Motion rant
Ok Motion officially sucks for real use. That was my original reaction, but then it grew on me. Well after trying for an hour to get a mask to behave properly, I'm really thinking of never touching the app again. I've always thought After Effects had the edge, but maybe it's more like a huge canyon between the two. I have two layers in Motion. Both are the same image, but one has some drawing on it. I want to animate the mask over time to reveal the drawing (essentially like you're watching the drawing happen. The desired effect is like in movies when they show a map and trace the route. No, I will not use the "roadtrip effect" application because it sucks for what I want to do. Anyways, I added the mask to the top layer and checked 'add', so that only what is inside the mask is shown from the layer. All other parameters are fine because I'm checked them countless times. Both layers will end up moving in the end, so there is some tracking to be done but it shouldn't be an issue. I have lots of After Effects experience but don't have the app because of funds. My point was that I know what I'm doing so they're not novice errors. Motion just sucks.
Comments
I actually accomplished exactly what you are describing, so Im positive it can be done in Motion.
Not being too familiar with Motion when I attempted this, I actually attempted at first to use a bitmap mask that I had created in Photoshop. This absolutely killed Motion's real time capabilities and I had numerous problems when rendering (I was actually animating an entire series of "trails" with several masks). When I switched to Motion's mask tool, it worked flawlessly.
The only difference between what you are trying and what I did was that your "road" is flattened on top of the image while mine was transparent. It would seem to me that it should work either way though...
One thing that is extreemly difficult when using Motion for this type of work is that it doesnt have a camera function (I believe AE does, but you would know better than I would).