If there is a tunnel that moves a lot of vehicles quickly then that will generate problems at the entrance where vehicles line up to enter and at the end where vehicles exit. This may work out, but it is likely that all tunnel interfaces will, like bridges, require a complex array of interfaces to surface roadways. That adds a lot of expense and takes up a large amount of space, but there is no other way to make fast tunnels actually work to anywhere near their capacity. This implies that making fast tunnels work involves much more than the fast tunnels alone.