Highway Accident Report

8 Motorcoach Median Crossover and Collision With Sport Utility Vehicle Hewitt, Texas February 14, 2003

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Highway Accident Report

NTSB/HAR-05/02

PB2005-916202

Notation 7727A

National Transportation Safety Board Washington, D.C.

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Highway Accident Report

Contents

Acronyms and Abbreviations $\ldots$ v

Executive Summary vi

Factual Information .................................................................... 1

Accident Narrative 1

Injuries 4

Survival Aspects and Emergency Response 5

Driver Information .................................................................................................................................... 9

Vehicle and Wreckage Information 10

Motorcoach 10

Chevrolet Suburban .................................................................................................................................. 13

Highway Information $\ldots$ 15

Meteorological Information 19

Operational Information $\ldots$ 19

Central Texas Trails .................................................................................................. 19

Federal Motor Carrier Safety Administration 21

Tests and Research 23

I-35 Pavement Friction 23

Tire Friction Testing 24

Sight Distance .....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Simulation 27

Other Pavement Information .................................................................................................................................................................. 28

Recommendation History $\ldots$ 30

Analysis 31

Exclusions 31

Accident Discussion 33

Sight Distance 33

Roadway/Tire Interaction 38

Macrotexture Pavement Depth .................................................................................................................................. 38

Rut Depths 38

Roadway Friction ...................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Tire Friction ......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Commercial Vehicle Tires—Special Considerations 45

Texas Wet Weather Accident Reduction Program .......................................................................................................................................................................................................................................................................................................................................................................................................................................................................

FMCSA Exemptions 48

Survival Aspects 49

Motorcoach 49

Suburban .................................................................................................................................................................... 50

Conclusions 51

Findings 51

Probable Cause 53

Factual Information

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Highway Accident Report

〔图〕2002 SUV1996 MotorcoachImpacareaMotorcoachtire furrowsMotorcoachtire marksUnidentifiedtire marks●3060120180FEET\n

Figure 1. Accident scene. (Source: Texas Department of Public Safety)

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Figure 2. Motorcoach postaccident position.

〔图〕35DallasFort WorthWacoHewilTemple40mies。40 km\n

Figure 3. Accident location.

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About 9:30 a.m., an accident had occurred on northbound I-35 at mile marker 328, causing traffic to back up on I-35. The backup extended to mile marker 326.3, where the 9:59 a.m. accident subsequently occurred.

The bus driver stated that he observed brake lights appearing in the traffic ahead. He believed at first that the brake lights indicated traffic was slowing, but as he got closer, he saw that the traffic had slowed almost to a complete stop or had stopped. He stated that he was in the right lane, where the queue was longer, so he decided to move into the left lane to avoid having to stop more quickly. He stated that he initially braked normally, slowed, and steered to the left, when another vehicle ahead also moved to the left, requiring the bus driver to brake harder. The rear of the bus then began to skid to the right. He said he reduced brake pressure and steered to the right to attempt to straighten the bus and regain control, but the bus continued to travel to the left into the median. (See figure 1.)

A witness traveling through the area just before the accident described the rain as being heavy, with visibility reduced to about 500 feet. This witness also stated that he saw brake lights as he crested the hill and that the queue of traffic was longer in the right lane, so he moved to the left lane to take advantage of the increased distance to stop. The witness also stated that he observed two trucks approach his vehicle from the rear. One was able to move to the right lane and come to a stop; the other ran off the road into the median, becoming mired in the mud.

Another witness described the rain as being “really hard.” He stated he could not see the stopped traffic until he reached the top of the hill on I-35. A witness in a business adjacent to I-35 showed investigators the location of the end of the traffic queue, which was about 350 feet north of the beginning of the motorcoach’s skid marks.

