The University of Eindhoven in the Netherlands is conducting research on a new material to use in surfacing roads that would clean itself and the air around the road. That sounds like a sweet deal for places with lots of pollution like California.
According to the university, the normal road surface is infused with some sort of material that can decrease the NOx in the air. The NOx is captured by some Titanium Dioxide particles in the pavement and then some sort of photocatalytic material converts the NOx to nitrate.
Said nitrate is then washed away when it rains. The material in the asphalt also breaks down algae and dirt somehow to keep the road clean. The only downside I see is that some locations, like Cali, don’t get much rain so washing away that nitrate could be an issue. The surface would be 50% more expensive than normal asphalt, but promises 25-45% less NOx in the air.