In VANETs, how to resistant from collusion attacks are still an open problem. There are more and more attentions in recent years for improving the trust management model to counter collusion attacks. In this paper, we first evaluate beacon based trust management system, BTM, and RSUs aided model, RaBTM, with different combination of collusion attacks. Both of these two systems aim to propagate message opinions quickly while thwart internal attackers in privacy-enhanced VANETs. To evaluate the reliability of these two systems under different combination of collusion attacks effectively, we conducted a set of simulations with colluding alteration attacks and bogus message attacks. The simulation results show that the RaBTM and BTM are highly resilient to collusion attacks and performs at least 15% better than comparison baseline.