Freeways consume enormous amounts of urban land, and the space they occupy, from the pavement itself to the buffer zones alongside it and the voids beneath elevated segments, represents one of the largest pools of underused real estate in most cities. As urban populations grow and land becomes scarcer, planners, ecologists, and communities are rethinking what freeway space can do beyond moving cars. The approaches range from capping highways with parks to tearing them out entirely, and the outcomes touch everything from air quality and neighborhood economics to wildlife migration and stormwater runoff.
Capping Freeways With Parks
One of the most visible strategies for reclaiming freeway space is the cap park, a deck built over a below-grade highway segment that supports a public park or mixed-use development on top. A study examining 18 completed and 9 proposed cap parks across 24 U.S. cities identified four distinct development models that cities worldwide can adapt to green over freeway trenches and stitch divided neighborhoods back together.1Cities. Put a park on it: How freeway caps are reconnecting and greening divided cities These caps vary widely in ambition: some are modest pedestrian bridges with landscaping, while others span multiple city blocks and include playgrounds, performance venues, and commercial space.
A frequent concern about cap parks is that they will drive up property values and push out existing residents. Research examining ten highway cap projects completed between 1992 and 2016 in nine U.S. cities found a more nuanced picture. Household income rose in neighborhoods around all ten projects, and poverty rates dropped in the majority. But there were no consistent racial demographic shifts across the projects, and in most cases the number of housing units stayed the same or increased while vacancies fell. The researchers concluded that highway caps may offer a way to reconnect communities without uniformly triggering gentrification.2Cities. Bridging the gap with highway cap: Reshaping urban landscapes without gentrification That finding challenges the assumption that any infrastructure investment in a low-income neighborhood inevitably displaces the people it’s supposed to help.
The Space Beneath Elevated Highways
Where freeways ride on columns above city streets, the ground beneath them often turns into a no-man’s-land: dark, noisy, and usually devoted to nothing more than parking or storage. These “lost spaces” have become a design challenge in their own right. Research into user preferences for spaces under elevated highways and bridges found that people prioritize safety, visibility, physical coherence with the surrounding neighborhood, and a sense of belonging.3Journal of Place Management and Development. Reviving the lost spaces under urban highways and bridges: an empirical study In other words, these aren’t just engineering problems. People want the undercroft to feel like a real place, not a leftover.
Public art plays a surprisingly large role in making that happen. Studies of user preferences in these spaces find that murals, graffiti art, and other eye-catching visual elements strengthen people’s attachment to an otherwise alienating environment.4INFOLIO. Users’ environmental preferences in spaces under elevated highways and urban bridges Cities including Miami, Philadelphia, and Toronto have turned under-highway zones into skateparks, farmers’ markets, and outdoor galleries, precisely because those uses bring the foot traffic and visual richness that combat the dead-zone feeling.
Tearing Out the Freeway Altogether
Some cities have taken the most radical approach: removing a freeway entirely and replacing it with a surface boulevard. The most studied example in the U.S. is the replacement of the Cypress Freeway in Oakland, California, with Mandela Parkway. Research on the air-quality effects of that project found annual average concentrations of nitrogen oxides dropped by about 38 percent and black carbon dropped by about 25 percent along the boulevard alignment compared to what the freeway had produced.5PubMed Central. Effects of Freeway Rerouting and Boulevard Replacement on Air Pollution Exposure and Neighborhood Attributes
The air got cleaner, but the neighborhood changed. The long-time Black population along Mandela Parkway shrank by 28 percent, and property values jumped by 184 percent, outpacing West Oakland as a whole.6PubMed Central. Effects of Freeway Rerouting and Boulevard Replacement on Air Pollution Exposure and Neighborhood Attributes Affordable housing near the boulevard has helped some low-income residents stay, but the Oakland experience illustrates a tension that runs through almost every freeway space project: improving the environment around a highway often makes the area more attractive to wealthier newcomers. The health benefits are real, but who actually gets to enjoy them depends heavily on housing policy.
