One woman who made her way out of the tunnel said she was with five other people in a van, but added: "I have no idea about what happened to the others. I don't know how many vehicles were ahead and behind ours."
Our correspondent says there will be serious question about how a major tunnel on one of Japan's most important traffic arteries could have failed so catastrophically, with the private company that runs the highway saying the tunnel had undergone a major inspection just two months ago and had been given a clean bill of health.
Japanese media say that for more than 30 years the company that owns the tunnel had relied on rudimentary visual inspections, with no reinforcement or repairs since construction.
An inquiry is under way after nine people died following the collapse of concrete panels that started a fire in the tunnel, 80km (50 miles) from Tokyo.
