You can either buy ads to get traffic or use SEO to get traffic from search engines. Directories are a small way of boosting rank in search engine.
PR does translate to traffic but not in a direct way. You get listed on a page with PR N/A but the site's main page has PR5. Some of that link juice flows to your link's anchor text. If you picked the right keyword, ranking high for that keyword will result in some nice traffic to your site. Next challenge: structuring your site and pages so it converts the traffic to sales or clicks.
It's the backlinks that count to improve your SERPs then traffic comes from the SERPs. Only problem is it makes you dependent on the SERPs.
Finally one reasonable head between many blind ones. PR is nothing and 90% of people who submit to directories don't know or don't check for pr. Try submitting inner pages (categories or detail pages for website with brandable names) to social bookmarks website in order to rank these pages in G and get traffic and sales. Just one idea to deliver more, try other ways that apply to other type of websites too.
Each link with keywords in it helps some. Some diredctories have good rank or will have after some time. This will help your site so it can help. Bookmarks and articles also help rank and site rank well in search engines.
everybody wants traffic ? so how much do you want and what are you prepared to pay for it ? (targeted) and be realistic or do you want free traffic ?
There are some niche directories out there that will give you very targeted traffic. Start with a list of high traffic directories then narrow it by your niche. You'll find some that will send you traffic...and if your site is any good then you'll also get some natural links that will generate traffic for you too. Once that happens you'll rise in the SERPs and get most of your traffic from the search engines.
Web Directories are a medium to get easy backlinks. If you want the traffic, you can try article marketing. Articles tend to get high ranking in search engines but you need to do keyword research before actually writing articles. you may submit your articles to ezinearticles.com, this is web's most famous Article Directory. Keep on building links along with other marketing tactics, this way you will get good ranking in search engines sooner or later. hope it helps....
My take is that you have to diversify your backlinks, directories are the most basic way to get backlinks and maybe PR but as others have said PR doesn't necessarily translate into traffic.. your best bet is to start doing article submissions (several of them), doing some A-B-C-D exchanges with relevant sites, creating a backlink profile from web 2.0 sites and keep on doing that until you reach your goal....