LOS ANGELES – Stick-thin starlet Nicole Richie was busted for driving under the influence of drugs early yesterday after she took a dangerous wrong-way ride on a busy highway, then stopped dead in the fast lane to make a phone call, cops said.

The 85-pound star of “The Simple Life” told cops she smoked pot and consumed the painkiller Vicodin before the incident, but she passed an alcohol breath test at the scene, California Highway Patrol Officer Todd Workman said.

“Someone could have been killed or seriously injured in a situation like this,” said Workman.

Richie, 25, who pleaded guilty to a heroin charge three years ago, submitted a sample of body fluid that will be tested to determine what she may have been on, police said.

But the weight of her arrest didn’t seem to faze the 5-foot-1 Richie, who was more concerned that cops got her body weight right on the booking sheet. “Her license says 100 pounds, but she told the booking officers she weighs 85,” Workman said.

Two concerned motorists called 911 to report seeing a dark Mercedes SUV barreling east in the westbound lanes of the 134 Freeway in Burbank around 12:30 a.m.

When cops caught up with the vehicle a few minutes later, they found it facing west, but fully stopped in the fast-moving “carpool” left lane with Richie at the wheel talking on her cell phone, Workman said.

“She must have done a U-turn to change direction from eastbound to westbound, but she was at a complete stop in the carpool lane. Public safety was endangered,” Workman said.

“She was alone in the vehicle. She said she was lost and had been trying to follow some friends, but there were no friends around,” Workman said.

It is unclear where Richie had been before the arrest or where she was going, cops said. But some pals believe she was leaving the home of her new squeeze, Good Charlotte rocker Joel Madden, who lives in Glendale, Calif., near the arrest scene.

It’s possible Richie may face further charges for driving infractions, Workman said. Her “Simple Life” co-star, Paris Hilton, was busted for drunken driving in Hollywood in September.

The skin-and-bones celeb was booked at 1:45 a.m. on suspicion of driving under the influence of drugs and was later released on $15,000 bail. No court date had been set yesterday.

Her rep, Nicole Perna, declined comment on the incident.

In recent months, the adopted daughter of singer Lionel Richie has become a poster girl for way-too-thin celebrities.

Richie insists she does not have an eating disorder.