We examine the impact of several factors, including union membership and port efficiency, on the number of vessel related accidents and the resulting economic damage at major US ports during the time period 2002-2012. We show that, on average, the union membership rate and port efficiency have a positive and a statistically significant impact on the number of vessel related port accidents . This could be possibly explained by the increased use of contract and casual workers, an issue exacerbated by the antagonistic relationship between management and the unions, at the ports.    We also evaluate the resulting economic damages of the port related accidents. In this analysis, contrary to our earlier results, we find that union membership has a negative impact on economic damages , while all other findings noted in the analysis on the number of accidents hold. Though this initially appears contradictory to the results on the number of accidents, it may be related, given the duties specifically performed by the aforementioned contract employees. We evaluate the robustness of the results from our initial models, which evaluated the number of accidents and economic damages based upon efficiency estimates obtained by employing Data Envelopment Analysis, by comparison with those models in which the efficiency is estimated using Stochastic Frontier Analysis (SFA). Our findings could help policy makers improve planning and to better manage port traffic risks.