A lorry has ploughed into the railway bridge at Ickleford forcing the closure of both approaches.
Paramedics are at the scene assessing the male driver, who is out of the vehicle, while officers have shut Arlesey Road either side of the bridge.
The road is a popular rat run between Letchworth GC and Hitchin.
The lorry, which hit the bridge shortly before 1.45pm, cannot be removed until an engineer from Network Rail has assessed the damage to the structure, a police spokesman said.
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