Tunnels covering motorways? Barmy! It won't happen. It would take years to implement and the cost would be enormous. If internal combustion powered cars are due to be phased out over the next twenty or so years, what's the point anyway? Commercial vehicles will no doubt be under pretty stringent emmissions controls by then too.
https://www.theguardian.com/uk-news/...sorb-pollution
http://www.dailymail.co.uk/sciencete...d-tunnels.html