The Taiwan Transportation Safety Board (TTSB) released the final report on the investigation of A Yu Feng Tourist Bus Rear-Ended Another Tourist Bus in the Suao Tunnel of Provincial Highway No. 9.

A commercial tourist bus owned by Yu Feng Transportation Co., Ltd. was carrying passengers on a tour. The bus carried 1 driver, 1 tour leader, and 32 passengers, for a total of 34 people. At 1515 on October 4, 2023, at 104K+200 northbound of Provincial Highway No. 9, the occurrence vehicle rear-ended another commercial tourist bus (carrying one driver, one tour leader, and 31 passengers, for a total of 33 people), which was slowing down for a red light. A total of 19 people in the 2 vehicles were injured in this occurrence.

In accordance with the TTSB is the independent transportation occurrence investigation agency responsible for conducting the investigation. The investigation team also included members from the Highway Bureau, MOTC, Yu Feng Transportation Co., Ltd., and Griffin Automotive Limited, Taiwan Branch (B.V.I.).

The draft for this investigation report was completed in December 2024. It was revised after preliminary review at the 71^st Board Meeting of the TTSB on January 10^th, 2025, according to procedures, and then sent to relevant agencies (institutions) for their opinions. The investigation report was published after review and approval by the 72^nd Board Meeting on February 14^th, 2025.

After comprehensive investigation and analysis of the factual data, a total of 8 findings and 3 safety recommendations were obtained.

I.     Investigation Findings

Findings Related to Probable Causes

1. The driver was speeding in the Su’ao Tunnel. When he noticed the road conditions ahead and wanted to slow down the vehicle, he first used the retarder and then the main brake. However, because the main brake was activated too late, the bus could not stop safely and hit the tourist bus in front.

Findings Related to Risks

1. The driver usually brings the retarder lever to position 2, but during this occurrence, he brought the retarder lever to position 5 (the highest position) all at once, showing that the driver would need to create a braking effect that is around 80% to 90% as strong as the main brake. Such a situation would be categorized as an emergency that necessitated braking and stopping the vehicle immediately. However, the driver did not use the main brake for emergency braking, which is inconsistent with correct braking operations.

Other Findings

1. The driver held a valid driver’s license issued by the Highway Bureau, MOTC; there was no evidence that indicated the occurrence might have been related to alcohol or drugs. The vehicle had a valid vehicle license issued by the Highway Bureau, MOTC; there were no abnormalities in the tires and steering system; the maintenance, repair, and inspection records of the vehicle were normally documented; there was no insufficiency in the air pressure in the main braking system before the occurrence, and the results of the braking force test after the occurrence were normal.
2. The Driver’s Manual provided by the vehicle manufacturer did not mention the meanings when the retarder’s activation indicator light flashes green. The driver was unable to clarify the cause of the flashing indicator light even after receiving guidance from the maintenance technician, and the flashing light still continued to occur in the vehicle even after inspection. Therefore, the flashing light may have caused the driver to mistakenly believe that there was an abnormality in the system, making it impossible for the driver to determine whether the vehicle would perform normally.
3. According to the test results of the retarder of the occurrence vehicle and the other vehicle of the same type, the operating performance of the retarder of the occurrence vehicle was normal; there was no significant difference in the output performance on the air pressure and oil pressure of the occurrence vehicle compared to the other vehicle of the same type. After the retarder’s activation indicator light started flashing, there was no significant drop in air pressure and oil pressure delivery.
4. The activation indicator light of the retarder of the vehicle went out at 1515:20.8 and did not light up again until the time the vehicle struck the tourist bus in front at 1515:24.3. This time period is consistent with the period the vehicle significantly reduced its speed. The retarder may have stopped operating because the driver was using the main brake to decelerate.
5. The occurrence took place on a road section with a gentle downhill slope inside a tunnel. The vehicle was in a higher gear, and the driver was attempting to decelerate and, therefore, had released the accelerator pedal. Consequently, the vehicle's engine speed may have been lower than 1,800 revolutions per minute (rpm). As a result, both the output performance of the brakes and the heat dissipation would have been relatively insufficient. This caused the retarder to underperform due to the low engine speed; the insufficient heat dissipation would have caused the oil temperature to rise, which would have led to the indicator light flashing.
6. A red light lit up on the dashboard of the vehicle prior to the collision, which was likely the collision warning light of the advanced emergency braking (AEB) system. The driver had fully pressed down on the brake pedal before the collision, which would overwrite the AEB's functionality, so it would not activate; only the collision warning would light up. This is in line with the description in the test report. In addition, the on-board diagnostic computer was read after the occurrence, and it showed no record of AEB activation. This confirms that the AEB was not involved in the emergency braking prior to the collision.

II.  Safety Recommendations

To Yu Feng Transportation Co., Ltd.

1. Strengthen drivers’ knowledge related to the correct operation of the main brake and auxiliary braking systems. Ensure that drivers will prioritize braking with the main brake in emergency situations, understand the correct method and time to use auxiliary braking systems, and promote compliance with speed limits.

To Griffin Automotive Limited, Taiwan Branch (B.V.I.)

1. Use suitable methods to convey information about the functions and lights of the retarder to owners of vehicles equipped with retarder, in order to ensure that vehicle owners and drivers have the correct understanding of the functions and operating principles.

To the Highway Bureau, MOTC

1. Supervise Yu Feng Transportation Co., Ltd. in strengthening drivers’ knowledge related to the correct operation of the main brake and auxiliary braking systems, ensuring that drivers will prioritize braking with the main brake in emergency situations, and understand the correct method and time to use auxiliary braking systems.

Full Final Report in Chinese is available for download at https://www.ttsb.gov.tw/

Ya-Ting Chang, Secretariat Office

Tel: +886-2-7727-6210

E-mail: yating@ttsb.gov.tw

Jen-Sung Tseng, Chief Investigator

Highway Occurrence Investigation Division

Tel: +886-2-7727-6270

E-mail: jstseng@ttsb.gov.tw