Many wheelchair users rely upon fixed route public transportation using large accessible transit vehicles (LATVs) for independent transportation to and from work, healthcare appointments and leisure activities. A substantial number of these wheelchair users may not be able to transfer from their wheelchair to a motor vehicle seat during transit. It is necessary to afford these wheelchair users the same level of safety as occupants seated in motor vehicle seats. Therefore, the Americans with Disabilities Act (ADA) requires that LATVs be equipped with wheelchair securement and occupant restraint systems (WTORS). The four-point tiedowns and occupant restraints are the primary means of WTORS on LATVs. Recent research has shown extremely high disuse and misuse rates for this wheelchair and tiedown occupant restraint system (WTORS). However, to-date, no studies have investigated the underlying causes of WTORS disuse and misuse. Perhaps more importantly, the consequences of WTORS misuse and disuse for wheelchair passengers aboard LATVs have not been determined. This study had three major phases aimed at describing consequences of WTORS misuse and disuse for wheelchair passengers on LATVs:​ I) A case series of adverse events involving wheelchair seated passengers on LATVs, II) Driving experiments using a wheelchair seated anthropomorphic testing device onboard a LATV, III) Development, validation and verification of a computer simulation of a wheelchair passenger onboard a LATV. This study revealed that the vast majority of wheelchair related instabilities are associated with minor wheelchair excursions. We also found when WTORS are properly used, the wheelchair passenger does not appear to be at increased risk of injury, while WTORS disuse and misuse are associated with a high frequency of wheelchair passenger ejection from the wheelchair, especially during an emergency braking maneuver. We found that a manual wheelchair brake can play a major role in limiting wheelchair excursions when WTORS are not properly used and that lateral wheelchair excursions can be limited by the folded-seat side barrier under the window.