Injuries

Table 1. Injuries.*

*Title 49 Code of Federal Regulations (CFR) 830.2 defines a fatal injury as any injury that results in death within 30 days of the accident. It defines a serious injury as an injury that requires hospitalization for more than 48 hours, commencing within 7 days from the date the injury was received; results in a fracture of any bone (except simple fractures of the fingers, toes, or nose); causes severe hemorrhages, nerve, muscle, or tendon damage; involves any internal organ; or involves second or third degree burns, or any burns affecting more than 5 percent of the body surface.

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Survival Aspects and Emergency Response

The driver of the motorcoach was ejected during the rollover and sustained serious injuries. He was not wearing the available lap/shoulder belt. Fourteen passengers in the motorcoach were either fully or partially ejected; one was ejected out of the front window, and the rest were ejected or fell through the broken windows on the right side of the motorcoach as it rolled over. Four of the ejected passengers sustained fatal injuries, and four sustained serious injuries. (See figure 4.) None of the passenger seats had restraints.

〔图〕(EJECTED)M-58:SF-66:MF-43:MM-66:S(EJECTED)F-46:MF-65:S2F-64:S(EJECTED)F-83:S3F-67:SF-78:F(EJECTED4F-70:M(EJECTED)F-71:F5

Figure 4. Seating diagram.

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The right side of the motorcoach came to rest over a drainage ditch that was filled with 12 to 14 inches of rainwater (see figure 5), in which several of the ejected passengers were submerged. Witnesses said these passengers were assisted in keeping their heads above water until they could be extricated; in one instance, a rescuer gave a passenger a piece of tubing to use as a snorkel to breathe. Because autopsies were not performed,$^{1}$ it could not be determined whether the fatally injured passengers who were ejected died from their injuries or from drowning; the coroner found that all sustained blunt force trauma.

〔图〕chloristky's Dell\n

Figure 5. Drainage ditch filled with water beneath motorcoach.

Damage to the motorcoach's interior was minimal. The headrest on the seventh row, right aisle seat, was bent forward. The interior padding on many of the headrests on the aisle had worn away, such that the headrests consisted of only a metal pipe frame covered by fabric. (See figure 6.) The headrests on the window seats had 2.5 inches of foam rubber padding covering the metal pipe under the fabric. The manufacturer's service manual did not provide information regarding headrest padding.

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〔图〕10456T11\n

Figure 6. Interior of headrest without padding.

Several passengers stated that they exited the bus through the roof hatch or slid out the windows and exited from under the motorcoach. Emergency personnel helped passengers exit the bus through the windshield, which shattered during the accident sequence.

The Suburban's dashboard was pushed rearward to within several inches of the front seats. (See figure 7.) The space between the passenger seating area in the rear and the front seat remained intact. (See figure 8.) The front airbags deployed during the accident. Emergency responders cut the lap/shoulder belts from the driver and two passengers during their extrication.

The Robinson Fire Department received the first 911 call at 9:59 a.m., followed by several calls to the Hewitt Fire Department and a motorist's notification to the Texas Department of Public Safety (Texas DPS) troopers investigating the other accident 2 miles north. The Robinson Fire Department arrived at 10:11 a.m., followed by the Hewitt Fire Department at 10:12 a.m., followed by Texas DPS troopers, Lorena Volunteer Fire Department and Woodway Volunteer Fire Department firefighters, Hewitt Police Department and Waco Police Department police officers, and McLennan County Sheriff's Department deputies. Police and fire department responders stated that their initial focus was on evacuating the bus, identifying and removing the injured from the interior, and partially raising the motorcoach with wooden blocks to stabilize it. Ambulances began to arrive about 10:17 a.m. Twelve ambulances transported 31 injured; 2 additional injured were airlifted by helicopter. The last transport occurred at 10:43 a.m.

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Figure 7. Suburban front seating area damage.

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Figure 8. Suburban rear passenger seating area.

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Driver Information

The 58-year-old bus driver held a valid Texas Class A Commercial Driver License with an expiration date of April 2, 2005. The driver also held a valid medical certificate issued January 14, 2003, with an expiration date of January 14, 2004. The driver had 23 years of experience driving commercial vehicles with no convictions or prior accidents. He had been working part-time or full-time as a driver for Central Texas Trails since 1980.

