Google has an AdWords tool for advertisers to better estimate the traffic they may get from a particular keyword phrase. The tool is named Google AdWords Traffic Estimator. Google themselves says;
All estimates are provided as a guideline, and are based on system-wide averages; your actual costs and ad positions may vary. To view estimates based on your keywords' performance history, use the Traffic Estimator within the appropriate Ad Group.
But PPC managers like GuyFromChicago in a DigitalPoint Forums thread says;
The estimator is for entertainment purposes only
(seriously, don't waste your time with it)
Others estimate that the estimator is "off by a factor of 3 usually."
Forum discussion at DigitalPoint Forums.