Pat Hasler:
As an add to this though i have found that with only 40 miles on the sea you can infact drive from the UK to the USA
Down to Dover and ferry to Calais, drive for days or even weeks to far eastern Russia, from there it is only 4 miles by ferry during winter (and even just a drive if the sea is frozen) to Alaska.
I remember this was tried a few years back by a group in some Ford Maverick 4x4,s. They drove througfh the tunnel before the train tracks were completed and among the many trials were having to use Russian army escorts and some big old eight wheel all terrain vehicles to get across the Siberian wilderness (not sure if this was the same programme that had fires lit under the engines to stop them freezing up overnight). I vaguely recall they were stumped by the gap to Alaska due to an early thaw.