This is part of CapRadio’s new series “Great Question!” where our we answer listeners' most pressing inquiries. Learn more at capradio.org/greatquestion.

Traffic on Interstate 80 between Sacramento and San Francisco has gotten much worse over the past decade. And it often comes to a crawl around Dixon for no apparent reason.

Consider the experience of Sacramento resident Andy Brenan: He hopped in his car on Sunday afternoon just off the freeway near the small town, headed on westbound 80 — and instantly hit traffic.

The stop-and-go continued past Dixon and on to Vacaville, which made Brenan scratch his head. Why does traffic flow at the speed limit — and then, suddenly, all one sees are brake lights?

That’s why he contacted CapRadio: His submission is the latest in our “Great Question!” series, where inquisitive listeners ask us to investigate.

“I’d be very curious to see if there is a reason” for the traffic jams in I-80 in Dixon, Brenan said.

Vince Jacala with Caltrans says they’ve gotten that question about Dixon before. He says the variations in traffic speeds along this stretch are well known, too.

“It actually speeds up and then it slows back down, and then going the other way, it slows down and then it speeds back up,” Jacala explained.

An aerial view of I-80 stretching through Dixon, California. The highway connects Sacramento with San Francisco.
Public Domain

The six-lane portion of freeway is decades old, he said, and there are no immediate plans to expand or alter it despite the fact that, between 2007 and 2017, Caltrans says traffic on this stretch of I-80 has increased 30%, to roughly 139,000 vehicles per day.

And as a wave of workers chooses to treat Sacramento as a suburb of the Bay Area, that number is only growing larger.

Daryl Fairweather with real estate data company Redfin says recent surveys indicate that more people are opting to call Sacramento home than any other urban area in the country. Meanwhile, San Francisco was No. 1 for the opposite reason: people moving out.

“This is part of a long-term trend,” Fairweather said. “Every single quarter that we have been tracking it … we’ve seen this trend of people leaving expensive coastal markets for more affordable inland markets.”

This population shift makes it more likely that drivers will experience what’s known as a phantom traffic jam, according to Massachusetts Institute of Technology professor Berthold Horn.

He says even the slightest tap of the brakes by one car will cause a slowing ripple effect. “The person behind notices that change and makes an adjustment, and it turns out that in order to avoid collisions that adjustment has to be slightly larger than the original change,” Horn said.

He says that magnifies from car to car. “Eventually, instead of slowing down one mile per hour, you’re slowing down 10 miles per hour, and then pretty soon you’re slowing down 60 miles per hour, and some cars are actually stationary.”

Brenan says the phantom traffic jam explanation could account for some of the back ups, although he doubts that’s always the cause.

Caltrans says the slowdowns don’t amount to much, on average about a two minutes delay.

And while two minutes doesn’t seem long, it is more than enough time to make you wonder, “Why am I sitting here?”

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