The A602 has now reopened following a four-vehicle crash this morning.
The road was shut in both directions after the collision near Watton-At-Stone at about 10.55am.
A silver Mercedes C220 was travelling behind a red convertible Audi TT, heading from Stevenage towards Hertford, when it was in a collision with a red Audi A4 travelling in the opposite direction.
A white Mercedes panel van also heading along the road from travelling from Stevenage then collided with the Audi A4.
The driver of the Audi A4, a man in his 20s from Stevenage, sustained serious injuries and was taken to Lister Hospital in the town.
The van driver, also a Stevenage man in his 20s received slight injuries and was taken to Lister Hospital. The other drivers were not injured.
PC Felicity Moody from the Bedfordshire, Cambridgeshire and Hertfordshire Road Policing Unit is investigating the collision.
She said: “We would like to establish the exact circumstances of what happened and as such would like to hear from anyone who may have witnessed the collision or the events leading up to it. Anyone who saw any of the vehicles prior to the incident should contact me.”
Anyone with information can call police on non-emergency number 101.
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