Ultra (for Urban Light Transit) is an ATN (or PRT) system developed by Ultra Global PRT of Great Britain.  After running on a Cardiff test track the first public system was opened at London's Heathrow Airport in 2011. It was made up of 21 autonomous vehicles running on a 2½ mile track.  To reduce its construction costs Ultra made use of many standard products such as rubber tires that run on top of an open guideway to achieve what it believes and economic solution.  The system has carried over 700,000 passengers and in May 2013 celebrated reaching its 1 millionth autonomously driven mile.