in 1894 Tower Bridge made it possible to have a bridge below London Bridge
without restricting access by ships. Originally steam powered the hydraulic
bascules are now oil powered. It is now possible to visit the towers and
high level walkways, which were originally designed to allow pedestrians
to cross while the bridge was open.
By the way, don't scoff at those old films showing the villain meeting a sticky end as he tries to escape the police by jumping the opening bridge in his Morris Oxford, because in 1952 it happened to a London double decker bus! (The bus made it).
When nondiscript London bridge was replaced some years ago, the old structure was sold to an American millionaire to be rebuilt in Arizona. It's a London urban myth that he believed he was to recieve the gothic magnificence of "Tower" rather than dull "London" bridge.