Elon Musk, not content to solve our (surface) automotive problems, our space problems and our solar power problems, also wants tunnels to reduce traffic. What a guy.
(Elon Musk wants tunnels @$1Billion per mile)
The company’s promotional video shows vehicles zipping along city streets before stopping on car-sized elevators built into the pavement. Underground tracks whisk them off to their destination at speeds of more than 130 miles per hour (200 kilometers per hour) before depositing them back at the surface to finish the last leg of their journey. He wants to do all this for one-tenth of today’s costs compared to the estimated $1 billion per mile price tag for Los Angeles’ new subway extension.
Fans of scifi movies of course recognize the care taken in making sure that you are NOT in control of your car; visualization follows.
(I, Robot chase scene - underground tunnel problems)
I should mention that underground tube systems have been popular for a long time (see the pneumatic tube station from Schachner and Zagat's Exiles of the Moon); an English engineer named George Medhurst patented one in 1825.