Impacts of Vehicle Emission Regulations and Local Congestion Policies on Birth Outcomes Associated with Traffic Air Pollution.

Introduction: In the United States, billions of dollars have been spent implementing interventions to reduce traffic-related air pollution (TRAP). These interventions are usually regulatory actions focused on reducing tailpipe emissions. However, they also include local programs to reduce traffic congestion and excess vehicle emissions, such as electronic tolls and roadway capacity improvements. Few health studies have empirically evaluated the direct impact of air pollution exposure reductions from these emission regulations and congestion reduction programs; no studies have examined infant health, an important population health outcome linked to air pollution exposures.

Objective: Assess changes in birth outcomes for all recorded births in Texas from 1996 to 2016 associated with (1) long-term cumulative regulatory improvements of motor vehicle emissions and resulting TRAP change and (2) local congestion reduction programs that may yield localized TRAP changes over shorter time periods.

Methods: We used Vital Statistics data in Texas from 1996 to 2016 (n = 8.1 million recorded births; n = 6,158,518 births analyzed after exclusions). We calculated diverse traffic-related exposure measures using residential addresses at the time of delivery. We implemented research triangulation methods using different study design and analysis approaches to test our primary hypotheses on the effects of long-term cumulative regulatory improvements and local congestion reduction programs on birth outcomes.

Results: Traffic-related exposure measures (nitrogen dioxide [NO₂] air pollution, traffic volume, congestion) were consistently associated with adverse birth outcomes over the 20-year study period. This finding is supported by an analysis of pregnant individuals living upwind versus downwind of the same major road, where living downwind within 500 m was associated with an 11.6-g decrease (95% CI: -18.01, -5.21) in term birth weight. For all pregnant individuals, NO₂ exposures decreased 59% from 1996 to 2016, while the total vehicle miles traveled (VMT) within 500 m of residential addresses (VMT_500m) remained relatively stable. We observed marked differences in TRAP exposure for pregnant individuals by sociodemographic characteristics. While levels of air pollution disparities reduced in absolute terms over the 20 years, relative disparities persisted, and large differences in traffic levels remained. The magnitude of associations between VMT_500m and adverse birth outcomes decreased for term low birth weight (-60%, OR in 1996: 1.08, OR in 2016: 1.03 for the highest vs. lowest quintile) and preterm (-65%) and very preterm (-61%) births, but not for term birth weight. A direct analysis of congestion exposure for 2015-2016 births, measured for all roadways in Texas using connected device data, showed that congestion was associated with decreased term birth weight, background traffic, and TRAP levels. When we examined local projects designed to reduce congestion as a natural experiment and applied a difference-in-differences (DiD) study design, we found little evidence that the implementation of tolling projects was associated with improved birth outcomes. For roadway construction projects, we observed increased congestion during construction and decreased congestion post-construction. This dynamic translated into increased odds of term low birth weight (OR 1.19; 95% CI: 1.05, 1.36) for pregnant individuals living within 300 m during construction but no consistent improvements in birth outcomes post-construction.

Conclusions: TRAP is an important environmental health and justice issue that affects pregnancy. Our results provide some evidence supporting that cleaning up the vehicle fleet was more impactful at decreasing adverse pregnancy outcomes than local programs aimed at reducing congestion.