Water Filtering Roads

So if you have ever been to a wastewater treatment plant, the system at one point goes through what is basically a giant brita filter.

It usually goes into a big pond where they let the biggest chunks settle out and/or float to the top, then they skim both top and bottom, and then chemical treat it, and then it goes to the giant brita filter.

I couldn't find a very good picture online, but here is something kinda close:



Except it starts with the largest rocks on the top layer, and goes smaller and smaller through gravel until it gets to charcoal. Well, a road could be made like that. Large rock/gravel on top, but with a porous cover over it so it isn't like driving on a dirt road, but then through the porous cover and then through the various layers of filtering rocks.

It wouldn't do for freeways, obviously, and probably not even major city streets, but all neighborhoods could have roads like this, and even larger roads could have shoulders like this. They could also potentially filter into the water run off pipes that already exist, but the water would be filtered before it got channeled out to our lakes/rivers/streams/oceans.