A half marathon is a road running event of 21.0975 kilometres (13.1094 mi). It is half the distance of a marathon and usually run on roads. Participation in half marathons has grown steadily. One of the main reasons for this is that it is a challenging distance, but does not require the same level of training that a marathon does. In 2008, Running USA reported that the half marathon is the fastest growing type of race. A 2010 article by Universal Sports echoed the growing popularity of the distance. It is common for a half marathon event to be held concurrently with a marathon, using almost the same course with a late start, an early finish or shortcuts. The half marathon is also known as a 21K, 21.1K or 13.1 miles, although these values are rounded and not formally correct.