Entrepreneur Elon Musk has unveiled a prototype underground tunnel in Los Angeles which is designed to transport cars at high speed around the city.
The tunnel is only a mile (1.6km) long at the moment but the goal is a network to ease chronic traffic congestion.
Modified electric cars would be lowered into the tunnel and travel at speeds up to 150mph (240km/h), Mr Musk says.
The tunnel has been built by Mr Musk s Boring Company, which boasts state-of-the-art engineering techniques
Mr Musk, best known as the head of Tesla electric cars and the commercial SpaceX programme, arrived at the launch on Tuesday evening in a Tesla car modified to work on the "loop" system.
He was cheered by a small crowd as he emerged from the car at one end of the tunnel bathed in green and blue interior lights.
The plan envisages modified cars being lowered into the tunnel network by lifts and then slotted into tracks on the "loop".
"The profound breakthrough is very simple: it s the ability to turn a normal car into a passively stable vehicle by adding the deployable tracking wheels, stabilising wheels, so that it can travel at high speed through a small tunnel," Mr Musk said.
He first unveiled the tunnel plan earlier this year, saying he wanted to alleviate Los Angeles s "soul-destroying" traffic congestion.
On Tuesday he said his Boring Company had built the tunnel segment for $10m (£8m), adding that traditional tunnel-building technology would have cost up to $1bn.
The tunnel runs beneath the municipality of Hawthorne, where the Boring Company and SpaceX are both based.