Police have opened an investigation after a tanker truck carrying liquefied petroleum gas rammed into a truck stuck in a line of traffic in Bologna.
The tanker immediately caught fire and then exploded a short time later across eight lanes, causing part of the raised highway to collapse.
Italy’s Prime Minister Giuseppe Conte has visited victims of the crash.
He said it was important to understand what happened to prevent future tragedies.
The truck’s driver was the only person killed.
"I am here today to demonstrate the closeness of the government to the people who are suffering, but also to understand why so that these things are not repeated," said Conte.
Police video shows by the time of the explosion, most vehicles had been evacuated from the highway.
Highway police said only the tanker driver died in the crash, revising earlier reports that there were two dead.
Dozens of people were injured, many with burns or cuts from flying glasswhen windows of buildings nearby were shattered in the blast.
The victim was identified as 41-year-old Andrea Anziolin of the northern city of Vicenza.
"Maybe it was a moment of distraction, or a wave of sleepiness," Bologna prosecutor Giuseppe Amato told RAI radio.
Conte will later travel to the southern region of Puglia, where a van packed with immigrant farm-workers overturned after colliding with a truck carrying tomatoes in the southern Puglia region, killing 12.
It was the second such fatal accident in the area in the space of three days.