It took 8 years, between 1886-1894, and 432 construction workers to build Tower Bridge.
Originally the only crossing over the Thames was London Bridge. More bridges were added over the centuries as London grew, although these were all west of London Bridge, since the area east of it had become a busy port.
In the 19th century London's East End became heavily populated and need mounted for a new bridge east of London Bridge, as road journeys south from the area were being delayed by hours. This could not be a fixed bridge because it would not accommodate access by tall-masted ships to the port facilities between London Bridge and the Tower of London.
Therefore the chosen design for Tower Bridge incorporated a tower at each end of the central span split into two equal halves which could be raised to allow river traffic through. The raising mechanism was originally powered by steam but since 1976 it has been driven by oil and electricity.
The bridge originally had high-level open air walkways between the towers above the central span, which could still be crossed when the bridge was raised. However, they became regularly frequented by undesirables, and in 1910 they were closed due to lack of use. Also, many people arriving at the bridge when raised preferred to wait at street level rather than climbing up the stairs often carrying heavy loads. In 1982,as part of the Tower Bridge Exhibition area, the walkways were reopened to the public, now fully enclosed. In 2014 a 11 metres long glass floor was installed in one walkway, 42 metres above the river.
Tower Bridge is now crossed by over 40,000 pedestrians, cycles and vehicles every day. It is raised around 1000 times a year. The river traffic is now much reduced, but it still takes priority over the road traffic.