Roadside vegetation design characteristics that can improve
local, near-road air quality
U.S. Environmental Protection Agency, Research Triangle Park, NC, USA
Highway landscape design
As public health concerns have increased due to the rising number of studies linking
adverse health effects with exposures to trafﬁc-related air pollution near large roadways,
interest in methods to mitigate these exposures have also increased. Several studies have
investigated the use of roadside features in reducing near-road air pollution concentrations
since this method is often one of the few short-term options available. Since roadside veg-
etation has other potential beneﬁts, the impact of this feature has been of particular inter-
est. The literature has been mixed on whether roadside vegetation reduces nearby
pollutant concentrations or whether this feature has no effect or even potentially increases
downwind air pollutant concentrations. However, these differences in study results high-
light key characteristics of the vegetative barrier that can result in pollutant reductions or
increase local pollutant levels. This paper describes the characteristics of roadside vegeta-
tion that previous research shows can result in improved local air quality, as well as iden-
tify characteristics that should be avoided in order to protect from unintended increases in
nearby concentrations. These design conditions include height, thickness, coverage, poros-
ity/density, and species characteristics that promote improved air quality. These design
considerations can inform highway departments, urban and transportation planners, and
developers in understanding how best to preserve existing roadside vegetation or plant
vegetative barriers in order to reduce air pollution impacts near transportation facilities.
These designs can also be used to mitigate impacts from other air pollution sources where
emissions occur near ground-level.
Published by Elsevier Ltd.
Numerous health studies have linked adverse health effects with spending signiﬁcant amounts of time near high-trafﬁc
roads with elevated air pollution levels of particulate matter, gaseous pollutants, and air toxics emitted by nearby motor
vehicle activity (HEI, 2010). The signiﬁcant impact of trafﬁc emissions on urban populations all over the world has motivated
research on methods to reduce exposure to these pollutants. While vehicle emission control techniques and programs
directly reduce pollutants emitted to the air from transportation sources, these programs often take a long time to fully
implement and may be offset by increases in vehicle activity. Thus, other mitigation options will also be needed to fully
and comprehensively reduce air pollution exposures for these urban populations.
Recent studies have investigated how roadside vegetation may provide an opportunity to reduce near-road pollutant
concentrations in urban areas. This roadside vegetation can include the preservation of existing trees and bushes, as well
1361-9209/Published by Elsevier Ltd.
E-mail address: Baldauf.Richard@epa.gov
Transportation Research Part D 52 (2017) 354–361
Contents lists available at ScienceDirect
Transportation Research Part D
journal homepage: www.elsevier.com/locate/trd
as planting vegetation, which may be some of the few near-term mitigation strategies available for urban developers and
facilities already subject to high pollution levels near roads. These mitigation methods, if successful, can complement exist-
ing pollution control programs and regulations, as well as provide measures to reduce impacts from sources that are difﬁcult
to control such as brake and tire wear and re-entrained road dust (EPA, 2016).
In general, vegetation and green infrastructure has been shown to have overall health beneﬁts including increased phys-
ical activity, lower obesity, improved mental health, overall improved birth outcomes, lower adverse cardiovascular illness,
and decreased mortality (James et al., 2015, 2016). Dadvand et al. (2015) found an improvement in school children’s cogni-
tive development associated with an increase in surrounding greenness, particularly at schools. The authors partly attributed
this association to reductions in nearby air pollution.
In addition to air quality and general health beneﬁts, roadside vegetation can improve aesthetics, increase property val-
ues, reduce heat, control surface water runoff, and reduce noise pollution (with dense, thick and tall stands). However, veg-
etation can also affect driver sight lines, protrude into clear zones along highway right-of-ways, contribute to debris on
roads, present ﬁre hazards, and be pathways for pests and invasive species. Thus, all of the beneﬁts and potential unintended
consequences of roadside vegetation need to be considered for any application.