Air Quality in the Freeway Zone
Even where a freeway stays in place, the space around it has a distinct pollution profile. Researchers have measured spatial gradients of nitrogen oxides, black carbon, ultrafine particles, and particulate matter at varying distances from freeway edges, finding that concentrations shift sharply over relatively short distances.7PubMed Central. Characterizing Determinants of Near-Road Ambient Air Quality for an Urban Intersection and a Freeway Site A synthesis of real-world monitoring data across many studies found that pollutant concentrations fell into three broad patterns with distance from the road. Some pollutants, like carbon monoxide and certain ultrafine particles, dropped by at least half within about 150 meters and then continued a slow decay. Others, like benzene and nitrogen dioxide, decayed more gradually over the entire measurement range. And some, like total particulate matter mass, showed no clear trend with distance at all.8PubMed. Near-roadway air quality: synthesizing the findings from real-world data
That 150-meter figure has become a rule of thumb in planning. It helps explain why buffer zones, setbacks for residential construction, and vegetative barriers along freeways can make a meaningful difference for some pollutants but not others. Schools, daycare centers, and senior living facilities built within that distance face elevated exposures to the pollutants that do fall off with distance, which is why many jurisdictions now discourage sensitive land uses immediately alongside major highways.
Heat, Noise, and Water
Freeways affect the space around them in ways that go well beyond tailpipe emissions. Roadways can make up a large fraction of urban land cover, contributing meaningfully to the urban heat island effect. A systematic review of the literature found that road design, landscaping, surrounding building form, and pavement materials all influence local temperatures, yet roadways remain relatively understudied within the urban heat island literature despite accounting for up to 35 percent of the urban landscape.9Urban Climate. Mitigating urban heat along roadways; systematic review of impact and practicability Reflective pavement coatings are one response. Large-scale projects using reflective materials on roads and other urban surfaces have shown that lighter-colored surfaces reduce both surface temperature and ambient air temperature in their vicinity.10Urban Climate. Using reflective pavements to mitigate urban heat island in warm climates – Results from a large scale urban mitigation project
Noise is another constant companion of freeway space. Noise barriers are the standard mitigation tool, but their effects on how people actually experience sound are more complicated than decibel readings suggest. A study of three highway noise barrier installations found that while measured daytime noise dropped by roughly 4 to 12 decibels at points close to the barriers, residents’ self-reported annoyance only decreased in one of the three cases. In that successful case, the acoustic environment was restored to a condition similar to low-noise reference areas. A broader analysis across all three sites did show that larger reductions in measured noise were associated with a higher likelihood of people reporting less annoyance, but the relationship isn’t automatic.11PubMed. Noise barriers as a mitigation measure for highway traffic noise: Empirical evidence from three study cases A wall that cuts noise by a few decibels on a meter might not change how irritating the sound feels if the frequency profile or intermittent character of the noise stays the same.
Then there’s water. Freeways generate large volumes of stormwater runoff laced with heavy metals, oil, and rubber particulates. The strips of land alongside highways are increasingly being designed to treat that water before it reaches storm drains. Grass swales with check dams or infiltration features are effective at reducing runoff volume and capturing sediment and heavy metals, while engineered bioswales outperform them at removing phosphorus and bacteria. Wet swales, which maintain saturated conditions, are the best performers for nitrogen removal.12PubMed. Next generation swale design for stormwater runoff treatment: A comprehensive approach These vegetated channels take advantage of the very land that was previously treated as wasted buffer, turning the freeway’s margins into a working piece of green infrastructure.
