  Transport simulations are very valuable for extracting physics information
from heavy-ion collision experiments. With the emergence of many different
transport codes in recent years, it becomes important to estimate their
robustness in extracting physics information from experiments. We report on the
results of a transport code comparison project. 18 commonly used transport
codes were included in this comparison: 9 Boltzmann-Uehling-Uhlenbeck-type
codes and 9 Quantum-Molecular-Dynamics-type codes. These codes have been
required to simulate Au+Au collisions using the same physics input for mean
fields and for in-medium nucleon-nucleon cross sections, as well as the same
initialization set-up, the impact parameter, and other calculational parameters
at 100 and 400 AMeV incident energy. Among the codes we compare one-body
observables such as rapidity and transverse flow distributions. We also monitor
non-observables such as the initialization of the internal states of colliding
nuclei and their stability, the collision rates and the Pauli blocking. We find
that not completely identical initializations constitute partly for different
evolutions. Different strategies to determine the collision probabilities, and
to enforce the Pauli blocking, also produce considerably different results.
There is a substantial spread in the predictions for the observables, which is
much smaller at the higher incident energy. We quantify the uncertainties in
the collective flow resulting from the simulation alone as about $30\%$ at 100
AMeV and $13\%$ at 400 AMeV, respectively. We propose further steps within the
code comparison project to test the different aspects of transport simulations
in a box calculation of infinite nuclear matter. This should, in particular,
improve the robustness of transport model predictions at lower incident
energies where abundant amounts of data are available.
