Emergency crews responded to a crash at Highway 16 and Floating Feather Road Monday night.
Officials say a Jeep Cherokee being driven by a 50-year-old woman from Emmett was traveling north on Highway 16.
Another car, with a 16-year-old boy from Star and 16-year-old girl from Eagle, was traveling west on Floating Feather and tried to cross Highway 16, when the two vehicles collided.
The teen girl is currently in critical but stable condition, while the teenage male's injuries are considered serious but not life threatening.
An air ambulance was sent to the scene to fly the older women to St. Alphonsus Hospital in Boise where she is currently in serious condition.
"We unfortunately have a lot of accidents at these intersections," says Sgt. Patrick Schneider of the Ada County Sheriff's Office. "So we ask people to be really careful about coming up to these stop signs when they cross Highway 16.
"It's a regular occurrence, unfortunately, that people run across Highway 16. With both vehicles traveling at 50-60 miles per hour it's catastrophic, as you can see."
Highway 16 was closed in both directions while crews investigated the scene, but re-opened a little before midnight.