Police are advising motorists involved in nighttime traffic stops to pull over in well-lighted areas and be vigilant about anyone who may be impersonating an officer, after a woman was sexually assaulted this week on the Capital Beltway.

The woman, 21, told investigators that she stopped on the outer loop near Landover Road about 2:30 a.m. Thursday when a man who was driving a dark blue, four-door Crown Victoria flashed a red and blue light on the dashboard and sounded a siren. The man ordered the woman into the back of his car and assaulted her, police said.

Sgt. Tammy Sparkman of Prince George's County police called the incident "scary" and said she could not recall any similar incidents being reported in recent years.

The assailant was described as black, about 20 years old, six feet tall and 200 pounds. He was wearing dark blue pants, dark boots and a blue, uniform-style jacket that did not have a badge, insignia or patches, police said.

Sparkman said it was unclear whether the man displayed a weapon.

The man released the woman after the attack, police said. She then drove to her home in the Bladensburg area and called police.

Sparkman said if someone claims to be a police officer and is not wearing a uniform, motorists have the right to ask to speak to the person's supervisor or request that a uniformed officer come to the scene.

She also said that during routine traffic stops, police do not ask drivers to get into the back seat of their cruisers.