LOS ANGELES — Billionaire entrepreneur Elon Musk on Friday posted video on social media of what he describes as an electric sled speeding through a tunnel, a test of a system he envisions for 3-D networks of underground passages for speeding traffic under Los Angeles’ congested roads.

He posted on Twitter and Instagram that such sleds could transport cars at 125 mph (200 kmh), with automatic switching from one tunnel to the next.

The video shows the sled, apparently riding a monorail, zipping through alternately dark and lighted sections of the tunnel. Musk warned that watching it may cause motion sickness or seizures.

The founder of the SpaceX rocket and the Tesla electric car companies told a recent TED Talk his ideas for improving the speed and cost-effectiveness of tunnel boring.

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