Caltrans is planning to create rumble strips through the center of 30 miles of Highway 1, and widen the roadway in 40 places, a $5 million effort to improve safety in Marin, according to the agency. Last week, the agency secured a permit for the project from the California Coastal Commission, whose analysis met the requirements for environmental review. The United States Fish and Wildlife Service has also greenlit the project, which is estimated to begin in May 2018.

The rumble strips, lines of indentations that make vehicles vibrate, are meant to curb collisions. The broader road shoulders—which Caltrans is undertaking in response to concerns about the rumble strips brought by the Marin County Bicycle Coalition—are meant to make traveling safer for bicyclists and pedestrians. 

The agency will largely avoid removing vegetation to create the wider shoulders, using existing impervious surface like gravel pullouts. But at one section of highway, around a fifth of a mile of road in Stinson Beach, Caltrans will fill in 700 square feet of wetlands. Since that work is considered a public service with no feasible alternative, Caltrans was granted an exemption from strict rules around filling wetlands, as long as it creates more wetlands than the project will destroy.

Most of the work is supposed to be finished in 50 days, but the Stinson Beach widening will take an estimated 160 days. There will be one-lane closures in areas under construction.

Rumble strips are created to warn drivers when they veer too close to the opposing lane. Steve Williams, a Caltrans spokesman, wrote in an email that about a third of accidents between 2008 and 2011 on Highway 1 in Marin were attributed to “cross-centerline movements.” Of those, 11 were head-on and over 30 caused injury. 

“Federal researches show that centerline rumble strips are low-cost countermeasure to decrease the potential of future cross-centerline crashes,” Mr. Williams wrote. “It can reduce at least 25 percent of cross-centerline accidents,” which include the two most severe types of crashes: head-on and “opposing-flow sideswipe.”

Yet the Marin County Bicycle Coalition worried in a 2014 letter to Caltrans about the inadvertent harms of rumble strips. The fear is that cars may avoid the center of the road and pass too closely to cyclists. In response, Caltrans analyzed where it might be possible to widen the shoulders, and gathered input from the coalition. 

“We had concerns, and it seems they did their best to respond and understand what safety impacts there might be to people biking, not just driving,” said Bjorn Griepenburg, the coalition’s policy and planning director.

For instance, the widening of shoulders will only occur on uphill or flat portions of the highway, since the coalition says it is safer for cyclists to use the whole road on downhill slopes, where they can come closer to keeping pace with cars. 

Mr. Griepenburg said that depending on various factors, the wider shoulders can act either as de facto bike lanes or areas for bicyclists to pull into to stop and let cars by. Construction will include tapering these 40 sites where they transition between more narrow shoulders. 

He added that the project is “relatively unique,” as there are “no other real examples of Highway 1 rumble strips where there are high volumes of bike riders.” For that reason, the coalition is asking Caltrans to monitor the project for how it affects safety.

The area planned for wetland fill, in Stinson Beach on either side of the entrance to the park service’s parking lot, is lined with willows and drainages to Easkoot Creek. To widen the road, Caltrans will permanently fill in 700 square feet of wetlands and temporarily disturb an extra 750 square feet during construction, according to a California Coastal Commission report on the project. 

That report says there may be red-legged frogs in the area; though there are no documented sightings, the species has been spotted less than four miles away near Redwood Creek. Coastal commission staff say the absence of sightings may be the simple result of a lack of surveys.

Filling wetlands is highly restricted in the coastal zone, but when a project involves a government agency’s public service project—in this case, improving safety—the commission makes exceptions when there are no reasonable alternatives. For Stinson Beach, the coastal commission report says that work is “necessary to maintain existing traffic capacity safely.” 

The Stinson Beach Village Association agrees, saying that people often walk along that stretch of road to reach town. Creating an actual bike lane has also been part of the community plan for decades. “As far as I know, everyone in town is happy that this is happening,” said Mike Matthews, the association president.

No wider shoulders are in store through the eight-mile Olema Valley Ranches Historic District, federal land managed by the Point Reyes National Seashore. The commission report said that widening in this section may be feasible in the future, but that Caltrans “was not able to include it in the currently proposed project due to unresolved visual resource protection concerns” on the part of the National Park Service. 

The coastal commission also stipulated that Caltrans must create a plan to install new signs “where public safety concerns are greatest.” Though it does not specify exactly what the signs should say, the bike coalition had some suggestions. A February letter it wrote to Caltrans requests signs that say “bicycles may use full lane” in areas where cyclists are “compelled to use the full lane,” such as sharp curves or where shoulders do not exist.

Mr. Williams, the Caltrans spokesman, said the “bicycles may use full lane” sign would be used, pending final sign approval from the coastal commission, “where no adjacent shoulders usable by bicyclists are present and travel lanes are too narrow for bicyclists and motor vehicles to operate side by side.”

The terms of the coastal permit also stipulate that Caltrans must create four times as much wetlands as it eliminates. It is still undetermined where that will take place.