An alert motorist on Highway 101 helped Petaluma police arrest a driver whose blood-alcohol level was more than four times the legal limit, police said Sunday.

Shortly before 10 p.m. Thursday, a driver on southbound 101 reported a vehicle weaving within the lanes and had almost struck another vehicle, Lt. Tim Lyons said.

The driver followed the vehicle as it took the East Washington Street exit in Petaluma, where police found it parked a short while later.

Police spoke with the driver, Megan Samuelsen, 47, of Cotati, and smelled alcohol on her breath, Lyons said. She was arrested on suspicion of drunken driving.

Samuelsen was on probation for a DUI, Lyons said.

Her vehicle was impounded and she was booked into Sonoma County Jail.