This paper provides insight into roadside vegetation design characteristics that have been shown to most effectively
reduce near-road air pollutant levels downwind of major highways in order to implement this feature as an air pollution
mitigation strategy. The recommendations focus on general considerations applicable to multiple development types and
scenarios, and do not address site-speciﬁc siting or permitting requirements that might be required in certain locations such
as planting along a particular highway right-of-way or within a city park. This paper also focuses on how the vegetation char-
acteristics impact air quality, but does not review models and other methods used to quantify these impacts, which is still an
area of active research.
2. Vegetation effects on air quality
Trees and other vegetation have been shown to reduce regional air pollution levels through the interception of airborne
particles or through the uptake of gaseous air pollution through leaf surfaces (Janhall, 2015; Gallagher et al., 2015). Pollution
, CO) by urban trees has been estimated across the continental United States (US) using
the US Forest Service’s i-Tree model, which suggested that nationwide vegetation can reduce air pollution levels by approx-
imately one percent (equating to over 10 million tons of air pollution removed) (Nowak et al., 2014).
Removal of gaseous pollutants by trees can be permanent, while trees typically serve as a temporary retention site for
particles. The removed particles can be re-suspended to the atmosphere during turbulent winds, washed off by precipitation,
or dropped to the ground with leaf and twig fall (Nowak et al., 2000). These removal mechanisms can impact local air, water
and soil pollution; thus, careful consideration of the land uses that surround roadside vegetation are needed when choosing
At the local level, trees can also act as barriers between sources and populations, although vegetation is inherently more
complex to study than solid structures. For example, the effectiveness of vegetative barriers at reducing ultraﬁne particle
(UFP) concentrations has been shown to be variable (Janhall, 2015; Tong et al., 2015; Hagler et al., 2012; Pataki et al.,
2011). This variability is due to a number of confounding factors. The complex and porous structure of trees and bushes can
modify near-road concentrations via pollutant capture or through altering air ﬂow, which can result in either lower dispersion
through the reduction of wind speed and boundary layer heights (Nowak et al., 2000; Wania et al., 2012; Vos et al., 2013), or in
enhanced dispersion due to increased air turbulence and mixing as the pollutant plume is lofted up and over the vegetation
(Bowker et al., 2007). Recirculation zones have also been observed immediately downwind of forested areas with a ﬂow struc-
ture consistent with an intermittent recirculation pattern (Frank and Ruck, 2008). Janhall (2015) summarized that vegetation
like hedges, can ﬁlter out PM when located close to an emission source such as a road, while higher, canopy vegetation like
trees can reduce mixing and turbulence and result in increased concentration levels near the ground. Thus, vegetation type,
height, and thickness can all inﬂuence the extent of mixing and pollutant deposition experienced at the site. The built environ-
ment also matters greatly – air ﬂow and impacts of trees are substantially different for a street canyon environment than an
open highway environment (Gromke et al., 2008, 2016; Buccolieria et al., 2009, 2011; Pugh et al., 2012; Li et al., 2016).
Several studies have shown signiﬁcant reductions in air pollution concentrations behind roadside vegetation barriers
(Tong et al., 2016; Al-Dabbous and Kumar, 2014; Brantley et al., 2014; Steffens et al., 2012). Each of these studies compared
air quality concentrations near a large roadway with and without the presence of roadside vegetation. For all of these studies,
the roadside vegetation was dense and a mixture of trees and bushes, with full coverage from the ground to the top of the
canopy, although the heights, thickness and species varied. Two recent modeling studies demonstrated that hedgerows can
improve air quality in street canyons if the bushes provide full coverage with no openings or gaps (Gromke et al., 2016; Li
et al., 2016).