Wildlife Crossings and the Ecological Footprint
Freeways are among the most effective barriers to animal movement ever built. The concrete, noise, light, and traffic create a wall across the landscape that most species cannot safely cross. Wildlife crossing structures, both overpasses and underpasses, attempt to repair that damage by giving animals a path over or under the road. A global assessment of these structures found that they allowed animals to cross in 98 percent of the data sets examined and improved movement rates in about 60 percent. But the structures prevented ongoing declines in wildlife movement in fewer than 40 percent of cases, and the researchers cautioned that for most structure types and species groups there still isn’t enough evidence to draw broad conclusions.13Journal of Applied Ecology. Do wildlife crossing structures mitigate the barrier effect of roads on animal movement? A global assessment
Width matters enormously. A study of 12 wildlife overpasses in western North America found that wider overpasses, roughly 40 to 60 meters across, attracted a more diverse set of species and had nearly twice the average crossing rate compared to narrower structures that didn’t comply with expert design guidelines. The researchers concluded that overpasses around 50 meters wide remain an ecologically sound and cost-effective solution, particularly for species that are sensitive to the width of the crossing.14PubMed Central. Wildlife overpass structure size, distribution, effectiveness, and adherence to expert design recommendations That is a lot of space, roughly half a football field, and the cost reflects it. But for migratory species like mule deer, crossings have succeeded in pulling large numbers of animals off the roadway, making the highway safer for motorists and reducing habitat fragmentation at the same time.15The Journal of Wildlife Management. Overpasses and underpasses: Effectiveness of crossing structures for migratory ungulates
The lesson from ecology is that freeway space doesn’t end at the pavement edge. The road’s influence radiates outward through noise, light, and chemical pollution, and any serious effort to manage that influence requires thinking about the land for hundreds of meters on either side. Wildlife crossings are one piece; vegetative buffers, reduced lighting, and seasonal road management are others.
Informal Uses and the People Who Live There
Not all uses of freeway space are planned. The sheltered, overlooked zones under overpasses and alongside on-ramps have long served as sites for homeless encampments. Oakland’s Wood Street Encampment, one of the city’s largest and longest-standing, grew beneath and around freeway infrastructure into what researchers described as an extensive and innovative informal community-building effort, surviving repeated partial evictions and a tangle of overlapping jurisdictions.16PubMed Central. Reclaiming Placemaking for an Alternative Politics of Legitimacy and Community in Homelessness These settlements are often treated as problems to be cleared rather than as evidence that the space has value precisely because it’s marginal, out of sight, and offers a degree of shelter.
The tension is genuine. Freeway undercrofts and margins present real safety hazards: poor air quality, noise, limited escape routes, and proximity to fast-moving traffic. At the same time, the fact that people persistently choose to inhabit them says something about both the failure of housing systems and the odd utility of freeway space as a refuge. Any redesign of these areas has to grapple with where displaced residents will go, not just what will replace them.
Freeway Corridors as Energy Infrastructure
As the transportation system itself changes, freeway space is being eyed for new technological roles. One emerging concept is the charging-while-driving lane, which would embed wireless or conductive charging infrastructure into the pavement of one or more highway lanes so that electric vehicles can recharge without stopping. This could mean either converting an existing lane or adding a dedicated EV lane to a freeway segment.17Transportation Research Part A: Policy and Practice. Charging while driving lanes: A boon to electric vehicle owners or a disruption to traffic flow The idea is appealing because the right-of-way already exists and the vehicles are already there, but the traffic-flow implications and construction costs are far from settled.
Highway medians, meanwhile, are being explored as sites for renewable energy generation. Researchers have developed portable wind-photovoltaic hybrid systems designed specifically for installation in freeway medians, capturing both solar energy and the wind energy created by passing vehicles.18Solar Energy. A hybrid wind-photovoltaic power generation system based on the foldable umbrella mechanism for applications on highways The concept makes intuitive sense: highways receive unobstructed sunlight for long stretches, and the turbulence from high-speed traffic represents a small but consistent wind resource. Whether the energy yield justifies the maintenance challenges of putting delicate equipment in a harsh, high-vibration environment is still being tested, but the idea reflects a broader shift toward treating freeway corridors as multi-use infrastructure rather than single-purpose transportation channels.
Why Freeway Space Keeps Getting Harder to Ignore
Urban land is finite, and freeways claim a disproportionate share of it. Most cities built their highway systems in the mid-twentieth century, when land was cheaper, car ownership was climbing, and the downstream effects on air quality, ecology, and neighborhood cohesion were either unknown or considered acceptable tradeoffs. Decades later, those tradeoffs look different. The buffer zones, medians, undercrofts, and rights-of-way that once seemed like necessary waste are now being reconceived as parks, stormwater systems, energy sites, wildlife corridors, and community spaces. The physical space hasn’t changed. What’s changed is the recognition that it was never empty to begin with: it was generating pollution, trapping heat, fragmenting ecosystems, and dividing neighborhoods the whole time. Reclaiming it means deciding which of those harms to fix first, and who gets to benefit from the fix.