Following the September 11 attacks in the United States in 2001 , and a reported increased terrorist threat to the United Kingdom , security was stepped up with occasional spot checks on vehicles entering the cordon , although not to previous levels . In December 2003 , the ring of steel was widened to include more businesses in the City . This was as a direct result of a police report that categorised a terrorist attack on the City as " inevitable " . Traffic entering the City is also monitored and recorded at the boundary of the London congestion charging zone , which covers a wider area
September 11 attacks
September 11 attacks
Traffic and Environmental Zone