The rider of a mobility scooter has been seriously injured after crashing into the back of a tractor.
Police said the scooter went into the tractor "for reasons unknown" on a pedestrian crossing in Royston, at about 13:20 BST on Monday.
The woman, in her 50s, had life-changing injuries, but the tractor driver was unhurt, Hertfordshire Police said.
Officers are appealing for witnesses or dashcam footage.
The crash happened on a crossing near the junction of Priory Lane and Newmarket Road.