This study investigates the relationship between occupant injury and a host of other factors, including traffic and weather conditions present at the time of crash, road design, vehicle type, and occupant characteristics. Crash and traffic-detector data from six Orange County freeways were used in an ordered probit model. Crash outcomes were classified for each occupant, as no injury, non-visible injury, visible injury, severe injury, and fatal injury. Higher design speeds (holding speed limits fixed) and speeding contribute to injury severity, while lighting and pavement surface conditions appear to play no role. Consistent with the literature, sideswipe and rear-end collisions result in less severe injury, relative to other collision types (such as broadside, hit-object, and rollover), while females and older persons are at higher risk of injury. Information on current traffic conditions proved very valuable for injury severity prediction. And a variety of design and policy recommendations can be drawn, to enhance highway design and roadway safety.