Study links traffic speed to dirty air

Despite cleaner cars on the road, a University of California, Riverside study shows that higher speed limits can make city air dirtier.

The study compares auto emissions data collected in both the Los Angeles and Salt Lake City metro areas before and during the pandemic. It focuses on two tailpipe gases: carbon monoxide, a dangerous air pollutant, and carbon dioxide, a greenhouse gas.

Published in the journal Environmental Science and Technology, findings show that real-world vehicle emissions of carbon monoxide declined in Los Angeles from 2013 to 2019, but not in Salt Lake City. The scientists set out to understand why.

All measurements in both cities and in both years were taken in summer, and during the daytime when more drivers are present. Using on-board emissions analyzers, the researchers drove the highways for roughly five hours a day for 10 days. This way, wind, variations in traffic levels, fog, and other factors could be accounted for. 

“Bottom line, when Salt Lake City increased the speed limit, the amount of carbon monoxide produced increased. We need to pay more attention to vehicle speed as a factor in air pollution,” said UCR environmental scientist Francesca Hopkins, who initiated the study.

The study also highlights limitations in how vehicle emissions are typically modeled. In California, state agencies rely on controlled lab tests, often capping simulated speeds at 70 mph. But real-world driving can push engines to performance levels that those models miss.

“Many vehicles on our roads are driving at speeds not captured in the lab, and real-life car technology might be worse than in controlled settings, as the Volkswagen scandal showed us,” Hopkins said. “Measuring what actually happens on the freeway is a crucial check on those models.”

Research team members, including a former UCR undergraduate and scientists from the University of Utah, took their instruments directly onto the road, measuring how much carbon monoxide and carbon dioxide cars released while driving.

While CO₂ is harmful for the environment, it is relatively harmless to breathe in small amounts. Carbon monoxide, however, can interfere with the body’s ability to carry oxygen.

“We were interested in how much carbon monoxide, or CO, is in the air because it will hurt you if you breathe it,” Hopkins said. “It interacts with other compounds in the atmosphere, like ozone, that are directly damaging to your lungs.”

Because all combustion engines emit carbon dioxide, the researchers used the ratio of carbon monoxide to carbon dioxide to quantify emissions trends. This ratio dropped by about 8% per year in Los Angeles between 2013 and 2019.

“Cars have gotten cleaner thanks to catalytic converters and regulations, especially here in California,” Hopkins said. “But the Salt Lake City area has seen a surprising increase in carbon monoxide relative to carbon dioxide. The most likely explanation is the speed limit increase.”

Further confirmation of driving speed as the source of the spike in emissions came from an unexpected natural experiment: the COVID-19 pandemic. When Los Angeles traffic disappeared in 2020, drivers moved faster. This caused a spike in the CO-to-CO₂ ratio, undoing years of emission improvements. But by 2021, as congestion returned and speeds slowed, emissions fell again.

“When you drive faster, your engine has to work harder, and it gets less efficient at removing pollutants like carbon monoxide,” Hopkins explained. “What we saw during the pandemic was a perfect demonstration of that.”

Hopkins credits collaboration with undergraduate students and regulatory experts for bringing the project to life. The researchers emphasized the importance of maintaining strong vehicle emissions rules, and also rethinking how speed policies may be undermining them.

“This work shows that California’s policies have made a real difference,” she said. “But we also learned something new: Speed matters — not just for safety, but for air quality.”