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An international consensus has emerged thatpeople living, working, and going to schoolnear roads with high volumes of traffic face

increased risks for adverse health effects (1), mostlikely from acute and chronic exposures to elevatedlevels of air pollution, including particulate matter(PM), gaseous criteria pollutants, and air toxics.

Field measurements conducted in the UnitedStates and throughout the world have shown that airpollution levels are highly elevated near high-vol-ume roadways (2). Pollutant concentrations areoften highest within the first 100 to 150 meters of theroad, and some pollutants are found in concentra-tions that have increased by an order of magnitude.Pollutant concentrations from traffic emissions canremain elevated as far as 300 to 500 meters or morefrom the road (1, 2).

Urban Form and Air QualityWith increased urbanization worldwide, the numberof people exposed to traffic emissions near high-volume roadways continues to increase. Moreover,urban form indirectly affects air quality and globalclimate conditions (3).

Public transportation and land use policies andpractices increasingly support sustainable develop-ment patterns by promoting compact growth in infilllocations along major transportation corridors. Anexample is transit-oriented development, a mix ofhousing and supportive land uses near transit, withaccess to jobs and services, intended to capture thebenefits of location efficiency (4).

The U.S. Environmental Protection Agency (EPA)is implementing policies to address the impacts ofmajor roads on nearby air quality. Recent revisions to

Integrating Vegetation and GreenInfrastructure into SustainableTransportation Planning R I C H A R D B A L D A U F , G R E G M c P H E R S O N , L I N D A W H E A T O N , M A X Z H A N G , T O M C A H I L L ,

C H A D B A I L E Y , C H R I S T I N A H E M P H I L L F U L L E R , E A R L W I T H Y C O M B E , A N D K O R I T I T U S

Environmental Sustainability in Transportation

Baldauf is Senior Scien-tist, Office of Researchand Development, U.S.Environmental ProtectionAgency (EPA), ResearchTriangle Park, NorthCarolina. McPherson isResearch Forester, U.S.Department of Agricul-ture Forest Service,Pacific SouthwestResearch Station, Davis,California. Wheaton isAssistant Director forIntergovernmentalAffairs, CaliforniaDepartment of Housingand Community Devel-opment, Sacramento.Zhang is Associate Pro-fessor, Mechanical andAerospace Engineering,Cornell University,Ithaca, New York. Cahillis Professor Emeritus ofPhysics, University ofCalifornia–Davis. Baileyis Physical Scientist, U.S.EPA, Ann Arbor, Michi-gan. Fuller is AssistantProfessor, School of Pub-lic Health, Georgia StateUniversity, Atlanta.Withycombe is Air Qual-ity Consultant, and Titusis CEO, Breathe Califor-nia of Sacramento–Emi-grant Trails, Sacramento. Example of a vegetation barrier along a highway; roadside vegetation barriers have been shown to improve

near-road air quality. Questions remain on the optimal design features for effectiveness.

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the monitoring rules for the National Ambient AirQuality Standards (NAAQS) require monitors for PM,carbon monoxide (CO), and nitrogen dioxide (NO2)near high-traffic roads in large metropolitan areas.

EPA’s transportation conformity rule requires themodeling of hot-spot concentrations of PM in theimmediate vicinity of large federal highway or tran-sit projects in nonattainment and maintenance areasthat have high levels of heavy-duty diesel vehicletraffic. Projects are required to model concentrationsat or below the NAAQS or to model the concentra-tions to be at lower levels after the project is builtthan they were before the project.

In California, three recent state laws have givenimpetus to sustainable development patterns.1–3

Under California Senate Bill 375, regional trans-portation plans of metropolitan planning organiza-tions must include “sustainable communitystrategies.”2 These strategies forecast developmentpatterns integrated with the transportation networkand other transportation measures and policies toreduce regional greenhouse gas (GHG) emissionsfrom automobiles and light trucks; the goal is toachieve regional GHG emission reduction targets by2020 and 2035 (5, 6).

