A nearly empty northbound 110 Freeway at 5:30 p.m. on March 24. (Mary Hawley/LAist)

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I've been reporting on how the COVID-19 crisis is affecting transportation — including its impact on air quality issues caused by transportation.

It's abundantly clear that there are way fewer cars on Southern California freeways in the age of coronavirus. Now, about a month into our region's official stay-at-home phase of the global health crisis, we are getting a better picture of the impact on traffic.

I reached out to Caltrans — which monitors and measures traffic flow on our freeways — to ask what differences they're seeing on the roads. The agency's Traffic Operations Division sent over sets of data measuring Vehicle Miles Traveled (VMT), which is calculated by adding up all the miles driven by cars and trucks on roadways in a specific time period.

The data broke down VMT by hour on freeways within Los Angeles city limits and across L.A. County as measured on March 2 (a simpler time) and April 6. The findings:

VMT on freeways within the city of Los Angeles fell roughly 35% , from about 38.3 million miles to 24.7 million

Vehicle miles on the 405 Freeway (within L.A. city limits) decreased an average of 45% in that time period, Caltrans reported

Countywide, freeway VMT dropped about 34% , from 96.2 million miles on March 2 to 63.1 million on April 6

, from 96.2 million miles on March 2 to 63.1 million on April 6 Caltrans also crunched VMT for heavy trucks specifically and found mileage dropped by a third, from 1.8 million to 1.2 million

(Dana Amihere / LAist)

The agency also measures hours of delay by calculating how long cars spend on freeways traveling less than 60 miles per hour. It showed that, from March 2 to April 6:

The average morning peak hour of delays dropped 80%

The average evening peak hour of delays fell 88%

So, there's some empirical evidence for you: our region's freeway traffic volume fell by a third in just over a month. But what effect is our decreased car- and truck-based pollution having on local air quality? That's still up in the air (zing).

Researchers with the South Coast Air Quality Management District have been working to understand how much of our clearer air is linked to reduced vehicle emissions, but there's one particular challenge: traffic started tapering off about the same time rainstorms rolled in — and you may have noticed the weather was especially wet and cold last week, too.

AQMD spokesman Bradley Whitaker explained to me recently:

"...we continue to see rain, cold weather and windy days, which can push pollutants out of the atmosphere and improve the air, making it challenging to separate the effects of weather and recent reduction in emissions using air quality models. Once weather returns to more typical conditions and we have a longer period of data, we can better analyze the effects of the reduced emissions."

In a general sense, yes, fewer cars on the roads means less air pollution. But it turns out atmospheric chemistry is playing out in more complicated ways, particularly in the inland regions of Southern California. I have more on that here:

EXPLORE MORE

Caltrans also provided this list, showing decreased VMT for all sections of freeway within L.A. city limits.

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