That would depend entirely on the person speaking. With mobile phones now, about anything is possible: from "I am on the road" to "I'm just about ready to start to head out the door".
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Anonymouson my wayThe literal meaning is that you are in the progress of making your journey, but in addition to that, you can also use it when you have not yet started as a way of saying that you will be literally on your way in a very short time, i.e., when you want to show that you will hurry.