On February 14, 2003, about 9:59 a.m., central standard time, a 1996 Dina Viaggio motorcoach, operated by Central Texas Trails, Inc., and occupied by a driver and 34 passengers, was traveling northbound on Interstate 35 near Hewitt, Texas. The weather was overcast with reduced visibility due to fog, haze, and heavy rain. As the motorcoach approached the crest of a hill, the bus driver said he observed brake lights ahead of him and began to brake lightly. The bus driver said that as he moved from the right lane into the left lane, another vehicle ahead of the bus also moved over, so he braked harder and the rear of the bus skidded. The bus driver was unable to maintain control of the bus as it departed the left side of the roadway, crossed the grassy median, entered the southbound lanes, and collided with a 2002 Chevrolet Suburban sport utility vehicle (Suburban) occupied by a driver and two passengers. The right mirror of a southbound 1996 Chevrolet C1500 Z71 pickup truck, occupied by a driver, was also struck by the motorcoach. The motorcoach then overturned on its right side, rotated, and slid to final rest facing south against a concrete embankment on the side of the road. The Suburban rotated 180 degrees, began to climb the embankment, slid back down, and came to rest facing north and against the roof of the bus.
Five motorcoach passengers, the Suburban driver, and one Suburban passenger sustained fatal injuries. The bus driver sustained serious injuries; the remaining passengers on the bus and in the Suburban sustained injuries ranging from minor to serious. The pickup truck driver was not injured.
The National Transportation Safety Board determines that the probable cause of this accident was Texas's decision to set a speed limit on Interstate 35, in the vicinity of the accident, that did not take into account the roadway's limited sight distance or its poor conditions in wet weather; as a result, the bus driver was unable to detect the stopped vehicles as he approached the traffic queue and lost control of the motorcoach due to low pavement friction. Exacerbating the poor roadway conditions were the minimum tread depths on the motorcoach's drive axle tires and differing tread depths on its front and rear tires, both of which were allowed under the Federal Motor Carrier Safety Regulations but reduced the friction available to the motorcoach. Contributing to the severity of the accident were the lack of a temporary or permanent median barrier, which might have redirected the motorcoach or reduced the speed at which it crossed the median into the southbound lanes, and the lack of an occupant protection system for the motorcoach passengers.
Major safety issues identified in this accident include:
- Sight distance and speed as they relate to roadway design;
- Roadway and tire friction interaction, particularly between commercial vehicle tires and wet pavement;
- Effect on vehicle stability of differing front and rear tire tread depths; and
- Need to better identify areas with a high risk of wet weather accidents and implement the necessary roadway improvements.
As a result of this accident, the National Transportation Safety Board makes recommendations to the Federal Highway Administration, the National Highway Traffic Safety Administration, the Federal Motor Carrier Safety Administration, and the Texas Department of Transportation.
As a result of its investigation, the National Transportation Safety Board makes the following safety recommendations:
To the Federal Highway Administration:
Issue guidance to your field offices describing the inadequate stopping sight distance that could occur when poor vertical geometries exist at locations with low coefficients of friction and speeds higher than the design speed and work with the States to inventory such locations. (H-05-12)
Once the locations in Safety Recommendation H-05-12 have been identified, assist the States in developing and implementing a plan for repaving or other roadway improvements. (H-05-13)
Issue guidance recommending the use of variable speed limit signs in wet weather at locations where the operating speed exceeds the design speed and the stopping distance exceeds the available sight distance. (H-05-14)
Conduct research on commercial vehicle tire and wet pavement surface interaction to determine minimum frictional quality standards for commercial tires on wet pavement; once completed, 1) revise the tire requirements for commercial vehicles operating on wet pavement at highway speeds, and 2) develop minimum acceptable pavement coefficients of friction and maximum permissible pavement rut depths as part of roadway maintenance requirements, as appropriate. (H-05-15)
Review State programs that identify and eliminate locations with a high risk of wet weather accidents and develop and issue a best practices guide on wet weather accident reduction. (H-05-16)
To the National Highway Traffic Safety Administration:
Conduct testing on the effects of differing tread depths for the steer and drive axle tires. (H-05-17)
To the Federal Motor Carrier Safety Administration:
Once the testing in Safety Recommendation H-05-17 is complete, modify the tread depth requirements for each axle to reflect the results of the research. (H-05-18)
To the Texas Department of Transportation:
Inventory highway locations where poor vertical geometries, combined with low coefficients of friction and speeds greater than the design speed of the roadway, may create a situation in which traffic has inadequate stopping sight distance, and develop and implement a plan for repaving or other roadway improvements. (H-05-19)
Install variable speed limit signs or implement alternate countermeasures at locations where wet weather can produce stopping distances that exceed the available sight distance. (H-05-20)
Change the Pavement Management Information System to increase its emphasis on roadways with low coefficients of friction in determining maintenance priorities. (H-05-21)
Revise and validate your Wet Weather Accident Reduction Program so that improvement priorities are not disproportionately influenced by the number of accidents that occur but also consider locations where surface conditions and roadway geometry lead to very low friction coefficients and dangerous conditions. (H-05-22)