Hi, Escobar...
We have a similar question going along on this post: http://videomaker.com/community/forums/topic/interviewing-people-on-the-street?replies=20#post-49184
about interviewing people on the street that you might want to look at. I added some links to Videomaker legal stories that might help you.
This story below addresses some of your questions about permits:
http://www.videomaker.com/article/809/
Basically, every town is different. Some don't expect you to pay for permits at all, they just want to know that you're trying to go about it legally. Larger towns, like New York City, have rules that are strict and sometimes rigid. We know of many guerrilla producers that just go in and out stealth-like and have no problems, the road-blocks arise when you have to stop traffic, put yourself or the public in danger, have a full crew of equipment, and have crew-members that generally excel in making nuisances of themselves.
Many cities actually invite filmmakers to use their streets as background, and you can call your local city hall and make queries.
As for your question about getting into trouble if you use street signs or city names in your film if someone sees them - not likely. Even without permission, these are public areas and no one can completely control who has taken images of public signs. They can, however, give you grief if you make fun of that city, or business that is seen in the background.
Check these What's Legal articles for more help:
http://www.videomaker.com/article/12917/
http://www.videomaker.com/article/13774/
http://www.videomaker.com/article/13773/
Jennifer O'Rourke, Managing Editor - Videomaker