Tesla’s Chief Executive Officer Elon Musk is working on a self-driving vehicle he says could replace buses and other public transport in order to reduce traffic in cities. But he’s keeping the development a secret.

“We have an idea for something which is not exactly a bus but would solve the density problem for inner city situations,” Musk said Thursday at a transport conference in Norway. “Autonomous vehicles are key,” he said of the project, declining to disclose more. “I don’t want to talk too much about it. I have to be careful what I say.”

Musk’s Tesla Motors Inc. has disrupted the transport industry with smarter software-packed electric cars and this month received 400,000 pre-orders for its latest product, a less costly model dubbed  Model 3 aimed at the mass market. Meanwhile the billionaire is exploring Hyperloop to transport people between cities via pods enclosed inside of tubes, and his aerospace company Space Exploration Technologies Corp. is working on one day sending humans to Mars.

“I very much agree with solving the high-density urban transport problem,” Musk said. “There’s a new type of car or vehicle that would be great for that and that’ll actually take people to their final destination and not just the bus stop.”

Musk in the past has mentioned building car tunnels as a fix for traffic congestion. “Gridlock is just a soul-destroying thing,” he said in May last year in an interview with Bloomberg Television. His response last August to a question about car-sharing prompted speculation about Tesla starting its own such service with electric vehicles.

Musk is not the only billionaire investing in the future of public transport: Warren Buffett has backed initiatives to roll-out all-electric buses to cities including Seattle.