Following a collision a young male who suffered a very badly broken leg was flown by Air Ambulance to St George's Hospital hospital in London at around 6pm.
Video of Police Cordon at both ends of North Approach and Helicopter taking off.
Police and Ambulances were at the scene, along with an East Air Ambulance helicopter which landed on the A405 North Orbital Road in Leavesden.
One witness who lives on the road said "I heard the sound of screeching tyres as the car sped off down the road. Shortly followed by Police Sirens.
One local says it looked like a black BMW that hit a kid on a moped.
She saw the car came down Harris Rroad with the front end totally smashed in.
Many locals via Social media hoped the boy is ok and that the BMW car and driver are found.
The police have since found the car unattended.
Many people wanting to get the the Hollywood Bowl had to find another way.
One pizza delivery driver had to ask police how to get there as both roads where closed.
It is unknown the exact location of the incident and why the road above was coned off.
Collision investigation officers remain at the scene while investigation continues.
More updates to follow.
Enquiries are continuing.
Police are appealing for anyone who was in the area at the time and witnesses to contact them as soon as possible by calling the non-emergency number - 101.
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