We derive the critical behaviour of a cellular automaton traffic Row 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 nu(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 ho - rho(c) and p in the vicinity of the phase transition point. [References: 14]
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