United States Department of Transportation
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Improving the roadside environment through integrating air quality and traffic-related data.
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2016-12-01
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Abstract:Urban arterial corridors are landscapes that give rise to short and long-term
exposures to transportation-related pollution. With high traffic volumes, congestion, and
a wide mix of road users and land uses at the road edge, urban arterial environments are
important targets for improved exposure assessment to traffic-related pollution. Applying
transportation management strategies to reduce emissions along arterial corridors could
be enhanced if the ability to quantify and evaluate such actions was improved. However,
arterial roadsides are under-sampled in terms of air pollution measurements in the United
States and using observational data to assess such effects has many challenges such as
lack of control sites for comparisons and temporal autocorrelation. The availability of
traffic-related data is also typically limited in air monitoring and health studies. The work
presented here uses unique long-term roadside air quality monitoring collected at the
intersection of an urban arterial in Portland, OR to characterize the roadside atmospheric
environment. This air quality dataset is then integrated with traffic-related data to assess
various methods for improving exposure assessment and the roadside environment.
Roadside nitric oxide (NO), nitrogen dioxide (NO2), and particle number
concentration (PNC) measurements all demonstrated a relationship with local traffic
volumes. Seasonal and diurnal characterizations show that roadside PM2.5 (mass)
measurements do not have a relationship with local traffic volumes, providing evidence
that PM2.5 mass is more tied to regional sources and meteorological conditions. The
relationship of roadside NO and NO2 with traffic volumes was assessed over short and
long-term aggregations to assess the reliability of a commonly employed method of using traffic volumes as a proxy for traffic-related exposure. This method was shown to be
insufficient for shorter-time scales. Comparisons with annual aggregations validate the
use of traffic volumes to estimate annual exposure concentrations, demonstrating this
method can capture chronic but not acute exposure. As epidemiology and exposure
assessment aims to target health impacts and pollutant levels encountered by pedestrians,
cyclists, and those waiting for transit, these results show when traffic volumes alone can
be a reliable proxy for exposure and when this approach is not warranted. Next, it is demonstrated that a change in traffic flow and change in emissions can
be measured through roadside pollutant concentrations suggesting roadside pollution can
be affected by traffic signal timing. The effect of a reduced maximum traffic signal cycle
length on measurements of degree of saturation (DS), NO, and NO2 were evaluated for
the peak traffic periods in two case studies at the study intersection. In order to reduce
bias from covariates and assess the effect due to the change in cycle length only, a
matched sampling method based on propensity scores was used to compare treatment
periods (reduced cycle length) with control periods (no change in cycle length).
Significant increases in DS values of 2-8% were found along with significant increases of
5-8ppb NO and 4-5ppb NO2 across three peak periods in both case studies. Without
matched sampling to address the challenges of observational data, the small DS and NOx
changes for the study intersection would have been masked and matched sampling is
shown to be a helpful tool for future urban air quality empirical investigations.
Dispersion modeling evaluations showed the California Line Source Dispersion
Model with Queuing and Hotspot Calculations (CAL3QHCR), an approved regulatory model to assess the impacts of transportation projects on PM2.5, performed both poor and
well when predictions were compared with PM2.5 observations depending on season.
Varying levels of detail in emissions, traffic signal, and traffic volume data for model
inputs, assessed using three model scenarios, did not affect model performance for the
study intersection. Model performance is heavily dependent on background
concentrations and meteorology. It was also demonstrated that CAL3QHC can be used in
combination with roadside PNC measurements to back calculate PNC emission factors
for a mixed fleet and major arterial roadway in the U.S. The integration of roadside air quality and traffic-related data made it possible to
perform unique empirical evaluations of exposure assessment methods and dispersion
modeling methods for roadside environments. This data integration was used for the
assessment of the relationship between roadside pollutants and a change in a traffic signal
setting, a commonly employed method for transportation management and emissions
mitigation, but rarely evaluated outside of simulation and emissions modeling. Results
and methods derived from this work are being used to implement a second roadside air
quality station, to design a city-wide integrated network of air quality, meteorological,
and traffic data including additional spatially resolved measurements with feedback loops
for improved data quality and data usefulness. Results and methods are also being used to
design future evaluations of transportation projects such as freight priority signaling,
improved transit signal priority, and to understand the air quality impacts of changes in
fleet composition such as an increase in electric vehicles.
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