Police and the air ambulance service were called to a crash on the South Mimms slip road of the A1(M).
The collision took place at around 6.30pm today, on the southbound carriageway of the A1(M) at Junction 1 for South Mimms.
The southbound and northbound carriageways have been closed to allow the air ambulance to land.
Once the ambulance helicopter leaves, the northbound carriageway will reopen, but drivers are still advised to avoid the area and seek an alternative route.
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