A tanker carrying a hazardous substance has overturned on the M56 in Cheshire, causing long delays.
The motorway is closed in both directions between junction 11, Daresbury and junction 14, Hapsford.
The non-explosive substance leaked on to the eastbound carriageway. Nearby motorists were advised to close their windows and turn off air conditioning.
Police have declared a major incident. Three causalities were decontaminated at the scene and taken to hospital.
They suffered minor injuries believed to be from the impact of the crash, the North West Ambulance Service said.
The driver of the tanker, which crashed at about 15:45 BST, is also believed to have suffered minor injuries.
Motorist Carl Rowson, who was earlier escorted from his vehicle, said: "The fire brigade told everyone to walk back about a mile because of the toxic fumes."
Cheshire Police, who are currently working with Public Health England, said the M56 was expected to remain closed for several hours and motorists have been being advised to avoid the area.
The force tweeted: "We have received reports of drivers driving down the wrong way on roads and slip roads. Drivers are asked to be patient #M56"
Supt Bev Raistrick said: "Highways England are patrolling the tailbacks to help anyone who is in need of assistance on the motorway.
"Motorists are strongly advised to avoid the area."
Diversions are in place but Highways England warned there was a low-bridge height restriction on the route.
A 400m cordon is in place and residents in the Plover Lane area of Helsby have been told to stay indoors, keeping their windows and doors closed.
The train line through Helsby has been reopened, although delays are expected on services between Runcorn, Chester and Liverpool.