The Marin-Sonoma Narrows is a 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.
