The incident happened around 8.35pm when a quad bike struck a black Skoda Superb taxi.
The 31-year-old male rider of the quad bike was taken to the Royal Infirmary of Edinburgh.
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His condition is described as critical.
No-one else was injured as a result of the crash.
Constable Jake Threadgold, of Police Scotland’s Road Policing Unit in Livingston, said: “We believe the area was busy around the time of the crash and would urge anyone who may have witnessed the crash to contact police.
“We are also keen to speak to the rider of a motorbike who we understand was riding in tandem with the quad bike.
“Anyone with information or dash-cam footage which could assist our investigation should call 101, quoting incident number 3442 of 1 April.”