actual profit, not to some vague promise by a government official who doesn't give a shit after he's elected.

And of course, the usual question - but what about the poor who can't afford it? Well, if a democratic system means that the majority of people agree that helping the poor is a good idea, I see nothing impossible in those very same people giving money to charities voluntarily, without any system forcing them to. Except that in a voluntary system the charities are also much more accountable than a government, because as soon as this charity fails to deliver what was promised, people stop donating.