Interesting. I never would have thought of this....My greatest suggestion is to put this in a black and white to make it more appealing/interesting. And I wish I could see the whole feet, perhaps from the side.
Very creative idea. My only two nits are: the paw is in sunshine and the foot in the shade, and the foot itself looks too purple. Did you try this with the feet on other surfaces too, maybe which are less busy than grass? :)
Comparative transportation - cute. Wouold be better with a more even background - golf course grass, moss, or even concrete? Human foot is a little blue, could have more definition with better lighting.