ISO 26262, a functional safety standard for motor vehicles, was published in November 2011. Although motorcycles are not included in the scope of application of the current edition of ISO 26262, it is expected that motorcycles will be included in the next revision. However, it is not appropriate to directly apply automotive safety integrity levels (ASILs) to motorcycles because the situation of usage in practice presumably differs between motorcycles and motor vehicles. In our previous study, we newly defined safety integrity levels for motorcycles (MSILs) and proposed that the levels of MSILs should correspond to levels one step lower than those of ASILs; however, we did not investigate the validity of their connections. Accordingly, in this research, we validated the connections. We defined the difference of levels of SILs between motorcycles and motor vehicles as the difference of target values of random hardware failure rates specified in ISO 26262-5. By taking into account the benefits of economy, convenience, and pleasure, it is considered that motorcycle and motor vehicle users accept the results of traffic accidents as upper limits on tolerable risks. Using recent traffic accident results, we compared accidental risks while riding motorcycles or motor vehicles. We demonstrated that the difference of levels of SILs between motorcycles and motor vehicles and the difference of upper limits on tolerable risk between motorcycles and motor vehicles balance each other. It is considered that this finding justifies setting the levels of MSILs one step lower than the levels of ASILs.
