Raised beds around trees

Now that summer is waning (-: I'm starting to plan for next year. Part of the parkway in front of my house has a big Norway maple in the center and two other weird shaped trees at the end. The parkway area is about 20' x 4' square (in the city a parkway is the land between the sidewalk and the street). Because of the mature trees, nothing has ever grown here, not even weeds and the roots are very dense due to the age of the tree. The city tree butchers came by last year and took down a limb from the norway maple (that I did not want them to) which now allows for more light in this area.
What I was thinking is putting in a raised bed, perhaps 12" in height, to have some fresh soil to perhaps plant some semi-sun tolerant herbs and other things. I heard it's not a good idea to put a raised bed right next to a tree because it does something to the roots and can kill the tree. Does anyone know how far from the trees I should keep the raised bed from affecting the tree's roots? I was hoping 2' on each side of the trees but I'm not sure about this.
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