Greening Public Spaces

While sustainable design has seemed to focus primarily on buildings, some of the most innovative work is taking place in the realm of public spaces - streets, sidewalks, parks, parking areas, and other open space.

Street trees absorb rainwater, provide shade, create a pleasant environment for people, and clean the air.

Much of the residential and commercial development that has taken place in the United States over the last 50 years or so has been auto-focused and land-intensive. A staggering amount of land has been cleared and paved over for new highways, suburban residential enclaves, regional enclosed shopping malls, and big-box stores, triggering a cascade of environmental problems. For main street districts, the two biggest problem areas - or areas of opportunity depending on how you look at it - are water and heat.


Towns and cities in the United States generally use one of two types of sewer systems: combined sewer systems or sanitary sewers.

Combined sewer systems generally date from the mid- to late-19th century, before cities began treating wastewater in centralized treatment facilities. Almost a thousand towns and cities in the United States have combined sewer systems, which serve approximately 40 million people. Combined sewer systems have separate channels for sanitary sewage - from toilets, sinks, bathtubs, etc. - and storm water runoff, but the two channels are linked in sections so that the sanitary sewer can be used to help with storm water runoff when storm water levels are high.

But there's a big problem with combined sewer systems: in heavy rain, the system can become inundated and discharge a combination of storm water and raw sewage into the river - or, in extreme situations, such as when the river floods, the whole system can back up into streets and buildings. Unfortunately, many communities have exacerbated the situation by clearing land for residential and commercial development and for highways, by paving larger surface areas with asphalt and concrete, and by allowing more new development than the sewer system can handle. Even in communities with sanitary sewer systems, the amount of land clearance, new development, and surface paving makes it easier for the sanitary sewer system to back up, increasing the likelihood of flooding. So, one of the keys to making communities - and main street districts - more sustainable is to relieve pressure on the sanitary sewer system by reducing the flow of rainwater into the system.


Large paved surfaces cause problems for more than just sewer systems; they also increase the temperature. Asphalt, concrete, bitumous roofs, and other hard-paved surfaces absorb heat, making it unpleasant to walk on the sidewalk in hot weather and increasing the strain on the air conditioning systems of the district's buildings. And, as hot air rises, it traps airborne pollutants, such as auto exhaust, close to the ground, adding to the misery of pedestrians. The Portland Cement Association estimates that the "heat island effect" of concentrated areas of paved surfaces impervious to water increases the temperature of the paved areas by an average of three to eight degrees. The most extreme increases take place in heavily paved areas, areas without shade, and areas paved with materials that don't reflect much light, such as asphalt. The heat island effect occurs in both small-town and urban commercial districts.

Improvements in either of these areas - heat or water - will improve the environmental health of your main street district, making it function better and turning it into a more pleasant place to visit. But the ideal scenario is to develop a sustainable public space plan that addresses each of these key issues, along with others, in an integrated way.

Public Space Plans

Four specific streetscape components are the core ingredients of sustainable main street public space plans:

Trees are essential to a sustainable main street district. They absorb rainwater. They provide shade, creating a pleasant environment for pedestrians and helping mitigate heat build-up at street level. And they help clean the air by absorbing carbon monoxide and producing oxygen.

There are some tricky aspects to planting trees in main street districts, of course. There must be adequate room for root structure. The trees selected shouldn't ooze sap on the sidewalk or shed their leaves too easily. Trees should be sited so they don't interfere with building facades, streetlights, or overhead wires. Finally, it's essential to plan and budget for ongoing maintenance. Be sure to work with a horticulturalist experienced with urban environments to select the type of trees best suited for your climate and your district's streetscape elements.

An example of permeable paving.

Permeable paving allows water to seep into the ground below, instead of draining into the municipal storm-water sewer.

Historically, most main street paving was permeable. In fact, until the late 19th century, streets were rarely paved at all and, when they were, the paving material was usually packed stone - gravel or cobblestones - or brick, both of which are permeable. Tarred road surfaces several thousand years old have been found in Europe and the Near East, so the technology has existed for millennia; but these roads were quite rare, and the technology more or less disappeared for 2,000 years. Modern asphalt was first introduced as a road-surfacing material in Paris in the early 19th century. In the United States, asphalt was first used for road paving in 1872 on Fifth Avenue and in Battery Park in New York City.

Almost all paved roads in the United States are now surfaced with asphalt. The American Geophysical Union estimates that the amount of impervious surface area in the nation - roads, sidewalks, and roofs - now covers more than 43,000 square miles, an area about the size of Ohio. According to the Center for Watershed Protection, about 65 percent of that paved area consists of streets, driveways, and parking lots. Fifty or 60 years ago, most rainwater was absorbed into the ground, pollutants were broken down by microorganisms, and clean water flowed back into the water table. Today, an alarming amount of rainwater pours into storm-water systems and, from there, streams into lakes and rivers, taking with it a host of pollutants, from antifreeze to pesticides.

