Why rain could make this week the most dangerous for Bay Area drivers in a long time

Following a summer of historic drought — the driest in more than 100 years in California — the downpours expected this week in Northern California could bring several inches of rain to parts of the Bay Area.

While the raindrops will dampen parched vegetation and aid firefighters looking to quell a nasty wildfire season, they’ll also present a new, albeit temporary hazard: slicked roads and highways that could lead to car crashes.

That the rain follows such a prolonged dry spell only adds to the danger, according to experts. And with traffic congestion making a comeback in the Bay Area, an increase in crashes could lead to more roadway snarls.

Meanwhile, wildfires have only worsened the road risks in the Sierra, where storm clouds recently dumped rain and snow.

“In areas like Lake Tahoe, wildfires burned up trees and grass,” said Janis Mara, a Caltrans spokesperson. “This means that there aren’t tree leaves and grass to collect the water, so more water goes onto the roadway, creating muddy conditions."

Here’s what happens when drivers get behind the wheel after the first rain, and how experts say you can reduce your risk on the road.

Why the danger increases

The first 10 minutes of rainfall present the highest risk of skidding and sliding on roads, said Aldo Vazquez, a spokesperson for AAA of Northern California.

Oil and other chemicals accumulate on the asphalt, and upon first rain, the water mixes in and creates a slick on the roads. Subsequent rainfall will wash the chemicals off the roadway.

The risk of an accident on a rainy day goes up the longer it's been dry, according to researchers at UC Berkeley. A 2003 study, which analyzed more than 1 million deadly car crashes in the U.S. from 1975 to 2000, found that the more rain or snow that fell during the month, the fewer fatal accidents occurred.

Also, the study found that the more time that elapses since the last day it rained or snowed, the higher the risk of a deadly crash the day precipitation falls.

Researcher Daniel Eisenberg noted in Science Daily that the study confirmed the effect of oil and debris accumulating on the road over long dry spells, making the roads slicker immediately after the first rainfall. He added that another factor could be at play: Drivers might take a few days to adapt to rainy conditions.

The hazards of hydroplaning

In the first few minutes of rain, especially on long-dry roads, water increases the risk of hydroplaning — which is when water comes between the road and the tire, causing it to lose its traction.

“Tires have to displace about a gallon of water a second (to meet the road), even if just one-twelfth of an inch of water is present," Vazquez said.

In severe situations, a driver can completely lose control of their car.

To help avoid hydroplaning, “Just go slow, follow the vehicle in front of you,” Vazquez said. “Their tires are already displacing water in front of you, so you’re reducing chances of hydroplaning.”

Also, certain parts of a highway may be safer to drive on than others.

“Stay in the middle lane because some highways have been designed to pull water to edges,” Vazquez said.

If your vehicle feels like it’s hydroplaning, AAA says you should stay calm, avoid braking and try not to make hard turns. Take your foot off the accelerator to slow your speed, and when your car again has traction, look and steer toward the direction you want to go.

Car maintenance: Tires and wipers

Drivers who aren’t used to inclement weather may have also neglected necessary maintenance in case of storms. Before heading out, check tire treads and windshield wipers.

The more worn down a tire’s treads are, the more likely the vehicle is to go out of control on the road. To check at home whether the tires are in good shape, grab a quarter, rotate it so that George Washington’s head is upside-down, and put it into the tread. If the top of his head can be seen, the tires are balding and in need of replacement.

After months with little or no rain, drivers may have also neglected their windshield wipers. Over time, the rubber protecting the windshield wiper blades disintegrates. They might just smear moisture over the windshield, or worse, break off while driving.

Car owners should replace their windshield wipers every six months, according to Vazquez. Clean the inside of the windshield as well to reduce smudging.

Gwendolyn Wu is a San Francisco Chronicle staff writer. Email: gwendolyn.wu@sfchronicle.com