The A405 had to be shut after a car flipped over.
Emergency services remain at the scene of the crash which has happened at the junction of Bucknalls Lane, between Bricket Wood and Garston.
Police say it involved a silver VW Golf and a blue Mini Cooper.
One lane remains closed, towards the direction of Watford, and both cars need recovering.
Paramedics are at the scene.
There are major tailbacks on the A405 all the way back to the Noke roundabout in Chiswell Green.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here