​The bus was transporting passengers despite having two tires with unacceptably low tread depths, which can reduce a vehicle’s traction with the road. There was a moderate rain at the time, so the roadway was wet, which would further reduce the available traction. Moreover, the bus was traveling at an excessive speed for the wet roadway conditions, given the suboptimal condition of the bus’s tires, which would have made it more difficult to recover from a loss of control event.

​As the bus neared the end of the bridge, the driver experienced a loss of traction, to which he responded by braking and steering. These inappropriate driver responses exacerbated the situation to a complete loss of control, so that the bus left the roadway to the right and rolled over. During the rollover, the integrity of the bus’s roof was compromised, so that the roof partially collapsed, and many windows were broken out, putting occupants’ safety at risk. Additionally, most of the 20 passengers were not wearing the available lap/shoulder belts, which further increased passengers’ risk of ejection and displacement.​

​We determined that the probable cause of the Pala Mesa, California, bus crash was the loss of vehicle control due to the combination of the low and substandard tread depth of the rear axle tires, the excessive speed for the wet roadway and vehicle conditions, and the driver’s inappropriate inputs before and during the loss of control event. Contributing to the crash was Executive Lines Inc.’s inadequate vehicle inspection process, which permitted the bus to operate in passenger service despite having two tires with treads below the minimum required depth. Contributing to the severity of the injuries were the National Highway Traffic Safety Administration’s failure to require roof strength standards for buses, Executive Lines Inc.’s failure to follow California’s requirement to inform passengers about the state’s mandatory seat belt use law, and the passengers’ limited use of the available lap/shoulder belts.

​​​​​As a result of this investigation, we recommended that the National Highway Traffic Safety Administration (NHTSA) sponsor research to determine the appropriate minimum tire tread depths necessary for safe operation of commercial vehicles, particularly buses, taking into consideration the effect on vehicle handling when different tread depths are used for tires on the steer (front) versus the rear axles. We issued additional recommendations to NHTSA and the Federal Motor Carrier Safety Administration to revise the relevant sections of the Code of Federal Regulations based on the outcome of the research.

We also recommended that the state of California revise state law to require that passenger-carrying motor carriers document the tread depths of tires on their vehicles during any required, periodic vehicle inspection and that the California Highway Patrol (CHP) revise its motor carrier self-inspection form to include the measured tire tread depths of all tires on inspected buses. We recommended that the CHP develop and implement an education program to increase passenger-carrying motor carriers’ awareness of the state’s requirements about informing passengers of the seat belt use law. Further, we recommended that the CHP use its regular terminal inspections of passenger-carrying motor carriers to verify that carriers are meeting the state’s requirements about pretrip briefings and/or signage in buses concerning the state’s mandatory seat belt use law. 

​​To ensure industry awareness of this crash investigation and its findings, we recommended that the American Bus Association and the United Motorcoach Association inform their members about the circumstances of this crash and encourage them to create a policy on speed and safe driving on wet roadways and under inclement weather conditions based on authoritative guidance. We also recommended that these two associations encourage their members to adopt the recommended tire rotation practices established by the U.S. Tire Manufacturers Association, and inform their members who operate in California of the requirements about pretrip briefings and/or signage in buses concerning California’ mandatory seat belt use law.

Finally, we reiterated NTSB Safety Recommendation H-21-2 to NHTSA, asking the agency to require all newly manufactured buses, other than school buses, with gross vehicle weight ratings above 10,000 pounds to meet a roof strength standard that provides maximum survival space for all seating positions and accounts for typical window dimensions.​