Motorists are warned to expect disruption on their morning journeys, with the A414 closed between the A405 Park Street roundabout and J7 of the M1 at Hemel Hempstead.
The closure is to enable investigation work to take place following an earlier accident, and there are severe delays reported in the area.
A diversion is in place taking drivers south down the A405, over the M25 roundabout, and then back to join the M1 northbound at J6.
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