The northbound George Washington Parkway has been shut down at Spout Run due to a vehicle that skidded off the road and slammed into a tree.
One southbound lane is being blocked by a fire truck. Northbound traffic is being diverted onto Spout Run.
The closures are causing major traffic problems, prompting Arlington County to issue an email alert.
The prolonged closure, with accident investigators on the scene, is usually a sign that at least one person has died or is in grave condition.