A hydrogen leak on a lorry on the A1 motorway in County Durham led to delays of up to 90 minutes during rush hour.
Highways England said the vehicle was carrying nine cylinders of gas, one of which had a crack and was leaking, and the only way to safely allow it to disperse was to let it vent.
An exclusion zone was set up around it and the road closed northbound between Chester-le-Street and Washington.
The motorway was fully reopened just before midday.
Diversions were put in place as the lorry could not be moved during the venting process as it presented a fire hazard, Highways England added.
Rob Cherry, from Durham and Darlington Fire and Rescue Service, said crews were alerted at about 23:20 GMT on Wednesday.