The bus driver's medical examination certificate was valid for 1 year, rather than the standard 2 years, because of the driver's chronic hypertension, according to the "Medical Examination Report for Commercial Driver Fitness Determination," which was completed by a physician. The driver was being treated and prescribed medications for his hypertension and osteoarthritis.$^{2}$ The driver told investigators that he used alcoholic beverages occasionally, was a nonsmoker, and did not use illicit drugs or abuse prescription medication. The driver's blood and urine were tested by the Texas DPS and the Civil Aeromedical Institute and found to be negative for alcohol and illicit or performance-impairing drugs. The driver reported that he was in good general health at the time of the accident. The driver was 5 feet 11 inches tall and weighed between 390 and 400 pounds. He reported that he snored; when the symptoms of sleep apnea were described to him, he denied experiencing any, other than snoring. He had never been screened for sleeping disorders.

The bus driver's 72-hour history, obtained during an interview with the driver, shows that he received 7.75 to 11 hours of sleep each of the 3 nights before the accident. He was off duty the 3 days before the accident. (See table 2.)

Table 2. Driver 72-hour history.

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Postaccident testing of the ABS electronic control unit (ECU), which controls the operation of the ABS by monitoring system status, revealed 25 fault codes.$^{9}$ Thirteen entries $^{10}$ were recorded relating to wiring system shorts in the vehicle's wiring system and were not internal to the ABS ECU. Whenever the ABS ECU records five identical faults, no matter what the cause, the ABS is designed to shut down. At the time of the accident, 5 of the 13 entries related to the wiring system shorts were identical, rendering the ABS nonfunctional.$^{11}$

Tires. The manufacturer-recommended tire pressures were 115 pounds per square inch (psi) for the steer axle and 95 psi for the drive and tag axles. Table 3 shows the tire inflation pressures and tread depths measured during the postaccident investigation (see figure 9).

Table 3. Motorcoach tire inflation pressures and tread depths.

The drive axle tires had been replaced in August 2001, when the hubometer read 144,175, indicating that they had been in use for 72,753 miles at the time of the accident. The steer axle tires had been replaced in January 2003 and had been in use for about 9,412 miles. The tag axle tires, which were replaced in August 2002, had been in use for 34,771 miles.

Federal regulations require that the tread depth for the front tires of motorcoaches be at least 4/32 inch and the tread depth for remaining tires be at least 2/32 inch.$^{12}$ This regulation has existed since 1969, when the FMCSRs were established. Commercial Vehicle

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Highway Accident Report

Safety Alliance (CVSA)$^{13}$ out-of-service criteria (sections 10(a)(1) and 10(b)(7)) state that a steer axle tire is out of service when the tread is worn to less than 2/32 inch in two adjacent tread grooves and that all other tires are out of service when the tread is worn to less than 1/32 inch in two adjacent tread grooves. The CVSA sets out-of-service criteria based upon the existence of imminent hazards, that is, a significant chance of a crash occurring. CVSA out-of-service criteria may differ from FMCSR criteria for the same items.$^{14}$

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Figure 9. Left drive axle tire treads.

Damage. During the accident sequence, the motorcoach was involved in multiple collisions. The first collision occurred when the vehicle crossed over the center median of I-35 and collided with the Chevrolet Suburban. Secondary impacts along the right side of the motorcoach occurred as both vehicles rotated while separating from each other and as the motorcoach rolled onto its right side and came in contact with the pavement. The most significant damage was to the right front of the motorcoach. The lower portion of the

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Highway Accident Report

passenger loading door was displaced to the rear about 30 inches. (See figure 10.) The impact also damaged the body panels and the frame support structures between the loading door and the front wheel opening, pushing the front wheel opening rearward 6 inches. From the front of the bus, the maximum deformation to the windshield frame was 33 inches rearward at the right-front corner.

〔图〕层第联\n

Figure 10. Motorcoach damage.