Reducing ExposuresAlthough development patterns that limit urbansprawl and vehicle miles traveled can have a majorimpact on reducing GHG emissions, these plans, aswell as similar proposals in other localities, concen-trate development along major transit corridors. The

result is to increase the local population’s exposure toemissions generated from the high-volume freeways.

Transit-oriented development and similar poli-cies increase the population’s access to services andtransportation options and lead to regional reduc-tions in vehicle miles traveled and air pollution.Nonetheless, these practices often bring peoplecloser to the sources of air pollutant emissions, suchas traffic activity. As a result, ways to reduce the expo-sure of people residing and working near high-vol-ume roadways are needed.

A workshop in Sacramento, California, on June5–6, 2012, gathered a multidisciplinary group ofresearchers and policy makers to discuss roadsidevegetation as an option for mitigating the healthimpacts of air quality near roads. The following is asummary of the workshop discussions, including anoverview of the role that roadside vegetation mayplay in reducing population exposures to air pollu-tants emitted by traffic. Roadside vegetation also isexamined as a sustainable mitigation option in thecontext of other potential benefits and disbenefits.

Vegetation BarriersResearch studies measuring and modeling theimpacts of vegetation barriers on near-road air qual-ity suggest that a barrier can lead to reductions inpollutant concentrations. Field measurements com-paring pollutant concentrations behind roadside veg-etation with the concentrations in a clearing at thesame distance and along the same stretch of limited-access highway generally show lower pollutant con-centrations downwind of the vegetative barrier, asillustrated in the example in Figure 1 (below).

The Clarendon neighborhood of Arlington, Virginia,just outside of Washington, D.C., features a mix ofhousing and business land uses within walkingdistance of Metro rail and bus stops.

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1California Assembly Bill 32, Global Warming SolutionsAct (2006), and Climate Change Scoping Plan (2008).2California Senate Bill 375, Sustainable Communities andClimate Protection, Ch. 728 (2008).3California Assembly Bill 1358, Complete Streets Act, Ch.657 (2008).

FIGURE 1 A study in North Carolina measured PM concentrations at a clearing andbehind a vegetation stand along the same stretch of highway and the samedistance from the nearest pavement edge. Substantial reductions in PMconcentrations occurred during morning time periods with light winds from theroad; however, as winds became variable, the vegetation did not effectivelyreduce PM concentrations, with some instances of higher concentrations behindthe vegetation than at the clearing (7, 8).

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The measurements suggest that the barrier led toan increase in air mixing, resulting in lower behind-barrier concentrations at ground level. Field andwind tunnel studies also suggest an enhanced cap-ture of PM by the vegetation; generally, the concen-trations of ultrafine and coarse-mode PM decrease,with limited reductions in fine-particle PM2.5 mass.

The field measurements, however, also indicatedthat under certain meteorological and design condi-tions, the PM concentrations could be higher behinda vegetative barrier than in a clearing. These resultssuggest that higher pollutant concentrations couldoccur behind a vegetation stand when wind speedsare low and the winds are parallel to or toward theroad. In addition, gaps in the barrier from dead treesor natural openings could cause wind stagnation,leading to higher downwind concentrations behindthe vegetation (9).

Computational ModelsResearchers have incorporated the representationsof the aerodynamic and deposition effects of vegeta-

tion barriers on transporta-tion air quality into acomputational modelbased on fluid dynamics.To explore the effects ofvegetation barriers on near-road air quality, the simula-tion results were comparedwith the data collectedfrom field studies (7, 8).

The models consistentlyreproduced the spatial vari-ations of pollutants behindbarriers under differentatmospheric stability con-ditions. With the accuracy

of the three-dimensional, detailed modeling verified,researchers are examining the effects of different bar-rier designs, wind speeds, and turbulence environ-ments (10).

