The reason it can be the culprit in two very different extremes is because of its effect on groundwater supplies. During heavy rains, most of the water runs off the surface of the road and into creeks, rivers and streams where it ends up downstream from where it's needed.

If the pavement weren't there, the rainwater would instead seep into the ground, where it would replenish wells and aquifers the way it did before development sealed the ground's surface. Instead, the result is that much more water draining into already overflowing streams, making flooding even worse.

One possible solution to this problem is porous pavement. This specially designed surface allows rain and melting snow to seep through small holes and down into a subsurface stone layer before draining off into the subsoil.

"Porous Pavement parking systems are gaining credibility 'cause the ones they put in about 20 years ago have proved they're still working," said Michelle Adams of Cahill Associates, a West Chester, Penn., environmental consulting firm. "The oldest systems in this area are at Morris Arboretum and the Seimans site in the Great Valley Corporate Center. These are large systems, more than 15 years of age and still working without clogging. They also tend to provide a very stable surface because of the stone base underneath."

Adams said there's about five or six dozen such installations in the Delaware Valley region.

A layer of stone provides a bed for rainwater to slowly drain through as it returns to groundwater supplies.

"It's not widely used enough," she said. "But it's certainly spreading."

Depending on how you compare costs, installing a porous pavement system can actually save a developer money.

"It's usually less expensive than or equal to the cost of an asphalt system," Adams said. "You have an additional cost due to the stone under the surface, but you reduce costs from catch basins and piping so that pretty much evens out, and there's no cost differential from the asphalt itself. If you factor in the cost savings of the land because you don't need a detention basin it's much cheaper.

"It does add to design costs, but that's a minor element in the construction costs."
These paving systems are best suited for flat or slightly inclined areas. If a hillside needs to be paved, terracing the pavement so it remains level is the only option.

One question that's frequently asked concerns the surface's viability when a lot of cars are parked on it. What's the effect of oil and gas leakage?

"On the older lots you can see spots where somebody had cars that leaked a lot," Adam said, "and there hasn't been much research in this country on what happens. But they have done some studies in France and virtually all the pollutants are bound to the first couple of inches of soil above the bed. They found no movement in the groundwater below it, but we have not had those kinds of detailed studies here.
"We use it for main parking all the time."

As an added bonus, it looks like the surface is capable of handling Pennsylvania winters.
"That was a surprise to us," she said. "You don't get black ice on a porous pavement. The stone base tends to retain heat and melts snow faster. Customers have said to us that they don't have to plow the lots as frequently."

One of the area's old such parking systems, is located at the Morris Arboretum in Philadelphia's Germantown section. This botanical garden is part of the University of Pennsylvania and first installed a porous pavement parking system about 15 years ago.
"Car parking is on porous pavement," said Tony Aiello of Morris Arboretum. "The area where the cars and buses drive is impervious and was designed with a slight crest so that water drains to the porous surface.

"You can watch it during a thunderstorm, the water bounces off the impervious surface and runs down to the porous surface and drains."

The organization is satisfied enough with their current system, that it's planning on installing another when it builds a new horticultural center and classroom, Aiello said.

"It's held up pretty well," he explained. "There are a couple of things that need to be addressed, sometimes chunks scraped off, also sometimes it gets silted up so there is some maintenance involved, but we haven't kept up with that as well as we should.

"It's held up pretty well all things considered. Fifteen years is a pretty good lifespan. That's what we expected to begin with."