A tanker carrying propane gas caught fire, shutting the M56 motorway near Chester and leading to traffic gridlock.
Firefighters have now brought the blaze under control, which broke out at 3pm on the westbound carriageway.
The motorway remains closed in both directions between junctions 12 and 15 near Stanlow and Frodsham.
Many of the surrounding road closures have now been lifted and the 1,600m cordon has been reduced.
The force said it had removed the motorway’s central reservation so motorists could turn around and get off the motorway.
Tom Jackson, who was stuck on the motorway, said: “I was about 20 cars back and we just saw the traffic start to slow and then come to a complete standstill.
“A huge plume of black smoke and then two small explosions, that may have been tyres, then there was a huge one which was like a giant bunsen burner going off way up into the sky.”
Five fire engines attended the scene and worked with both the police helicopter unit to check the tanker’s temperature and specialist staff from the tanker company.
Air traffic controllers were also advised of the incident to ensure flights at Liverpool and Manchester airports avoided any smoke.
The tanker caught fire on the hard shoulder of the M56 at junction 14, a few miles from Ellesmere Port.
The Manchester to Chester railway line has reopened but the line between Helsby and Hooton remains closed.
Earlier, stretches of the M56 became gridlocked, with traffic backing up as far as Liverpool, with a knock-on effect into north Wales.
Photos posted on Twitter showed motorists leaving their vehicles to sunbathe or play football on the opposite side of the carriageway, which was empty of traffic.