2.1. Roadside vegetation barrier physical design characteristics
The seemingly contradictory results of previous ﬁeld studies investigating how roadside vegetation affects near-road air
quality actually provide useful insights on the characteristics needed for such a barrier to improve local air quality. The fol-
R. Baldauf / Transportation Research Part D 52 (2017) 354–361 355
lowing sections review how height, thickness and the porosity of the vegetation interact to either provide an effective barrier
against air pollution impacts or potentially have no or even a negative effect. These recommendations and characteristics
focus on the presence of vegetation along highways and motorways that carry large volumes of trafﬁc but are not located
in urban street canyons. These characteristics will also be pertinent for other similar air pollution source types where emis-
sions occur near ground-level. Although there are similarities in results from street canyon studies to this type of application,
other recent publications provide more detailed information on designs for street canyons (Gallagher et al., 2015; Gromke
et al., 2016; Li et al., 2016).
Generally, a vegetation barrier along a high-volume highway should be tall, thick, and dense to achieve greater reductions
in downwind pollutant concentrations. Each of these factors is discussed below with an emphasis on roadside vegetation
characteristics that promote improved near-road air quality.
2.1.1. Vegetation barrier height
Near-road vegetation barrier studies in non-street canyon settings that measured air pollution reductions behind the veg-
etation typically had heights ranging from 4 to 5 m or higher (Al-Dabbous and Kumar, 2014; Brantley et al., 2014; Steffens
et al., 2012). At these heights, the barrier will be above the exhaust release height of typical motor vehicles operating on the
adjacent roads, forcing the pollutant plume to loft above and over or pass through the vegetation. Heights lower than
approximately 4 m may allow for the pollutant emissions to proceed downwind of the low barrier unimpeded, although
studies evaluating varying heights of vegetation barriers have been minimal.
Vegetation barriers will only be effective at reducing air pollution in urban areas with limited space for planting if there’s
full coverage from the ground to top of the canopy. Ornamental trees with large openings under the canopy can result in
higher downwind concentrations by allowing the plume to pass through while also reducing wind speeds as in the example
of Tong et al. (2015). Thus, the vegetation barrier should be similar to a solid noise barrier by impeding the entire plume air
ﬂow from the highway. Solid noise barrier studies also suggest walls at least 4 m or taller provide a sufﬁcient height for air
quality improvement near roads (Baldauf et al., 2008, 2016; Heist et al., 2009; Finn et al., 2010). As discussed in Gallagher
et al. (2015) and Li et al. (2016), vegetation heights for barriers in street canyons may be effective at lower heights due to the
differences in air ﬂow patterns.
2.1.2. Vegetation barrier thickness
The thickness of the barrier will provide the residence time to allow for particulate removal by impaction or diffusion, as
well as reduce turbulence and wind speed, increasing the amount of air ﬂow blocking. The vegetation thickness also forces
air ﬂow over the barrier for a longer distance, as well as provides increased distance from the air pollution source to the
downwind receptors. The thickness of the barrier needed for effective air pollution mitigation will vary depending on the
porosity/density of the vegetation. In general, studies reporting decreased near-road pollutant concentrations with vegeta-
tion were a minimum of approximately 5 m thick, with most approaching 10 m or more (Neft et al., 2016).
