You have to have a passion, time, and money to participate. Here's the basic rule: Old bikes only.
The Motorcycle Cannonball puts age restrictions on the bikes that can be entered. In 2010, only pre-1915 period-correct bikes could be entered. The second Cannonball in 2012 allowed pre-1929 bikes, and the third reprise was in 2014 when pre-1936 bikes could be ridden. For 2016, in honor of the 100th anniversary of Cannonball Baker’s record-setting cross country ride, the Motorcycle Cannonball registrants could only ride on pre-1916 bikes again.
As you can imagine not all make it across the country. Some break down, some catch on fire and burn down...
People come from around the world to participate, not that there's a huge number of participants, just because or the rarity of the bikes. And, the route is unknown as each event is different.