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If you are only targeting for US traffic then I would definitely set GWT to target the US. You will never be able to completely cut off foreign traffic and you shouldn't want to however if you want to increase the visits to the site you should build backlinks from US sites, blogs etc. and start using social networks and target Us nationals.

Hello, Yes google uses many signals to identify the related searches for a query, but the most important factor is user behaviour on google searches. When many users type "x" and then are not satisfied with resutls they browsed, than they type "Y". Google will identify "y" as a related search for "X". So influence related searches for G, you have 2 solutions: 1. Push/guide users to search for Y query after X. 2. Code a script that can do some manipulation (grey hat stuff, i did it 2 times for some clientd of mine ). If you have further questions, get back to me. Thanks