2.1.3. Vegetation porosity/density
The porosity or density of the vegetation comprising the barrier will determine air movement through the interior of the
barrier. Generally, the lower the porosity (or higher the density) and thicker the barrier, the more air ﬂow forced over the
structure. At extremely low porosities, the vegetation will affect pollutant transport and dispersion in a similar manner as
a solid noise barrier. At higher porosity, the vegetation can reduce wind speeds, allowing pollutants to stagnate within or
behind the vegetation, potentially leading to higher pollutant concentrations near ground-level. Thus, the vegetation poros-
ity should be high enough that the combination of particle loss within the vegetation and the particle removal mechanisms
dominate the lowering wind speed and stagnation effect, leading to reduced concentrations behind the barrier. Since the
measurement of porosity and/or density along the horizontal air ﬂow from a highway is very difﬁcult, a quantitative tech-
nique in the ﬁeld has not been implemented. Modeling and wind tunnel analyses have used Leaf Area Index (LAI) and Leaf
Area Density (LAD) to estimate the porosity/density of vegetation (Tong et al., 2016; Steffens et al., 2012; Neft et al., 2016; Lin
and Khlystov, 2012). These studies suggest thicker and denser vegetation promotes increased pollution reductions, although
none provide a quantitative relationship that has been shown effective in the ﬁeld. Fig. 1 shows examples of effective and
not-effective vegetation barriers that provides a qualitative understanding of the porosity and density needed for pollution
2.1.4. Vegetation barrier coverage
As described, gaps in vegetation barriers, whether from high porosity, missing or dead trees, or space under ornamental
trees, can lead to increased pollutant concentrations downwind, sometimes higher than concentrations are if no barrier were
present. These increases can occur because pollutant emissions from the road funnel through the gaps or cause winds to
stagnate. Thus, the vegetation should provide full coverage from the ground to the top of the canopy as shown in Fig. 1. This
characteristic is important in planning the barrier design as well as maintaining existing or planted roadside vegetation. In
order to achieve sufﬁcient coverage, multiple rows and types of vegetation may be most feasible. For example, a barrier could
consist of a row of bushy plants and hedges followed by a row of trees to enable a barrier with full coverage from the ground
to top of canopy at the initial planting, yet achieve higher canopy heights than feasible by bushy plants alone. In addition,
rows of multiple vegetation types may allow for sufﬁcient downwind pollutant removal while the vegetation grows over
356 R. Baldauf / Transportation Research Part D 52 (2017) 354–361
time after ﬁrst planting. This approach will ensure sufﬁcient density for pollutant removal at the initial planting, while
allowing for increased pollutant removal as the vegetation matures. This process will also limit concerns of promoting plant
2.1.5. Vegetation barrier Length
In addition to passing through gaps, pollutants can also meander around the edges of a roadside vegetative barrier. Thus,
if a vegetative barrier will be constructed for a speciﬁc neighborhood or facility (e.g. school, daycare, nursing home), it should
extend sufﬁciently beyond the area of concern. Research on solid noise barriers suggests that the barrier should extend at
least 50 m laterally beyond the area of concern in order to maximize reductions in downwind concentrations (Baldauf
et al., 2008, 2016). If extending the barrier laterally is not feasible, extending it perpendicularly from the road, wrapping
around the area of interest, has been shown to be effective as well (Li et al., 2016; Brantley et al., 2014); however, if air pol-
lution sources are present upwind of the road, this design may trap pollutants emitted from this source when winds emanate
from this source toward the road.
2.2. Roadside vegetation species considerations
Certain types and species of vegetation will provide more air quality beneﬁts compared to other types of vegetation.
When considering the design and construction of a vegetation barrier, optimal species’ physical characteristics should be
favored to the extent feasible. However, given the vast number of vegetation species, and the regional differences in the fea-
sibility and effectiveness of speciﬁc species for a roadside barrier, speciﬁc recommendations cannot be made in this docu-
ment. The U.S. Forest Service’s i-Tree model (see Nowak et al. (2014)) can provide a list of potential species that best
meet the factors listed below for the United States and select other parts of the world. In addition, users need to identify
whether particular vegetation types can survive and prosper in a particular area of interest. Key factors to consider include.
2.2.1. Seasonal effects
The vegetation chosen for a barrier should not be subject to signiﬁcant changes in characteristics and integrity during
changing seasons. Deciduous trees that lose leaves during the cold season should not be considered for a barrier to mitigate
air quality impacts year-round. Instead, trees that are not subject to signiﬁcant seasonal changes, such as evergreen and
other similar coniferous plants, should be considered. Other shrubs and bushes that are not subject to seasonal changes
can also be considered as part of a roadside barrier.
