Apparently the game will make 'trails' when you walk over a piece of land over and over, say from nook to your house...
http://www.nintendowiifanboy.com/2008/11/14/iwata-asks-development-folk-about-animal-crossing/
http://www.nintendowiifanboy.com/2008/11/14/iwata-asks-development-folk-about-animal-crossing/