Some guy at the end of the bar swears it’s gotten hotter out by the Narrows since they widened U.S. Highway 101. Normally you let that kind of thing slide, somewhere between the fish that got away and the cousin who almost went pro.
Not this time. He’s right.
He’s talking about the Marin-Sonoma Narrows — the long-bottlenecked run of Highway 101 that Caltrans spent more than a decade widening through central Sonoma County. The ground within a half mile of the new lanes ran almost 4 degrees hotter the year after Caltrans finished the first stretch — the biggest jump of any of 11 Northern California freeway projects that researchers at UC Santa Cruz, San Jose State and Northeastern University checked by satellite.
Here’s the deal.
It’s a textbook urban heat island, the same reason a parking lot bakes while the park across the street stays bearable. Asphalt drinks up the sun and hands it back as heat. The grass, brush and trees that a road project rips out would have cooled the air by giving off water vapor. More pavement, fewer plants, equals more heat. And before the guy two stools down jumps in — this is the ground heating up, not the air coming off your bumper. So you can’t blame the extra cars. Blame the extra asphalt.
The hot strip isn’t out in the weeds, either. It runs straight through Petaluma, Cotati, Rohnert Park and the south end of Santa Rosa — towns that have soaked up most of the county’s new houses over the past 20 years, with some of its hottest summer weather. The warming caused by the widening was strongest right by the lanes and faded the farther out you went, which is exactly what you’d expect if the pavement is the culprit.
Where’s the shade?
Now, give the bar its due: nobody misses the traffic chokepoint. The Narrows used to be a parking lot at the worst of times, and the widening fixed that. Even the scientist who wrote a recent UC/Northeastern study of satellite heat data isn’t out to kill freeways.
“I’m not trying to argue we shouldn’t build highways,” said Serena Alexander, the study’s senior author and an associate public policy and engineering professor at Northeastern University in Boston, Massachusetts. “We have to be aware of these impacts and where we can mitigate them.”
And the fix is cheap as fixes go: strips of grass, shrubs and shade trees along the road, and lighter-colored paving that doesn’t hold as much heat. Shade, basically. The thing the project took out.
Here’s the part that may bug you. Nobody made anyone check for how the project might impact ground temperature. California gives big road projects a hard environmental look, but since a 2013 law, those environmental lookovers mostly count how many extra miles a project makes people drive — not how much it heats the ground next to the freeway. The state’s own gas-tax money that paid for the study that says, hey, maybe start counting the heat too. So far, that’s a suggestion, not a rule.
(Two asterisks, because the guy two stools down deserves an honest answer.) This is surface temperature — the heat off the ground — not the air-temperature number on your phone, though the two travel together, and hotter ground means bigger cooling bills, worse smog and more heat sickness. And the study is a two-year before-and-after snapshot, not a promise the strip is 4 degrees hotter forever.
On the way to being cool
One more thing, while we’re being honest: this isn’t every freeway everywhere. The team looked at 11 projects from Sutter County down to Fresno, and several corridors actually came out cooler than the land around them. Sonoma was the standout — the clearest warming of the bunch, and the only place where two separate widenings both ran hot.
As for the people who could say more about why this happened, it’s been quiet. Bo Yang, the UC Santa Cruz assistant professor who ran the satellite work, punted any questions about it to Alexander. Alexander didn’t answer a direct email inquiring about it.
On May 19, Caltrans spokesperson Jeffrey Weiss said he’d sent questions to people in the agency, like has Caltrans read the study? Does heat figure into its road planning? Is any cooling work coming for the Sonoma stretch? No answer yet.
So next time the guy at the end of the bar starts in about the Narrows getting hotter, you can tell him he’s onto something. Then tell him the good part. The first win is already in the bank — the traffic moves, and nobody’s giving that back. The second win is just looking for a budget: a row of shade trees and some lighter pavement, and the corridor everybody finally drives through fast could be one people don’t mind living next to, too.