2.2.2. Leaf surface characteristics
Leaf surfaces can also enhance particulate removal through diffusion and interception. Trees and bushes with waxy and/
or hairy surfaces have been shown to preferentially remove and retain particulates compared to smooth leaf surfaces. In
addition, vegetation with smaller leaves as well as leaf and branch structures that provide increased surface area for particle
diffusion are preferred (Tong et al., 2016; Petroff et al., 2009).
2.2.3. Vegetation air emissions
When selecting vegetation for a roadside barrier, especially at locations where sensitive populations may be spending sig-
niﬁcant amounts of time, care must be taken to choose species that do not emit compounds which increase allergic
responses or air pollution. Compounds that can be emitted by vegetation include high-allergy pollens as well as volatile
organic compounds (VOCs) which can contribute to the formation of ozone. Both can exacerbate respiratory effects and
should be avoided for roadside barriers, especially in areas where sensitive populations may be present, such as children
and the elderly. The i-Tree model provides vegetation air emission data for many species found throughout the world.
2.2.4. Resistance to air pollution and other environmental stressors
Vegetation implemented in a roadside barrier must also be resistant to air pollution and other trafﬁc stressors since con-
centration levels will be high. If the vegetation is not resistant and cannot maintain its integrity, gaps will form in the barrier,
potentially leading to increased pollutant concentrations downwind as discussed previously. Air pollutants emitted by trafﬁc
can include the typical tailpipe emissions like CO, NO
, and particulates; materials from brake and tire wear; re-suspended
road dust; and salt and sand used for road surface treatment during winter weather conditions.
2.3. Other vegetation barrier considerations
In addition to air quality considerations, other potentially beneﬁcial and adverse aspects of vegetation need to be consid-
ered in the development and use of a roadside barrier. These considerations include general physical and species-speciﬁc
factors. While location-speciﬁc factors will need to be addressed on an individual basis, some general considerations include.
2.3.1. Vegetation maintenance
The roadside vegetation will need to be maintained in order to provide a protective barrier from air pollution exposures
yet not lead to safety concerns from reduced visibility or falling debris. Maintenance requirements will depend on vegetation
R. Baldauf / Transportation Research Part D 52 (2017) 354–361 357
type and species, so a plan should be in place when selecting and constructing the barrier for optimal long-term perfor-
mance. These requirements include watering and fertilization needs, trimming and other pruning requirements, and overall
plant care. Maintenance should also include vegetation replacement due to die-off, disease, or damage from accidents. Prop-
erly designed roadside vegetation may also minimize the need for extensive mowing and trimming, saving money and
reducing air pollution emissions.
2.3.2. Water runoff control
An additional beneﬁt of a roadside vegetation barrier can be the control and containment of surface water runoff from the
impervious road and supporting infrastructure. Roadside barriers constructed to provide water runoff control can prevent
localized ﬂooding as well as improve water quality in the area. For certain regions of the country, drought resistant vegeta-
tion that can also resist high-water events may be most appropriate.
2.3.3. Native and non-invasive species
Whenever feasible, native species should be considered for implementing the roadside barrier. Native species will usually
be more robust and resistant to local climatic conditions. The vegetation barriers should also not be constructed from inva-
sive species that may not be contained within the project area of interest, and may create problems at other locations or at
the roadside. Non-poisonous species should also be used, especially if present near children or in locations that have the
potential to cause harm in other ways.
2.3.4. Roadway safety
Planting on or near a highway right-of-way (ROW) requires consideration of potential safety issues. In most cases, the
applicable highway department will require approvals for planting near roads due to these issues. Concerns may include cre-
ating undesirable wildlife habitat near roadways (e.g. deer and other animals that can exacerbate auto accidents), preserving
safe lines-of-sight and viewshed standards for drivers on the road, maintaining clear zones and horizontal clearance for dri-
ver safety, ensuring compatibility of the chosen vegetation species with existing species, and not obstructing outdoor
Fig. 1. Examples of (a) effective vegetation barriers that are dense with full coverage from the ground to the top of canopy and (b) ineffective roadside
barriers due to gaps and high porosity.
