A woman was badly hurt in what police may have been a hit-and-run incident.
She was taken to hospital with serious facial injuries after the incident this morning in Gledholt, Huddersfield.
The road was sealed off for several hours after the 6am incident.
Police said they were making extensive inquiries.
Det Insp Jim Griffiths, of Kirklees CID said: “A 24-year-old woman was injured in a road traffic accident at 6am at the junction of Gledholt Road and Quarry Road, Huddersfield.
“A passer-by has given her first aid and called an ambulance which has taken her to Huddersfield Royal Infirmary.
“She has been left with quite a nasty facial injury.
“It would appear that the motorist involved did not stop. However, we are at an early stage of the investigation and are trying to piece together how this lady came to be injured.”
Police accident investigators were called to the scene and spent a considerable time taking measurements of the road and taking photographs.
Commuter traffic using the road which runs from Longroyd Bridge to Marsh were diverted until the road re-opened shortly after 10am.
Anyone who has information or witnessed the accident should ring 101.
In unrelated investigations, police would like to speak to the people pictured in the image gallery below. The people pictured in the images may be witnesses as well as suspects. If you recognise anyone, contact police on 101 quoting the reference number on the image caption.