Since 2015, there have been more than 35,000 fatalities annually due to crashes on United States roads [1], [2]. Typically, road departure crashes account for less than 10% of all annual crash occupants yet comprise nearly one third of all crash fatalities in the US [3]. In the year 2020, road departure crashes accounted for 50% of crash fatalities [2]. Road departure crashes are characterized by a vehicle leaving the intended lane of travel, departing the roadway, and striking a roadside object, such as a tree or pole, or roadside condition, such as a slope or body of water. One strategy currently implemented to mitigate these types of crashes is the use of roadside barriers. Roadside barriers, such as metal guardrails, concrete barriers, and cable barriers, are designed to reduce the severity of road departure crashes by acting as a shield between the departed vehicle and more hazardous roadside obstacles. Much like new vehicles undergo regulatory crash tests, barriers must adhere to a set of crash test procedures to ensure the barriers perform as intended. Currently, the procedures for full-scale roadside barrier crash tests used to evaluate the crash performance of roadside safety hardware are outlined in The Manual for Assessing Safety Hardware (MASH) [4]. During roadside barrier tests, the assessment of occupant injury risk is crucial, as the purpose of the hardware is to prevent the vehicle from colliding with a more detrimental roadside object, all the while minimizing, and not posing additional, risk to the occupants. Unlike the new vehicle regulatory crash tests conducted by the National Highway Traffic Safety Administration (NHTSA), MASH does not require the use of instrumented anthropomorphic test devices (ATD). Instead, one of the prescribed occupant risk assessment methods in MASH is the flail space model (FSM), which was introduced in 1981 and models an occupant as an unrestrained point mass. The FSM is comprised of two crash severity metrics that can be calculated using acceleration data from the test vehicle. Each metric is prescribed a maximum threshold in MASH and if either threshold is exceeded during a crash test the test fails due to high occupant injury risk. Since the inception of the FSM metrics and their thresholds, the injury prediction capabilities of these metrics have only been re-investigated in the frontal crash mode, despite MASH prescribing an oblique 25-degree impact angle for passenger vehicle barrier tests. The focus of this dissertation was to use EDR data from real-world crashes to assess the current relevance of roadside barrier crash test occupant risk assessment methods to the modern vehicle fleet and occupant population. Injury risk prediction models were constructed for the two FSM-based metrics and five additional crash severity metrics for three crash modes: frontal, side, and oblique. For each crash mode and metric combination, four injury prediction models were constructed: one to predict probability of injury to any region of the body and three to predict probability of injury to the head/face, neck, and thorax regions. While the direct application of these models is to inform future revisions of MASH crash test procedures, the developed models have valuable applications for other areas of transportation safety besides just roadside safety. The final two chapters of this dissertation explore these additional applications: 1) assessing the injury mitigation effectiveness of an advanced automatic emergency braking system, and 2) informing speed limit selection that supports the safe system approach. The findings in this dissertation indicate that both the FSM and additional crash severity metrics do a reasonable job predicting occupant injury risk in oblique crashes. One of the additional metrics performs better than the two FSM metrics. Additionally, several occupant factors, such as belt status and age, play significant roles in occupant risk prediction. These findings have important implications for future revisions of MASH, which could benefit from considering additional metrics and occupant factors in the occupant risk assessment procedures. / Doctor of Philosophy / Every year, there are more than 35,000 fatalities due to crashes on United States roads. While there are many different types of crashes, there is a small collection of crash types that are responsible for the majority of these fatalities. One of the worst crash types is a road departure crash. Road departure crashes describe when a vehicle leaves the roadway and collides with an object off the roadway (such as a tree, pole, or ditch). Road departure crashes typically comprise 10% of crashes but are responsible for more than 30% of the annual crash fatalities. In 2020, road departure crashes were responsible for 50% of the 39,000 fatalities. One strategy that is currently used to reduce road departure fatalities is the use of roadside barriers. Common roadside barrier types include metal guardrails, concrete barriers, and cable guardrails, and are used to prevent vehicles that are departing the roadway from hitting an object that would be more dangerous than the barrier. To ensure barriers successfully protect the vehicle and vehicle occupants from heightened danger, they are crash tested in scenarios that are designed to mimic real-world crashes. The Manual for Assessing Safety Hardware (MASH) is the document that currently outlines the details necessary to conduct one of these crash tests. During roadside barrier tests, it is crucial to determine whether occupants are at risk of injury or fatality. For a variety of reasons, barrier tests do not use the traditional crash test dummies, which are designed to replicate human presence in a crash vehicle. Instead, MASH recommends using vehicle velocity data to assess how much risk is posed to an occupant. Using this velocity data, two values can be computed and if either value exceeds the maximum values provided in MASH, the crash test fails due to high occupant risk. The suggestion to use velocity data to assess occupant risk was first introduced in 1981. Since then, there have been significant advances in vehicle design, barrier design, and occupants' willingness to partake in safe habits, such as wearing seatbelts. Therefore, it is necessary to determine if the occupant risk values used in MASH are still applicable today. The focus of this dissertation was to use real-world crash data to assess the current relevance of roadside barrier crash test occupant risk values. The results presented in this dissertation can be used to select new occupant risk values in future versions of MASH. The findings within this dissertation show that the current methods in MASH do a good job estimating an occupant's risk of injury. Additionally, the findings show that certain occupant factors, such as the age of an occupant and whether the occupant is belted, help to more accurately estimate occupant injury risk. This finding has important implications for MASH, which does not currently consider different occupant conditions.