Chevrolet Suburban

The 2002 Chevrolet Suburban was examined by Safety Board investigators for mechanical factors that may have contributed to the Suburban driver's inability to avoid the accident. The inflation pressures for tires not damaged in the collision were near the manufacturer's recommended pressures.$^{15}$ The steering, suspension, and brake systems were examined, and, while damaged, no anomalies were found. The vehicle was equipped with a four-wheel ABS.

The Suburban also had a sensing diagnostic module (SDM), with the primary function of sensing changes in vehicle speed and determining when to deploy the vehicle's airbags. The SDM stores data for airbag deployment and near-deployment events. In this accident, the driver and passenger frontal airbags were deployed. The data retrieved from the SDM revealed that the driver's seatbelt was buckled. The SDM further indicated that 5

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$seconds^{16}$ before the airbags' deployment, the Suburban was traveling 54 mph; about 2 to 3 seconds before the crash, the driver released the throttle and applied the brake; and less than 1 second before the crash, the Suburban's speed decreased to 40 mph. The SDM indicated that the Suburban experienced a longitudinal velocity change of about 48.9 mph over a period of 110 milliseconds.

The tires recommended for the Suburban were P265/70R16, which have a radius (distance from the center of wheel rotation to the ground) of 15.3 inches. The tires found on the Suburban were P285/50R20, which have a radius of 15.6 inches.

Damage to the Chevrolet Suburban encompassed the entire vehicle. The two primary areas of contact damage were to the left side of the front end and to the rear section of the roof panel. (See figure 11.) The maximum penetration to the front end was 68 inches. The left side of both axles had shifted rearward, with the greatest displacement to the front axle, which moved about 8 inches; the rear axle moved about 2 inches rearward. Near the end of the accident sequence, the right side of the motorcoach made contact with the roof panel of the Suburban. The vehicles came to rest with the motorcoach's roof panel against the right side of the Suburban. As a result of the contact, the left-rear corner of the Suburban's roof panel was crushed downward about 19 inches and was displaced to the left about 15 to 18 inches. It is unknown whether recovery or extrication efforts altered the measured damage from the original postcrash configuration.

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Figure 11. Suburban damage.

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Highway Information

The accident occurred on I-35 at mile marker 326.3 in McLennan County, Texas, approximately 5 miles south of the Waco city limit line. I-35 is a divided, two-way, four-lane urban principal arterial paved asphalt roadway bordered on either side by a two-way frontage road. A sloped Portland cement concrete wall separates the southbound lanes from the frontage road. The paved portion of both the north- and southbound roadways consists of one 10-foot-wide right (outside) shoulder, two 12-foot-wide travel lanes, and one 6-foot-wide left (inside) shoulder. The north- and southbound lanes are divided by a 28-foot-wide depressed earthen median, which did not contain a temporary or permanent barrier at the time of the accident. (See figure 12.) A crest vertical $curve^{17}$ consisting of a 1.965 percent foreslope (uphill) and a -3.826 percent downslope (downhill) is located south of the accident site. The crest of the curve occurs about 158 feet before the motorcoach's first skid mark.

〔图〕Motorcoachtire marksVehicle finalrest positions\n

Figure 12. Accident location. (Source: Texas DPS)

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I-35 was first constructed in 1917$^{18}$ and expanded to a four-lane, divided highway in 1955. The horizontal alignment and vertical profile of the roadway have not changed since 1955, when the design speed was 60 mph. The most recent rehabilitation was completed in September 1992 and consisted of asphalt concrete repavement in the right lane of the southbound lanes and rut filling in the right lane of the northbound lanes. All north- and southbound lanes received microsurfacing.$^{19}$

Pavement texture depth ultimately determines most tire/road interactions, including wet friction, noise, splash and spray, rolling resistance, and tire wear. The pavement texture depth consists of microtexture, the component for surface friction, and macrotexture, the component for removing water to obtain better surface friction. For the area encompassing 0.5 mile north and south of the accident location, the average estimated macrotexture pavement depths ranged from 0.024 to 0.029 inch in the left lane and from 0.017 to 0.018 inch in the right lane. Research conducted in Great Britain concluded that the effect of texture depth on friction loss was greatest below depths of 0.028 inch.$^{20}$ The Texas Department of

