Permeable tarmacadam or asphalt is also known as porous or pervious, though in the trade it is almost always referred to as open-graded tarmacadam and in simple terms is merely the standard hot-mix product with reduced sand or fines (finely graded aggregates) which enables water to drain through it.
The reduced fines in the permeable tarmacadam mix provides interconnected but stable air pockets within the layers of tarmacadam which allow rainwater to flow through it. To be effective however, permeable tarmacadam relies on the aggregate sub-base layers also being porous whilst still supporting the tarmacadam. Properly installed permeable tarmacadam will reduce both the rainwater run-off volume and run-off rate, and also reduce pollutants entering storm water drainage systems.
Permeable tarmacadam systems can be designed to collect or harvest rainwater for later use, or allow the surface rainwater simply to infiltrate the ground. In terms of a basic installation there should be no real difference in cost between a permeable tarmacadam driveway or patio and a standard one, and both offer similar durability in a domestic installation.