Detached Garage- Floating Floors Needed?

My first post, be gentle. Okay, so I've been reading a lot of different books and surfing the interweb and this is the best place I've found, hands down. I'd like to thank you all. I'm building a small studio in a detached garage here in Seattle, WA. The house is a half block off the interstate here, so noise is less of a concern. All the comments I've heard so far is that if you're starting from a concrete slab, you don't need to build floating floors. True? The garage is between my house and a neighbor, who doesn't care. The noise off the interstate is like your next to the ocean at all hours of the day. If I don't need floating floors, then what should I put down on the concrete to deaden the room a bit? Thanks in advance