Trail running is a sport which consists of running and hiking over trails. In the United Kingdom and Ireland it is called mountain or fell running. It differs from road running and track running in that it generally takes place on hiking trails, often in mountainous terrain, where there can be much larger ascents and descents. It is difficult to definitively distinguish trail running from cross country running. In general, however, cross country is an IAAF governed discipline that is typically raced over shorter distances (rarely over 12 kilometers), whereas trail running is loosely governed, and run over longer routes. The ranks of trail runners are increasing annually. They have grown from 4.5 million to more than 6 million in the United States alone between 2006 and 2012. The amount of organized trail races has grown over the past few years throughout the world, now well into the hundreds in North America alone. Runners often cite less impact stress compared to road running, as well as the landscape and non-urban environment, as primary reasons for preferring trail running. This move to nature is also reflected in a large increase in competitors in non-traditional/off-road triathlons and adventure racing over the past five years.