A run of Bay Area roadway carnage that began early and ominously on Black Friday was part of a tragic 47.6 percent increase in road deaths statewide compared to Thanksgiving weekend last year.

Statewide -- from Wednesday evening through Sunday morning -- the California Highway Patrol marked 31 fatalities in crashes across the state, compared to 21 for the same five days in 2011. In the Bay Area alone last year, according to authorities, there were two such deaths, only a third of the six that happened this year.

Instead of being a result of negligence wrought by drinking and drugs, officials believe the fatal crashes may stem in part from the seasonal havoc that holiday festivities wreak on sleep patterns, work and vacation schedules and driving routines. Forgetting or choosing not to wear seat belts also seemed to have been a devastating factor.

"It was just a tragedy," said CHP spokeswoman Sgt. Diana McDermott of the weekend totals. "You don't want your family on the news."

One clear pattern also emerged from the string of fatalities: Of the 21 people who died on CHP-patrolled roadways, 13 were not wearing seat belts.

Different schedules

Among those numbers were likely two San Bruno sisters, headed home with four other family members after a marathon night of Black Friday shopping in Gilroy. They were killed in a car crash on Highway 101 near Palo Alto early Friday. Officials say it appears Nisha Tandel, 24, and Sheetal Tandel, 20, were not buckled in when the SUV driven by their father, Arvind Tandel, 48, crashed and flipped. Investigators suspect the father may have fallen asleep at the wheel.

McDermott said the holiday season suddenly throws people out of their routines -- and that can be deadly. Arrival of the holidays can produce a rush of new duties, errands and responsibilities. This time of year often means late-night parties or marathon shopping -- which result in less sleep. A normal commute might be in stop-and-go traffic while a long drive to special shopping venues or to myriad evening Christmas parties might mean lots of open roads and the temptation to hit the gas.

"All of those are factors," McDermott said. "You have to be more attentive than usual over the holidays."

The crashes over the weekend also included a mix of strange or particularly horrific collisions. In San Jose, a 59-year-old local man headed the wrong way on San Tomas Expressway died Saturday night after plowing into an oncoming car. Police didn't say how the man ended up driving north in the southbound lanes at Williams Road around 9 p.m.

Other accidents

In Contra Costa County, a 23-year-old El Sobrante man died after his BMW slammed into a big rig early Sunday morning on Interstate 80 and burst into flames. That crash in the westbound lanes near Willow Avenue about 2:20 a.m. remains under investigation. And in South San Francisco, a woman was struck and killed by several cars Saturday around 8:20 p.m. as she walked on Highway 101 near Oyster Point Boulevard. While at least two drivers stopped, the first driver to hit the woman sped away and remained unidentified.

Whether drunken or stoned drivers had a hand in the weekend's bloody results remains unclear. Authorities said they had made 224 driving-under-the-influence arrests from Wednesday evening to Sunday morning, as compared to 227 last year. All of the fatalities remain under investigation, but as of Sunday afternoon the CHP didn't report connections to booze or dope.

Police also reported arresting a driver who allegedly caused a chain-reaction crash that killed a 67-year-old man standing at a San Mateo bus stop Saturday.

Police say the suspect, Josue Lopez, 26, of San Mateo, lost control of his speeding car around 11:30 a.m. and veered into a concrete garbage can at the bus stop on the 1700 block of South Delaware Street. The impact blasted the can into the victim, Reynaldo Aguiniga, who ended up pinned against a tree. Police say Lopez took off and Aguiniga later died of his injuries.

However, witnesses passed enough of a description of the car, driver and license plate to police that they arrested the suspect the same day.

Contact Joshua Melvin at 650-348-4335. Follow him at Twitter.com/melvinreport.