Cobenefits and DisbenefitsUrban forestry and landscape ecology offer insightson potential additional advantages and disadvan-tages of implementing vegetation to mitigate near-road air quality impacts. Vegetation in urban settingscan provide benefits beyond improvements in airquality—these include carbon sequestration, tem-perature and storm water regulation, noise reduc-tion, aesthetic improvements, and opportunities forphysical exercise and the experience of nature. Thesecobenefits, known as ecosystem services, have beenassociated with improved physical and mental healthand community vitality.

Positive associations between personal health andphysical or visual access to green space have beenobserved in children, the elderly, persons with lim-ited mobility, and families in military and low-income housing. Trees also have been shown to havedirect health benefits (11). In addition, the servicesprovided by urban vegetation can yield significanteconomic returns, such as averted energy and med-ical costs, increased worker productivity, andincreased property values (9).

Near-road vegetation, however, has some poten-tial disbenefits, such as pollen production, waterdemand, introduction of invasive or nonnativespecies, channeling of invasive pests and fire intothe urban environment, and expanding the urbanfootprint by distancing buildings and other land useactivities from roadways. Trees also may obstructroadway visibility, cause damage or injury by falling,and create slippery conditions from dropped debris.

Barrier Design ConsiderationsMeeting participants agreed that further explorationof vegetative barriers to mitigate adverse air qualityis worth pursuing; the design process should maxi-mize the potential benefits and avoid the disbenefitsto the extent feasible. Successful designs wouldmatch plant species with each site and with the site’spurpose, to achieve optimal performance for the ser-vice life of the project. Many sites and designs areunique, with no single recipe for effectiveness.

Roadside vegetation barriers designed to reduceharmful PM concentrations, for example, should betall and wide enough to enhance particle depositionand dispersion—a minimum width of 5 meters hasbeen suggested (12). A closed canopy over roadways,however, can trap source particles and increase con-centrations below the canopy unless prevailing

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Wildflowers at I-40 andUS-421 in Forsyth County,North Carolina. Besidesaesthetic benefits, nativevegetation alonghighways improveecosystems, air quality,and stormwaterregulation.

Too-heavy tree canopycan obstruct roadwayvisibility and trap sourceparticles.

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winds continuously flush out the pollutants (13). Vegetation barriers with a porosity of 20 to 40 per-

cent have been suggested, because higher or lowerporosities are likely to reduce efficiency in capturingpollutants (12). A porosity of less than 20 percent canincrease turbulence, so that the vegetation acts morelike a solid structure.

A multirow barrier combines shrubs along the edgeto protect young trees from exposure and reduce sub-canopy air flow for deciduous and coniferous trees.Plants can be staggered to eliminate gaps horizontallyand from the ground level to the canopy top.

In terms of performance, conifers are superior todeciduous trees because of their year-round foliageand greater amounts of leaf and stem surface area perunit of land. Their demand for water, however, maybe greater for the same reasons (14). A diverse mixof well-adapted species increases the barrier’s long-term resilience to drought, pests, storm damage, andother urban stressors (15).

Barriers also may be designed to accomplish otherenvironmental objectives, such as carbon storage,rainfall interception, and reduction of contaminatedstorm water runoff. Desired characteristics for treesinclude high wood density values, large crown pro-jection areas, long life spans, and tolerance to inun-dation.

Addressing Negative EffectsAn important design goal is to minimize the poten-tial negative impacts of roadside plantings throughthe judicious selection and placement of species. Sin-gle-vehicle collisions with trees account for nearly 25percent of all fixed-object fatal accidents each year(16). Improving driver visibility and providing a safedistance between travel lanes and trees through clearzones can alleviate this threat.

Avoiding plant species that have invasive qualitiesand shallow roots can reduce long-term maintenancecosts. Tree species with small leaves and open crownsare less likely to clog drains during rain storms or toslow the ice melt from paved surfaces in winter.

