The main causes of 18 wheeler accidents are driver error, mechanical failure, and weather conditions. Driver error is the most common cause of 18 wheeler accidents, accounting for about 56% of all accidents. The most common type of driver error is speeding, followed by distracted driving, fatigued driving, and driving under the influence of drugs or alcohol. Mechanical failure is the second most common cause of 18 wheeler accidents, accounting for about 30% of all accidents.