ExpertAdWords

If you’re using Google Local Search as well as a national campaign, there is a little fact you should know.

Google ranks Local Search above a nation-wide search regardless of the quality score.  What this means to advertisers is that their local search will trump their national search despite quality score and bidding.  If your nation-wide search has a higher quality score and appears in the third position and your local search has a lower quality score and shows in the seventh position, the local search will show to users despite the quality score or bidding of the national campaign within that local area.

Theoretically, the relevance of the local search will become better because the ads for that area can be personalized and users will see the additional copy and click on the ads.  The quality score should improve. At the beginning of the campaign be aware that there will probably be degradation until the local quality score improves.

There is no way I know of to prevent this from happening.