Many types of permeable paving work well in main street districts. Some paving systems consist of mortarless bricks, concrete pavers, or paving stones laid on a permeable sub-base of gravel and sand or an engineered material. Other systems consist of cast concrete or composite grids that, when set in the ground, allow grass, moss, or other ground cover to grow through the openings in the grid squares.

By the way, permeable paving isn't the same as pervious paving. The former lets water drain between its joints and openings; the latter drains water through the material itself.

Rain gardens are essentially unpaved areas planted with trees, shrubs, flowers, succulents, or other vegetation that helps absorb and filter storm water. In this way, they are similar to green roofs and permeable paving. Like trees, however, they can also provide shade and absorb heat, making the district more inviting for pedestrians.

A rain garden's substrate and plant materials should be designed and selected specifically for the site. The substrate needs to drain water cleanly, with materials and microorganisms that will filter and clean rainwater as it trickles through the substrate. Plants must be suited to the environment and the local climate. Some should have roots deep enough to aerate the substrate, while others should have shallow, wide root structures to help prevent erosion.

Downtowns and neighborhoods that use rain gardens extensively have generally had great success with them. Alexandria, Virginia, uses rain gardens to treat about 20 percent of its storm water. Portland, while the Green Streets program in Portland, Oregon, incorporates storm-water planters between curbs and sidewalks. The planters look like regular sidewalk planting beds, except that the surrounding curb slopes into the street. Water that sheds from the street filters into the planters where it is absorbed by the plants. The City of Portland has found that the planters reduce water runoff by at least 70 percent.

Green roofs are more of a building component than a public space component, but they contribute so much to the environmental performance of a main street district that they're worth mentioning here (see section on green roofs on page # in "Greening Main Street Buildings"). As well as helping to insulate individual buildings, they offer the district some of the benefits of trees, permeable paving, and rain gardens, making them worthwhile candidates for public-sector investment and incentives.

On the Horizon

Quite a few public- and private-sector researchers are exploring ways to improve the sustainability of public spaces. Some of the more interesting, and viable, recent developments include:

Lunar-resonant streetlights. Developed by the Civil Twilight Design Collective, these streetlights contain light sensors that dim the lights on nights when moonlight is bright and turn them up on very dark nights. This not only cuts down on light pollution but also saves a considerable amount of energy: streetlights use 38 percent of all lighting-related electricity in the United States. The streetlight design won Metropolis magazine's 2007 Next Generation Design Competition.

Heat from the street. If you've ever walked barefoot across an asphalt street or parking lot on a hot summer day, you know just how hot asphalt gets. During the summer, the surface of asphalt streets can reach almost 115 degrees Fahrenheit. Two European companies - one in the Netherlands, one in Scotland - are turning this heat into energy.

Ooms, the Dutch company, embeds water pipes in asphalt streets. The pipes pump heated water to an underground storage tank, where heat exchangers convert the heat to electricity for the company's headquarters building. Cooled water is then pumped to a second storage tank and used to help cool the building during the summer. In winter, hot water from the storage tank is pumped  back to keep the street icing over.

In Scotland, a 1,300-square-foot parking lot generates 108 megawatts annually for Invisible Heating Systems, providing heat for the company's 5,000 square foot building.

According to an Ooms representative, its Road Energy Systems can produce 270 kilowatts annually from each square meter of asphalt paving - about half the amount produced by a roof-mounted solar panel, but at less than one-tenth the cost. He claims that, if all of the Netherlands' highways used the embedded water pipe energy system, the amount of energy created would surpass that produced by the nation's power plants., then click on "Innovations," then "Road Energy Systems."

Rubber sidewalks. When 26 mature street trees were cut down in her Gardena, California neighborhood in 2001 because their root growth was tearing up the neighborhood's sidewalks, Lindsay Smith was outraged. She began looking for solutions. By coincidence, the State of California was offering research grants to communities and companies to find ways to reuse old tires - Californians discard 20 million of them annually. With encouragement from a municipal sidewalk inspector in Santa Monica, Smith launched Rubber Sidewalks, a company that turns tires into, well, rubber sidewalks. The sidewalk slabs look like concrete, but they don't restrict tree root growth, they don't get as hot as concrete, they're easier on pedestrians' knees and ankles, and chewing gum doesn't stick to them! If one slab needs to be replaced, you simply pry it up and replace it. The newest model includes grooves on the underside of each slab that help guide tree roots and efficiently direct water to them. More than a hundred towns and cities in the United States, Canada, and Ireland have installed rubber sidewalks.

Self-powered lights, parking meters, and other equipment. Although they have been standard fare in Europe and parts of Asia for a decade, self-powered lights, parking meters and other equipment have only begun showing up in North America in the past few years. Solar cells are the most common energy source, supplying on-the-spot power for sign lights, streetlights, directional signs, parking meters, and parking kiosks. In addition to using sustainable energy, the solar-powered equipment also has lower maintenance costs, requiring no underground wiring (and thus no sidewalk demolition).