Transportation (TxDOT) conducted research in 1970 that suggests a 0.035-inch macrotexture pavement depth be used for design purposes.$^{21}$ Other research recommended minimum pavement texture depths of 0.040 inch $^{22}$ or 0.015 to 0.031 inch.$^{23}$

The cross slope of the right lane ranged from approximately 1.70 percent to 1.85 percent at the accident site. The rut depth in the right lane was measured near the accident site and found to be between 0.40 to 0.42 inch in the left wheel path and 0.25 to 0.35 inch in the right wheel path.

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The speed limit on I-35 was 70 mph during the day and 65 mph at night. TxDOT conducted a speed study on February 17, 2003, and found that the 85th percentile speed was 74 mph during the day. The average daily traffic count for I-35, provided by TxDOT, was 63,000 vehicles for north- and southbound traffic in 2001. The most recent vehicle classification data were from 1999: 57.2 percent of the vehicles were passenger cars, 26.6 percent were single-unit trucks, 15.9 percent were tractor-semitrailers or double trailer combination vehicles, and 0.30 percent were buses.

Accident data for the area encompassing 5 miles north and south of the accident scene, including the frontage roads, were obtained from TxDOT. (See table 4.) About 16 percent of the accidents occurred on wet pavement while being exposed to precipitation about 2.35 percent of the days.$^{24}$ Ten fatal accidents occurred from 1996 through 2002: 3 median crossover accidents, 5 rear-end accidents, and 2 sidewipe accidents. The National Highway Traffic Safety Administration's (NHTSA's) General Estimate System and Fatality Analysis Reporting System indicate that approximately 11 percent of all accidents on the interstate highway system occur in wet weather. For the southern region of the United States, 12 percent of all accidents on the interstate highway system occur in wet weather.

Table 4. Accident data on I-35 in the vicinity of the accident.

The FHWA requires that all States have a skid accident reduction program. To identify sites that are overrepresented in wet weather accidents, TxDOT established the Wet Weather Accident Reduction Program (WWARP) in 1999. In Texas, a site is considered to be overrepresented if it had five or more wet weather accidents in a 1/10-mile roadway segment in 1 year. Overrepresented segments are upgraded to improve the skid resistance of the roadway surface. The 2000 WWARP report did not indicate that I-35 in the vicinity of this accident had five or more accidents in the 1/10 mile surrounding the accident site. It did report 467 locations that had five or more accidents within 1/10 mile; 410 of these locations were in the districts with the highest populations (Austin, Dallas, Fort Worth, Houston, and San Antonio). These districts comprised 63 percent of the Texas population and covered 15 percent of Texas's land area.

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At the time of the accident, I-35 was part of an experimental project by TxDOT to evaluate the privatization of highway maintenance and operation. TxDOT's 5-year contract with Virginia Maintenance Services, Inc., which began on September 1, 1999, covered 876 lane miles from Johnson County, Texas, to Williamson County, Texas. As part of the contract, Virginia Maintenance Services was required to perform monthly condition assessments of the highway and conduct “all maintenance and repair required to

insure the highways are kept in their designed and constructed or updated condition."$^{25}$ The maintenance requirements further specified that no ruts be more than 0.5-inch deep.

TxDOT's Pavement Management Information System (PMIS) allows highway engineers to evaluate current pavement conditions, monitor trends over time, estimate pavement needs, analyze the impact of limited funding on current and future conditions, and fine-tune treatments.$^{26}$ PMIS scores roadways in five areas: distress, ride, condition, structural strength, and skid. For each category (except skid), the ratings range from A ("very good") to F ("very poor"). Skid ratings, which are numerical, are based on a standard deviation from the mean. Skid numbers between 36 and 99 are at the mean or above. Skid numbers between 13 and 20 are 1.0 to 1.5 standard deviations below the mean, and skid numbers between 1 and 12 are 1.5 standard deviations below the mean. Table 5 shows the PMIS ratings for I-35 in the accident area.

