Is there an Adwords Traffic Estimator?

Yes, but you need to be an Adwords advertiser to access the tool. 

If you are setting up a new campaign or Ad group, or modifying keywords in an existing Ad group, you have the opportunity to get estimations of traffic. After you enter new keywords you will see the 'Estimate Traffic' button. Clicking on it will bring up data on 'Clicks/Day', 'Average Cost-Per-Click', 'Cost /Day', and Average Position. Unlike Overture, Google keep their traffic data under wraps. 

Third party tools like Ad Words Analyzer (which uncover targeted keywords with high search volume and very little competition) are often used by successful advertisers in preference to the standard tool.