Using five years of motor vehicle crash data from Shandong Province, China, vehicle types were divided into three categories—passenger cars, trucks, and motorcycles—to study the changes in factors influencing injury severity on workdays and non-workdays. Taking driver injury severity as the subject of study, three levels of crash injury severity were considered: no injury, minor injury, and severe injury. To capture potential unobserved heterogeneity, random parameters logit models with heterogeneity in the means and variances (RPL-HMV models) were employed for modeling. Three likelihood ratio tests were conducted to assess the transferability of model estimation results from workdays and non-workdays and from one vehicle type to other vehicle types. Subsequently, marginal effects were calculated to investigate the instability between the explanatory variables. The modeling results indicate that the estimated models are not transferable between different models on workdays and non-workdays and that there are differences in the factors affecting crash severity. Separate modeling is required between workdays and non-workdays, as well as between different vehicle types, because the influencing factors of each model are different. However, there were some variables such as male, $\mathrm{age}\ge 51\mathrm{years}$, visibility at 50–100 m, proceeding straight, multivehicle crashes, national/provincial roads, urban roads, rural roads, no streetlights at night, and streetlights at night that showed relatively stable effects on the probability of injury severity for certain vehicle types on workdays and non-workdays. This study could provide a theoretical basis for traffic management departments to manage traffic safety for different vehicle types for workdays and non-workdays, helping to reduce the occurrence of severe crashes.

In terms of driver characteristics, passenger car, as well as motorbike, drivers $\mathrm{aged}\ge 51\mathrm{years}$ were involved in crashes, increasing the severity of injuries. To avoid severe crashes, the media could encourage drivers in this age group to drive less on non-workdays and to choose public transport for traveling. Traffic management authorities should also conduct regular driving skills tests and medical examinations for passenger car and motorbike drivers in this age group and recommend revocation of driver’s licenses and provisions of driver training and education for those who fail the tests. However, truck drivers $\mathrm{aged}\ge 51\mathrm{years}$ reduced the number of severe injury crashes, so freight companies may consider hiring more experienced older drivers to improve driving safety in terms of vehicle and crash characteristics. Rear-end crashes in trucks and motorbikes can cause more severe injuries, and to prevent the occurrence of such crashes, a predictive emergency braking system can be installed. To implement distance monitoring between vehicles, when the distance is less than the safety distance, the system alarms and emergency braking is applied. When driving, motorbike drivers should pay attention to the distance and situation ahead, maintain a safe distance, and consciously wear a helmet. Passenger car crashes into fixtures can cause severe injuries, so the traffic control department can install energy-absorbing buffer devices on the surface of roadside fixtures to reduce the severity of the crash. In terms of road characteristics, a crash between the three types of vehicles while traveling on rural roads will result in more severe injuries to drivers, and the probability of injuries occurring on non-workdays is greater. The road management department should maintain rural roads better, increase the construction of security facilities on key hidden road sections, and install additional warning signboards. Traffic control departments should make unlicensed passenger cars and motorbikes the target of special rectification and set up more rural motorbike examination centers to reduce the number of unlicensed drivers. Government departments should go into villages to preach and increase traffic safety education for participants. On non-workdays, traffic police can be assigned to stand guard and patrol in areas prone to crashes. In terms of environmental characteristics, the absence and presence of streetlights at night increase the severity of crashes for motorcyclists, so it is recommended that traffic control authorities improve lighting facilities for areas without streetlights at night, check whether the brightness of streetlights in areas with streetlights at night is reasonable, and install active light-emitting signs at night. A visibility of less than 100 m can have a severe impact on passenger car and motorbike drivers, so when driving on roads with low visibility, drivers should slow down, keep a safe distance, and turn on lighting equipment such as headlamps and fog lamps to improve visibility. Truck drivers, whose severity of injuries is increased by crashes in the early hours of the morning and in the morning on non-workdays, should be ensured adequate rest and freight companies should be required to have two drivers for some long-distance shipments to switch driving shifts.