The A41 has been closed northbound this morning (Tuesday) after a van overturned, causing an oil spill.
Police were called at 9.30am to reports of an road traffic collision on the A41 King Langley bypass northbound.
A spokesman said a transit van had overturned, but no other vehicles involved and no injuries were reported.
The road remains closed due to oil on the carriageway which is being cleaned up.
Hertfordshire Constabulary are advising motorists to avoid the area.
More to follow.