A workman had his arm amputated in hospital after he was knocked down a manhole by a car which failed to stop.
It happened at a roundabout between Marlowes and Combe Street in Hemel Hempstead on Thursday night shortly before 22:30 GMT.
Hertfordshire Police said the man, who is in his 40s, was working on the manhole when he was hit by a small white Kia type car.
Officers believe the car was then driven along Leighton Buzzard Road.
Police said the vehicle may have a possible oil leak and damage to the bodywork.
They want to hear from anyone who's been asked to repair a similar car or has any information about the crash.