The A602 is shut in both directions after a motorcyclist sustained serious injuries when coming off his bike this morning.
The crash took place on Ware Road at about 5.50am, close to the roundabout for the A119 and Watton-at-Stone.
The motorcyclist involved has been taken to Lister Hospital in Stevenage with serious injuries.
The A602 is currently closed in both directions between Stevenage and the junction for Watton-at-Stone while emergency services work at the scene, and motorists are being advised to avoid the area.
Traffic is being diverted into Knebworth and is said to be heavy.
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