The question this research address is if voluntary certification with ISO 9000 standards can reduce the number of accidents large trucks have. Certification might have a positive effect depending upon the actions a company takes as a result of being certified, for instance, a company may change driver training and maintenance and how it carries out corrective action. We identified 40 ISO 9002: 1994 certified U.S. trucking companies and compared their safety performance before and after ISO certification. We also compared them to a group of motor carriers that had similar characteristics but were not certified. The safety performance of the certified carriers was significantly better after certification than before, and it also was significantly better than that of non-certified carriers, findings that we believe can be useful to carriers, shippers and regulators who wish to better understand the value of certification.