The elevator stations. And the tunnels are straight shots. Watch the interview with him. Basically this allows you to drive an electric car potentially of any make/model at 150 MPH and then take an "off ramp" to exit.
See the bottle neck there? The elevators are slow, the "off ramps" will get congested and backed up, then the main flow will as well. (Or if a car breaks down). Hopefully there aren't many if any bends or that 150mph car may not see the backup in time.
Sure this may help with extremely long straight stretches for a while, until it becomes popular that use increases. It has the same issues as current roadways. And when repairs need to be made, then what? You'll have a ton of people backed up in a single line in a tunnel, blocking off ramps etc because no one knows how to use two lanes and zipper to not make the backup even worse.