It gaves the first explanation for "traffic jam out of nowhere".

The cells represents in the NaSch-model a street of equal sections. This cells are empty or contains a car with a defined, discrete speed. The speeds are one of six different values (from 0 upto 135 in steps of 27 km/h). The initial state is given by random. You can define the density of cars under menu item "options"(15% is default).

A car accelerates, if it has enough empty cells in front and maximum speed isn't reached. A car slowed, if it has less cells in front. Additional there is an unmotivated slowing by a part of the cars. The amount of unmotivating slowing cars is defined by the "dawdle factor", which you find in options too.

state |
representing |

0 |
cell of street without car |

1 |
car speed 0 km/h |

2 |
car speed 27 km/h |

3 | car speed 54 km/h |

4 | car speed 81 km/h |

5 | car speed 108 km/h |

6 | car speed 135 km/h |

Driving direction is left to right, the timing sequence is top-down. You can see: traffic jam moving to the left, though the cars moving to the right.

More information you'll find in the related Wikipedia entry.