Clustering trees within shrub borders can reducedamage from mowing. Understory plantings, how-ever, may limit access and may conceal encamp-ments in certain areas. When the flammability ofplantings is a concern, designs should avoid contin-uous planting strips and “ladder fuel” plantings thatallow fires to climb from the ground to the treecanopy via branches touching the ground or via highgrasses and underbrush that extend into the trees.

Nut and fruit production from trees near pavedsurfaces also can be a nuisance. Emissions of pollenand biogenic volatile organic compounds, which arehighly species-specific, can adversely affect human

health and air quality (17). Most of these effects canbe avoided.

Site CharacteristicsUnderstanding how a site’s microenvironments willchange over time and influence plant growth is fun-damental to good barrier design. Grading for optimalsurface drainage before planting will promote thesurvival and growth of the vegetation. Soil samplingis an important first step, followed by subsoiling, orripping, to reduce compaction and address the nutri-ent deficiencies in the soil. Chloride content, soilpH, and concentrations of metals may change withthe use of deicing salts and other road- or vehicle-generated contaminants.

Designing the barrier to create and protect healthysoil over the long term often reduces maintenancewhile promoting survival and growth. Traffic vol-umes influence the dispersion of pollutants, as wellas the drying effects on roadside vegetation fromlocal turbulence. Slope and direction also influenceplant stress from heat and wind and should be con-sidered in planning and designing the barrier.

Planting TreesTrees generally are planted into augured holes fromcontainers or bare root stock (caliper of 2.5 cen-timeters or more), liners (1.2-centimeters caliper), oras seedlings (30 to 45 centimeters tall). The tree sizeat the time of the planting appears to be related tosurvival—smaller stock, although less expensivethan larger stock, is more vulnerable to physicaldamage from mowers and animals and to competi-tion from weeds. Larger-stock trees will provide amore immediate barrier for mitigating the impacts ofpollutants soon after construction.

Newly planted treesalong SR-542 inWashington State. A mixof tree varieties canbalance disease and pestresistance, water use,foliage sizes, and rootdepths.

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Mulch helps to conserve soil moisture around thetree. Too much mulch, however, can become aseedbed for weeds and fungus. Most trees requirestaking for support and protection at planting.Removing stakes after trees have become establishedand self-supporting is an important maintenance taskbecause of the damage that vestigial stakes can causeto trees, through girdling and wounds.

Watering trees during the establishment period iskey to long-term success, as is controlling weeds bymechanical or chemical means. In some cases, acover crop can control weeds effectively while thewoody plants become established. Care must betaken to avoid plants that are invasive or that attractdeer and other animals that pose a threat tomotorists. Monitoring the barriers is also critical totheir performance and survival.

Pilot Studies NeededRoadside vegetation barriers can improve near-roadair quality and can affect the public health positivelyfor populations near high-volume roadways.Although questions remain about the optimal designfeatures for vegetation barriers, the current scientificunderstanding warrants pilot studies to investigatethis potential strategy for mitigating air quality.Three-dimensional modeling of PM transport anddeposition in roadside barriers, combined with fieldmonitoring and verification studies, are contributingvaluable new knowledge to the design and manage-ment of effective barriers.

References1. Traffic-Related Air Pollution: A Critical Review of the Liter-

ature on Emissions, Exposure, and Health Effects. SpecialReport 17, Health Effects Institute, Boston, Massachusetts,2009.

2. Karner, A. A., D. S. Eisinger, D. A. Niemeier. Near-Road-way Air Quality: Synthesizing the Findings from Real-World Data. Environmental Science & Technology, Vol. 44,No. 14, 2010, pp. 5334–5344.

3. Our Built and Natural Environments: A Technical Review ofthe Interactions Between Land Use, Transportation, and Envi-ronmental Quality. Office of Sustainable Communities, U.S.Environmental Protection Agency, Washington, D.C.,2001.

