It might be the most obviously unfinished motorway in the whole UK, let alone in London. The M23 was supposed to link London with Crawley - but its urban section was a problem that couldn't be solved.
Today you can catch a train to France. You can even put your car on it, and be there in less than an hour. But in the 1980s, Euro Tunnel had competition - including plans to build a motorway across the English Channel.