Imagine several people riding motorcycles in the plane without any obstructions or boundaries. Each one has a fixed starting position and a fixed velocity. At time 0 they all start moving and leave tracks. When a motorcycle meets a track it crashes and cannot move any further. If two motorcycles collide, both of them crash and can no longer move. When all the motorcycles have either crashed or moved away, their tracks form a planar graph called a motorcycle graph. In the 1982 science fiction film Tron, the motorcycles behaved like this. This Demonstration lets you choose up to five bikers; drag the black disks to set their positions and the tips of arrows to set velocities.
 J. Erickson. "Crashing Motorcycles Efficiently." (Jul 22, 2013) www.cs.uiuc.edu/~jeffe/open/cycles.html.
Wolfram Demonstrations Project
Published: July 23 2013