Officer Kevin Smale tells The San Diego Union-Tribune someone spotted the bound girl in the car's backseat on southbound Interstate 5 Monday evening and called 911.
Officers stopped the car in the city of Carlsbad.
Smale says in a statement that investigators determined the girl's 67-year-old father and 21-year-old sister forced her into the car with the intent to take her to drug treatment.
Smale also says the 17-year-old admitted to using methamphetamine for the past year.
The girl was taken into protective custody. No charges have been filed but the case is under investigation.
Information from: The San Diego Union-Tribune, http://www.utsandiego.com