A lorry carrying compressed gas caught fire on the M4, causing long delays for Easter bank holiday travellers.
The road was closed both ways between junction 17 (Chippenham) and 18 (Bath) for about an hour from 12:30 BST.
The westbound carriageway has fully reopened, while the eastbound carriageway has two lanes open.
Earlier the eastbound tailbacks extended as far as junction 19, where the M4 meets the M32 near Bristol, about 14 miles from junction 18.
Jade Jones told the BBC she had been stuck in traffic for two hours about a quarter of a mile away from the tanker.
She said: "I was travelling from Bristol to Bournemouth to meet a friend for her bank holiday birthday party. I needed to be in Bournemouth by 2pm.
"I left Bristol at 11:30 and hit the M4 at 11:55 and have been there ever since.
"We didn't see a fire engine arrive for 25 minutes. We're still an hour and 50 minutes away from our destination."
Travellers were advised to stay in their cars due to the clouds of black smoke from the lorry. The tanker contained argon which is an inert gas used in the production of titanium and in welding to prevent oxidisation.
Earlier, Highways England tweeted: "We've opened another lane past the scene of the HGV fire on #M4 E/B J18-J17 and 2 lanes are now running. This should help ease congestion."
Debee Horwood tweeted: "Tailbacks Westbound on the #M4 go back as far as 4 miles from Membury Services after Gas Tanker Fire between Junction 17-18. #trafficnews"
Tim Shires tweeted: "Any Bristol City & QPR fans heading to todays game using the M4 - its CLOSED in both directions between J17 & J18. Find alternative route!"
Hayley, using the Twitter handle @LilacRaindrops said: "Still at a standstill Eastbound on the #M4 due to the tanker fire. Been at a standstill now for nearly 1hr 20 mins... #BankHolidayProblems"