4. Infrastructure Financing Options for Transit-OrientedDevelopment. Office of Sustainable Communities SmartGrowth Program, U.S. Environmental Protection Agency,Washington, D.C., 2013.

5. Update to the General Plan Guidelines: Complete Streets andthe Circulation Element. Office of Planning and Research,State of California, 2010.

6. Regional Transportation Guidelines. State of CaliforniaTransportation Commission, 2010.

7. Hagler, G. S. W., M. Y. Lin, A. Khlystov, R. W. Baldauf, V.Isakov, J. Faircloth, and L. E. Jackson. Field Investigationof Roadside Vegetative and Structural Barrier Impact onNear-Road Ultrafine Particle Concentrations Under a Vari-ety of Wind Conditions. Science of the Total Environment,Vol. 419, 2012, pp. 7–15.

8. Baldauf R. W., L. Jackson, G. S. W. Hagler, V. Isakov, G.McPherson, D. Nowak, Y. A. Cahill, K. M. Zhang, C. R. Bailey, J. R. Cook, and P. Wood. The Role of Vegetation inMitigating Near-Road Air Quality Impacts from TrafficEmissions. Environmental Manager, January 2011, pp.30–33.

9. Steffens, J. T., Y. J. Wang, and K. M. Zhang. Exploration ofEffects of a Vegetation Barrier on Particle Size Distributionsin a Near-Road Environment. Atmospheric Environment,Vol. 50, 2012, pp. 120–128.

10. Steffens, J. T., D. K. Heist, S. G. Perry, and K. M. Zhang.Modeling the Effects of a Solid Barrier on Pollutant Dis-persion Under Various Atmospheric Stability Conditions.Atmospheric Environment, Vol. 69, 2013, pp. 76–85.

11. Donovan, G. H., D. Butry, Y. Michael, J. Prestemon, A.Liebhold, D. Gatziolis, and M. Mao, The RelationshipBetween Trees and Human Health: Evidence from theSpread of the Emerald Ash Borer. American Journal of Pre-ventive Medicine, Vol. 44, 2013, pp. 139–145.

12. Islam, M., K. Rahman, M. Bahar, M. Habib, K. Ando, andN. Hattori. Pollution Attenuation by Roadside Greenbelt inand Around Urban Areas. Urban Forestry & Urban Green-ing, Vol. 11, 2012, pp. 460–464.

13. Pugh, T., R. MacKenzie, J. D. Whyatt, and C. N. Hewitt.Effectiveness of Green Infrastructure for Improvement ofAir Quality in Urban Street Canyons. Environmental Science& Technology, Vol. 46, No. 14, 2012, pp. 7692–7699.

14. Peters, E., J. McFadden, and R. Montgomery. Biological andEnvironmental Controls on Tree Transpiration in a Sub-urban Landscape. Journal of Geophysical Research, Vol. 115,No. G4, 2010, pp. 2005–2012.

15. Raupp, M., A. Cumming, and E. Raupp. Street Tree Diver-sity in Eastern North America and Its Potential for TreeLoss to Exotic Borers. Arboriculture & Urban Forestry,2006, pp. 297–304.

16. Wolf, K. The Urban Forest in the Roadsides: Public Valuesand Transportation Design. Proceedings of the 9th NationalConference of the International Society of Arboriculture, Aus-tralian Chapter, Launceston, Tasmania, 2005.

17. Benjamin, M.T., and A. M. Winer. Estimating the Ozone-Forming Potential of Urban Trees and Shrubs. AtmosphericEnvironment: Urban Atmospheres, Vol. 32, 1998, pp. 53–68.

Tree species such as oakscan produce gutter-clogging leaf drops in thefall.

Massachusetts DOTreplanted 20 linden treesas part of a 2010 bridgerehabilitation project.Mature trees are lesslikely to experiencetransplant shock andbegin to mitigatepollution effects morequickly than young trees.

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