Using the Google Keyword Tool, since search volume numbers are now available, you can estimate (with a degree of success) the kind of traffic you'd get for key phrases that you optimize for. In a particular webmaster's experiment, he ranks 1 and 2 for a specific phrase. The keyword tool reported 2500 monthly visitors. In reality, the webmaster only gets 600 visits (which is about 25% of what the Keyword Tool reports).
In another experiment, he's seeing only 10% of the visitors that the Keyword Tool reports. He suspects that there could be other factors in play, such as the newness of this site or the fact that these results may vary by market.
Another webmaster reports that he has only seen 5% of reported visits.
Still, though, it's interesting to see how the data from the keyword tool comes into play. Surely, there's a lot of factors, and it may possibly also relate to the accuracy of the keyword data altogether.
Forum discussion continues at WebmasterWorld.
