Total Traffic Estimation

It's not really that simple, unfortunately.

First of all, Google's own approximations of search volume are just that - approximations; and sometimes those approximations are woefully inaccurate.

And don't forget that if a keyword did receive exactly 1000 searches per month at Google and you were ranking in position #1, it doesn't mean you'll receive exactly 1000 visitors/month. Some users may not click on anything before conducting another search; others will click on sites further down the list, below yours, if the title/description/url looks more appealing and relevant; others will click AdWords ads at the top/side of the page.

Then, of course, these figures you see bandied around the web that profess to inform as to the overall proportion of web-searches that Yahoo and Bing command, are also estimations. And even if they are pretty accurate in a general sense, search volumes often fluctuate by season, and it doesn't mean that these overall proportions apply equally to individual searches, anyway. Indeed, they certainly won't - that much is a given. There are sure to be some keyword phrases for which Bing and Yahoo command far more searches than Google. Yet to attempt to extrapolate these figures yourself based on Google's "estimated searches", and the search-share figures you see on the web, you'd figure that Yahoo and Bing receive far less of them.

Go figure. These things are never 100%.