Yeah, usually high traffic ranks have higher page ranks, but there is no actual relation between page rank and traffic, except that one is calculated while the other is organic(get the drift??).
Page rank is determined by the number of back links to your website and the page rank of the back link.
If a site has high page rank, there will be more links to the site and since there are billios of people online, some are bound to click them, thus resulting in more traffic.