  We derive the critical behavior of a CA traffic flow model using an order
parameter breaking the symmetry of the jam-free phase. Random braking appears
to be the symmetry-breaking field conjugate to the order parameter. For
$v_{\max}=2$, we determine the values of the critical exponents $\beta$,
$\gamma$ and $\delta$ using an order-3 cluster approximation and computer
simulations. These critical exponents satisfy a scaling relation, which can be
derived assuming that the order parameter is a generalized homogeneous function
of $|\rho-\rho_c|$ and p in the vicinity of the phase transition point.
