Bicycles with infant seats have become a common means of transportation for infants and drivers. Although studies on road surface evaluation considering infant seating have considered road surface roughness as a concern for passengers, but the magnitude of local road surface irregularities that causes road surface roughness is not yet clear. In this study, focusing on the steps that generate vehicle vibration, which is a risk to infants riding in infant seats of bicycles, the objective was to understand the scale of local road surface irregularities through vibration evaluation by driving tests and investigation of road surface profile analysis methods. As a result, the relationship between vibrations and step height that poses a potential risk to an infant was clarified. We used differential filter that considers the vibration characteristics of infant seats in the measured road profile in this study and demonstrated that localized pavement irregularities can be visualized.