This impressive arch was constructed in 1826-28 as a grand entrance to Green Park - we see traffic on Constitution Hill, running through the park, in the background. In 1846 a controversial, 28-foot statue of the Duke of Wellington was hoisted on top of the arch; it was removed to Aldershot in 1883.
At that time, to ease ever increasing traffic congestion, the arch was moved piece by piece to its present location, some 60 feet from its original site. The current monument was placed on top of the arch in 1912.
In the 1960s the current gyratory system was built at Hyde Park Corner, leaving the arch in the middle of what is now basically a huge roundabout.
By my reckoning, on the extreme left of this image Basher has captured the steps up to number 46, from which address Lord Lucan disappeared 40 years ago today.