Fig. 2. Examples of effective combinations of vegetation with solid noise barriers. Panel (a) shows vegetation behind the barrier (as studied in Baldauf et al.
(2008) while panel and (b) shows bushy vegetation in front of the barrier (no empirical evidence available).
358 R. Baldauf / Transportation Research Part D 52 (2017) 354–361
3. Vegetation with noise barriers
Noise barriers combined with mature vegetation have also been found to result in lower ultraﬁne particle concentrations
along and away from a highway compared to an open ﬁeld or a solid noise barrier alone (Bowker et al., 2007; Baldauf et al.,
2008). For vegetation planted with a solid noise barrier, the overall considerations should be the same as for vegetation
alone. However, for the vegetation to have an additive effect for pollutant reductions, the vegetation should extend beyond
the top of the solid barrier by a sufﬁcient height in order to allow air ﬂow through and over the plants to enhance pollutant
Factors affecting the effectiveness of roadside barriers in mitigating near-road air pollution impacts.
Height 5 m or higher (or extend 1+ meter above an
existing solid barrier)
The higher the vegetative barrier, the greater the pollutant reductions. A
minimum of 5 m will provide enough height to be above typical emission
elevations for vehicles on the road (4 m if little to no trucks use the road).
However, heights of 10 m or more would provide additional pollutant
Thickness 10 m or more The thicker the vegetative barrier, the greater the pollutant reductions. A
minimum thickness of 10 m should provide enough of a barrier to remove
particulate and enhance dispersion. However, gaps in the barrier should be
avoided. Multiple rows of different types of vegetation (e.g. bushes, shrubs,
trees) should be considered for maximum coverage and pollutant removal
during all stages of the barrier. A thickness of as little as 5 m may be sufﬁcient
with low porosity (high density) vegetation
Porosity 0.5–0.9 Porosity should not be too high to allow pollutants to easily pass through the
barrier or cause wind stagnation. As the porosity gets lower, the vegetation
barrier will perform similarly to a solid barrier, which may limit the amount of
particulate removal since air is forced up and around the plants
Length 50 m or more beyond area of concern Extending the barrier beyond the area of concern protects against pollutant
meandering around edges. May also consider constructing the barrier
perpendicular from the road depending on land availability
Seasonal effects Vegetation not subject to change by season Vegetative barrier characteristics must be consistent throughout all seasons
and climatic conditions in order to ensure effective pollutant reductions
Leaf surface Complex waxy and/or hairy surfaces with high
Leaf surfaces with complex and large surface areas will capture and contain
more particulate pollutants as air passes through the structure
Air emissions Vegetation with low or no air emissions Vegetation used for roadside barriers should not be sources of air pollution,
either at the local or regional scale
Resistant to effects of air pollution and other
Vegetation must be able to survive and maintain its integrity under the high
pollution levels and stress that can occur near roads in order to provide
effective pollution reductions from trafﬁc emissions. In addition to air
pollution, other stressors can include salt and sand for winter road
conditioning and noise impacts
Maintenance Plan must be in place to properly maintain
Proper vegetation maintenance must be provided in order for the barrier to
survive and maintain its integrity to provide effective pollution reductions
from trafﬁc emissions
Water runoff Contain surface water runoff and improve water
Roadside vegetative barriers constructed appropriately can provide an added
beneﬁt of controlling and containing surface water runoff from the road, which
can also improve local water quality
Choose species resistant to drought and ﬂooding Many regions face climatic conditions of extended drought followed by
localized ﬂooding. Vegetative barrier must maintain its integrity under these
conditions in order to provide effective pollution reductions
Native species Choose native species Native species will be more robust and resistant to climatic conditions in the
area of interest; thus, maintaining its integrity under these conditions in order
to provide effective pollution reductions
Non-invasive Choose non-invasive species The use of non-invasive species will ensure effective pollutant reductions
without potential unintended consequences from invasive species adversely
effecting nearby land uses
Non-poisonous Choose non-poisonous species if sensitive
populations will be nearby
Non-poisonous species are strongly encouraged and should be used if the
barrier will be at a location with sensitive populations, such as elementary
schools, parks, and recreation ﬁelds where small children may be active and in
Roadway safety Maintains safety for drivers on the road;
conforms to local safety and permit requirements
Prior to planting, ensure vegetation plan will meet all safety and other local
permit requirements (e.g. local highway department, city planning
department) to preserve sight-lines and vegetation compatibility while
avoiding potential wildlife/auto accidents and obstruction of outdoor
R. Baldauf / Transportation Research Part D 52 (2017) 354–361 359
removal and air mixing. Vegetation in combination with a solid barrier will also likely be effective at higher porosities than
for vegetation alone since the addition of the vegetation primarily enhances particulate removal through diffusion or impac-
tion as the solid barrier already enhances turbulence and mixing of the trafﬁc plume.
