  Analytical investigations are made on BML two-dimensional traffic flow model
with alternative movement and exclude-volume effect. Several exact results are
obtained, including the upper critical density above which there are only
jamming configurations asymptotically, and the lower critical density below
which there are only moving configurations asymptotically. The jamming
transition observed in the ensemble average velocity takes place at another
critical density $p_{c}(N)$, which is dependent on the lattice size $N$ and is
in the intermediate region between the lower and upper critical densities. It
is suggested that $p_{c}(N)$ is proportional to a power of $N$, in good
agreement with the numerical simulation. The order parameter of this jamming
transition is identified.
