These "cushions" and the barrels you depict are usually designed in a way that they collapse when a vehichle hits them - slowing down the vehichle while causing as little damage as possible. For example: the barrels are usually filled with water! Thought this fact might help in your idea development.
Very cool idea, and it seems to be a pretty well developed concept.
Here in Oregon, we get a lot of rain, so my #1 concern would be whether these mats will retain too much water, or if you plan on having them drain into conventional roof drains?
If you did plan on having them retain water, do you know how much? At roughly 8lbs per gallon, you can add a lot of weight in a hurry on a large roof - which may or may not be structurally sound enough to handle this. Just a thought!