Solid barriers can vary in height; research on air pollution reductions from these structures has been conducted for
heights between 4 and 6 m as previously discussed. A vegetation barrier should extend at least 1 m above the barrier,
although the higher and thicker the plants, the greater the downwind reduction as suggested by Hagler et al. (2011). For
shorter solid barriers, vegetation should extend above the barrier to a height of at least 6 m to maximize the potential for
downwind pollutant reductions. Fig. 2 provides examples of combinations of vegetation with solid noise barriers that could
lead to increased reductions in downwind air pollutant concentrations.
Previous research is based on vegetation planted behind the noise barrier (opposite side from the road), although bushes
or trees in front could provide an added reduction if sufﬁciently away from the solid barrier to allow air to ﬂow through the
plants. Some modeling studies suggest that ‘‘green walls” such as ivy or other climbing vegetation that grows on the solid
barrier surface may also improve local air quality (Pugh et al., 2012); however, as noted above, air ﬂow through the vege-
tation would be needed to enhance particulate diffusion and impaction.
No research has been done on whether gaps or spaces in vegetation along solid walls can lead to increased downwind
concentrations. Since solid noise barriers alone can reduce downwind pollutant concentrations, gaps in accompanying veg-
etation would likely not have the same detrimental effects as with vegetation alone, although the gaps would likely limit the
added beneﬁt of particulate removal from the adjacent vegetation.
Research shows that roadside vegetation can affect nearby air quality in both a positive and negative way. If properly
designed, vegetation barriers can be used to improve near-road air quality, either alone or in combination with solid barriers.
Many factors must be considered in designing effective roadside vegetative barriers that are applicable for use by transporta-
tion departments, urban planners and local developers as summarized in Table 1. These factors can be evaluated along with
site-speciﬁc information to guide the planning and development of roadside vegetation barriers that improve local air qual-
ity while also providing other beneﬁts associated with increased green infrastructure in urban areas.
Special thanks go to the many experts who provided advice and comments for the development of these recommenda-
tions as detailed in EPA (2016). These experts include David Nowak (U.S. Forest Service), Greg McPherson (U.S. Forest Ser-
vice), Kevin Jefferson (Urban Releaf), David Ralston (Bay Area Air Quality Management District), Tom Hanf (Michigan DOT),
Drew Buckner (Michigan DOT), Gorette Yung (Michigan DOT), Kevin Sayers (Michigan DEQ), Sheila Batka (U.S. EPA), Ken
Davidson (U.S. EPA), Bob Newport (U.S. EPA), Laura Jackson (U.S. EPA), Sue Kimbrough (U.S. EPA) and Vlad Isakov (U.S. EPA).
Al-Dabbous, A.N., Kumar, P., 2014. The inﬂuence of roadside vegetation barriers on airborne nanoparticles and pedestrians exposure under varying wind
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Frank, C., Ruck, B., 2008. Numerical study of the airﬂow over forest clearings. Forestry 81, 259–277.
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