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There are three main types of speed cameras which are used in the UK. They are all different and include lots of different technology.
St Stephen's tower is, perhaps, the best known of London's landmarks. Equally, Big Ben, the largest of the clock's bells, is world renowned as a symbol of regularity, due to its chimes sounding across London every hour. At Midday and Midnight Big Ben rings out twelve times, however, using a bit of mathematics of sound and distance it is possible to hear the famous "bong" thirteen times.