General Discussion Topics, page-5772

Elon Musk believes vehicles flying overhead will not reduce peoples'
anxiety - you're out walking the dog and a hubcap or vehicle part takes
your head off LOL.
He has designed an underground transport system where first, he explains,
multi-lane road tunnels are not wanted. Instead, his concept is for vehicles
not to be driven, but rather carried on individual electric shuttle platforms,
or “car-skates”, along narrower, single lane road tunnels. To that end, Musk
is looking to cut the diameter of road tunnels by at least half the current
standards for a single lane road tunnel of 7.9m-8.5m (26ft-28ft) diameter,
including allowance for safety, ventilation, access and emergency facilities.

https://www.tunneltalk.com/Discussion-Forum-May2017-Elon-Musk-talks-tunnelling.php
Well worth finding the one hour or long video of Musk discussing same. (2017)

And here, in Dec 2018 Elon Musk unveils prototype high-speed LA transport tunnel.
https://www.bbc.com/news/world-us-canada-46616902