LOS ALTOS HILLS — In the long shadows of early morning, Kathryn Harrold pulls her car onto the shoulder of I-280. While her friends are in their breakfast nooks enjoying their first cups of coffee, she stops behind the lifeless body of a raccoon, flattened by a passing motorist, and snaps a picture with her cell phone. It’s a routine she’s followed for years as a volunteer, and it’s starting to pay dividends.

Harrold will add the photo to more than 30,000 other entries in what has become the largest roadkill database in the country: the California Roadkill Observation System (CROS), a citizen science website that plunks dead animal sightings onto a map of the state. Data from CROS show that many Bay Area highway sections are roadkill hotspots, the hottest of which include Highway 13 in Oakland, U.S. Highway 101 in Marin County and Highway 4 in northwest Contra Costa County. Highway 17 near Los Gatos and Interstate 280 near Farm Hill Boulevard in Redwood City also see significant animal fatalities.

Researchers and wildlife advocates hope the data will convince state highway authorities to construct more animal crossings or barriers to reduce future collisions. Some projects already offer potential solutions, like a Highway 17 tunnel being built as a mountain lion corridor in the Santa Cruz Mountains. But ecologists want more.

“It’s relevant to planning better transportation,” said Fraser Shilling, co-director of the UC Davis Road Ecology Center, who created CROS in 2009 to monitor wildlife on roadways. “And it’s relevant to protecting wildlife populations.”

But it’s a hard sell to state highway officials weighing the costs and benefits. Suzanne Melim, an environmental planner at Caltrans, said it’s not clear how many dead animals are too many, or how many accidents an underpass would prevent.

“Is one deer-car collision enough to spend X number of dollars, and is it a safety issue?” Melim asked. “There’s no manual out there with the answers to those questions.”

Tens of thousands of animals end up as roadkill each year in California, hordes of deer, skunks and snakes that met their fate on more than 300,000 miles of roadways. While most of us pass roadkill without a second thought, Harrold and a thousand other volunteers diligently log every animal casualty they come across on roadways. Thanks to their data, Shilling has compiled a list of nearly 70 of the state’s most dangerous roads for animals.

Volunteers include biologists, historians and wildlife enthusiasts interested in making the roads safer for everyone. Some embark on weekend field trips to spot roadkill; others scan the roads during their commutes. The most prolific roadkill hunter, an ecologist from Oakland, has logged nearly 4,000 dead creatures — including two elephant seals, one run over on Highway 1 near Cambria and the other on Highway 84 in Newark.

Harrold, who lives in the San Jose area, became interested in roadkill a few years ago when she worked at nightclubs in San Francisco. Driving home late, she would pass animals killed in the hours before the highway patrol could clean them up.

“What I would see was deer smashed all over the road,” she said. “It really ate me alive.” Mule deer are the third most common species reported on CROS, with over 2,200 logged since 2009. They cross busy roads to forage and to reproduce — and they’re known to be particularly fearless around cars.

Heartbroken, Harrold dedicated her mornings and evenings to roadkill research. She wrote to Caltrans and talked with local biologists about what she’d seen. In 2009, a researcher connected her with Shilling, and she helped him and Caltrans win a grant to study how animals use wildlife crossings already in place along Bay Area highways.

Now, a few days a week, she leaves at 5 a.m. to comb I-280 for highway casualties before work. When she spots one, she pulls over and snaps a photo, which her phone tags with GPS coordinates. When she gets home, she uploads it to CROS along with information about traffic direction, speed limit and species.

But a citizen science system is only as accurate as its volunteers. Where they don’t drive, there’s no data. Plus, they’re on the honor system.

“We’ve had a very small handful of joke contributions, like a polar bear or ‘my dog,'” said Shilling. But he estimates that 97 percent of the logged species are correct.

One distinct advantage of CROS is that volunteers cover a huge area, said Seth Riley, a National Park Service ecologist who studies the interactions between wildlife and human development. And getting involved can be inspiring.

“People can really get a sense of participating in the work,” hesaid, “and therefore that they’re maybe doing something valuable.”

But Shilling’s report may paint an overly negative picture, Riley cautioned.

“It doesn’t necessarily tell you about the places where animals successfully do get across roads.”

In some cases, roadkill hotspots occur where populations of animals, like mule deer, simply don’t shy away from cars. In others, they indicate where roads cut through animals’ habitats.

“That’s one of the greatest risks that roads have for wildlife,” said Shilling. “They break populations away from one another.”

But roadkill is a problem for humans, too. According to data from transportation agencies, drivers report about 1,000 accidents involving wildlife on California highways every year. Each deer collision causes an average of $6,700 worth of damage, according to one study. And then there’s the possibility of injury or death to the motorists.

One strategy for preventing collisions is to change drivers’ behavior — for instance, by reducing the speed limit. But when that doesn’t work, said Shilling, “you have to keep animals off the road or give them a way to cross.”

Neither of those options is easy. Fences have to be tall to keep out leaping deer, and in an ideal world they’d stretch for thousands of miles. Under- and overpasses have been shown to be successful, but it’s difficult for transportation agencies to determine exactly where they’re needed.

Until then, Shilling hopes his work will galvanize the public to get involved with issues related to road ecology. Raising awareness is important, he said, because we just don’t interact with wildlife very much.

“If you think about the last time you saw an animal that wasn’t a bird,” he said, “it was probably roadkill.”

Contact Kerry Klein at kklein@mercurynews.com and follow her on Twitter at @einekleinekerry.