It happened about 8:30 a.m. Friday along Interstate 495 in Lanham in Prince George's County.
Troopers say 18-year-old Sebastian Reyes, a cadet in the commercial vehicle enforcement division, was driving an unmarked SUV on a routine assignment.
Police say Reyes used a crossover for emergency vehicles to head in the opposition direction.
Officers say Reyes lost control of the vehicle and sideswiped a car driven by 59-year-old Karen Holt-Williams of Temple Hills.
Authorities say Holt-Williams' car traveled across the interstate and hit a jersey wall. Reyes' SUV hit another car.
Holt-Williams died at a local hospital. The crash is under investigation.
Troopers say Reyes has been placed on leave.