An overturned car has led to a main road being closed in both directions.
Stanmore Hill is closed in both directions following the incident at the junction with Fallowfield with traffic building in Stanmore and along The Common towards Bushey.
The female driver was helped out of the car by a member of the public and was taken to hospital by ambulance.
Police have closed the road at around 2pm as the black Hyundai is recovered from the scene.
The 142 bus is also on diversion.