Dear Umbra,

What is the most eco-friendly driveway material?

Jean K.
Newington, Conn.

Dearest Jean,

Other than “no driveway because no car,” dunno. I can tell you all the characteristics possessed by a good eco-friendly driveway material, but no single material pulled ahead of the rest and parked itself in first place. Since I don’t know what type of drive you have — a long and winding road? A mere blip? — a face-saving answer would be “it depends.” But I get a little tired of writing “it depends” all the time. Better to admit out-and-out ignorance sometimes, don’t you think?

Not just for cars.

Still and all, I have a few concrete suggestions, and some non-cement ones, too. The constructed driveway protects our land from the depredations of our car(s). When we don’t have a designated path — one that stands up well under duress — the weight and scouring tires of our giant vehicles wear down the landscape and reduce it to a mud trench. Asphalt-y driveways are certainly durable, fighting off rain and oozing car fluids year after year. The trouble is, in wet weather, the fluids run off the driveway (an impermeable surface), and on sunny days the asphalt becomes incredibly hot (a “heat island“). The runoff damages the land and presents a water management problem in both urban and rural settings. The heat island effect, multiplied by the countless paved surfaces around us, raises local and regional temperatures and creates air pollution and health problems; it also bumps up greenhouse-gas emissions, particularly in urban areas (heat archipelagos, as it were), where soaring temps create soaring energy consumption for air conditioning.

A good driveway material, then, is permeable and not a heat sink. Grass, other tough plants, or dirt are obvious candidates, but they don’t meet the durability requirement, so most green driveways combine natural elements with a hard material. A traditional and effective option is gravel, particularly the angular-cut variety, which will hold on to its pebbly neighbor while leaving plenty of space for water to burble through it and into the ground.

At this point, I’m sure some readers are thinking, “There’s no way I could have a gravel driveway!” Don’t despair: There’s more than one route to an earth-friendlier driveway. The Environmental Protection Agency and municipalities in Vancouver, B.C. offer tidy roundups of green pavement options — some of which end up looking like lawns. These include lattice-like concrete pavers and plastic honeycomb-shaped grid systems, which are laid on a sturdy base such as gravel or crushed stone. Openings in the pavers are filled with soil or sand, and planted with grass. The plastic grid systems [PDF] I came across are made of recycled high density polyethylene.

Some folks are achieving a similar grassy effect with traditional brick or stone, leaving sand-filled gaps between each piece and allowing plants to grow (or not) as they wish. Other options include permeable paving stones and bricks, made from special concrete mixes that allow water to pass through. (Note, however, that cement is an ingredient in concrete, and since cement is made in giant kilns that emit toxic mercury, avoiding concrete when possible is always a good idea.)

Every new driveway material, not surprisingly, brings new ecological considerations. Is the material reclaimed or recycled, and will you be able to reclaim it, in turn, when it needs replacement? What maintenance is required? Is the supplier giving any manufacturing information to help you measure the product’s impacts? When it comes to “cool pavement” options or things like pervious concrete, what are the manufacturers claiming in terms of reduced greenhouse emissions? Sadly, I think some of these questions are outside the scope and space I have here. The best choice for you depends on your climate, the size and slope of your driveway, and also (likely) your budget. (Gravel is cheap; permeable paving stones are not.) Go forth and find an expert, and pave your own way in this exciting new field of green.