# Analysis of the Severity of Heavy Truck Traffic Accidents Under Different Road Conditions
## 5 Mechanism Analysis of Heavy Truck Traffic Accident Severity
### Mechanism Analysis of Highway Accidents  
Heavy truck accidents on highways are significantly correlated with six variables: visibility, roadside protection measures, intersection section types, vehicle driving status, inter-vehicle accident types, and road safety attributes. And age, driving experience, accident forms, road surface conditions, physical isolation conditions, alignment, weather, lighting conditions, and violations are not significant factors. The results are shown in Table 7.  
1. Vehicle aspects: The driving status of the vehicle and inter-vehicle accident types are significant factors affecting heavy truck traffic accident severity on highways. When the vehicle is driving straight, the probability of fatal accidents is 1.57 times that of non-straight driving. For accident types, side collisions have the highest probability of causing fatal accidents, being 1.294 times that of non-vehicle accidents. Non-vehicle accidents (i.e., vehicle-pedestrian accidents) have a greater probability of fatal accidents, being 1.241 and 2.571 times that of rear-end and other direction collisions, respectively. In China, many highways pass through villages and towns, leading to numerous minor roads connecting to these main roads. The majority of intersections between main roads and minor roads lack signal control, which increases the probability of serious traffic accidents caused by heavy trucks colliding with vehicles suddenly emerging from minor roads.
2. Road conditions: Roadside protection measures are significant factors affecting heavy truck traffic accident severity on highways. When there are no roadside protection measures, the probability of fatal accidents is 1.792 times that of protected roads. If a heavy truck loses control or veers off the road for other reasons, the lack of protective measures on the roadside will fail to absorb impact energy, resulting in a significantly increased force of impact during the collision, which exacerbates the severity of the traffic accident. Furthermore, many rural roads are located near water or cliffs, and the absence of roadside protection measures increases the risk of severe traffic accidents, such as trucks falling into water or off cliffs. For intersection section types, the probability of fatal accidents at road sections is 1.506 times that at intersections. For road safety attributes, the probability of fatal accidents on roads with hazards is 1.57 times that of roads without hazards.
3. Environmental aspects: Visibility is a significant factor affecting heavy truck traffic accident severity on highways. As visibility decreases, the probability of fatal accidents increases. When visibility is below 50 m, the probability of fatal accidents is 2.732, 3.534, and 5.181 times that of visibility at 50-100 m, 100-200 m, and over 200 m, respectively. As visibility decreases, drivers find it more challenging to observe the road traffic environment and have poorer ability to respond to sudden events, thereby increasing the likelihood of severe traffic accidents.  
The analysis of the mechanism of heavy truck traffic accident severity on highways reveals that straight driving status, side collision accident type, no roadside protection measures, driving on hazardous road sections, and visibility below 50 m are risk factors leading to fatal accidents on highways.