Table 5. PMIS ratings in the vicinity of the accident site.

Before the accident, TxDOT planned to conduct improvements on I-35 from August 2003 to January 2004 using Federal funds. The improvements, which did occur, consisted of the installation of a 42-inch-high permanent concrete median barrier 4 feet from the edge of the northbound left lane. $^{27}$ TxDOT resurfaced the lanes in both directions in the vicinity

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Table 6. Motor carrier safety rating factors.

The rating for the first five factors and the accident rate (number of recordable accidents per million miles traveled in the 12 months before the compliance review) are entered into a rating table, which is used to establish the motor carrier's overall safety rating (see table 7). Each of the six factors receives equal weight.

Table 7. Motor carrier safety rating table.

In 1991, the FHWA's Office of Motor Carriers (FMCSA's predecessor) issued a memorandum requiring inspectors to indicate whether violations or accidents occur during interstate or intrastate operations. The FHWA stated it should enforce interstate violations only, leaving the enforcement of intrastate violations to the States. However, according to the memorandum, both interstate and intrastate violations would be recorded and considered in determining a motor carrier's overall safety rating.

On April 19, 2002, the FMCSA issued a memorandum modifying the 1991 policy allowing intrastate data to affect a motor carrier's overall safety rating, stating that the FMCSA's "safety rating process shall only incorporate data over which it has jurisdiction; the FMCSA has jurisdiction over motor carriers that operate a commercial motor vehicle in interstate commerce." Further,

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Pavement friction tests were conducted at 50 mph for the treaded tire and 40 mph for the smooth tire for 1 mile on either side of the accident site in accordance with American Society for Testing and Materials (ASTM) Standard E274-97.$^{44}$ The resulting coefficients of friction can be found in table 8.

Table 8. I-35 Coefficients of friction.

Tire Friction Testing

In addition to determining the friction of motorcoach tires on a wet roadway under conditions similar to those at the time of the accident, investigators also determined the difference in friction values for the newer tires on the front axle and the worn tires on the drive axle. Testing was conducted at General Dynamics Company Tire Research Facility $^{45}$ under the direction of Safety Board investigators. The tires tested were the left- and right-front tires and the four drive axle tires from the accident motorcoach. Testing was conducted at three water depths: 0.02 inch, the depth of water at which the pavement friction testing is conducted; 0.11 inch, the estimated depth of the water as it flowed along the roadway at the accident site; and 0.19 inch, based on the heavy rainfall, which may have caused more water to have accumulated in the ruts. The pavement frictions used to calibrate the General Dynamics equipment were the pavement friction testing wet pavement friction numbers for a smooth tire in the right wheel path of the right lane (0.16), obtained in on-scene testing. Table 9 shows the resulting longitudinal and lateral coefficients of friction at which the tires began to slide. See appendix C for complete tire friction test results.

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Table 9. Tire friction test results at 60 mph.

Sight Distance

According to AASHTO,

Sight distance is the length of roadway ahead that is visible to the driver. The available sight distance on a roadway should be sufficiently long to enable a vehicle traveling at or near the design speed to stop before reaching a stationary object in its path.”$^{46}$

Using the formula recommended by AASHTO to calculate available sight distance, the calculated available sight distance on I-35 on the approach to the accident site was 732 feet, based on a 7.5-foot driver eye height,$^{47}$ a 2.0-foot object height,$^{48}$ and the measured slopes (foreslope and downslope) of the crest vertical curve.

A witness who was in a business next to the interstate indicated to investigators the location of the end of the queue, which was then measured to be about 350 feet beyond the beginning of the skid marks left by the motorcoach. Postaccident measurements taken by

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Safety Board investigators indicate that, from a distance of 767 feet, a motorcoach driver could see a 4-foot-high vehicle stopped in the right lane of the interstate at the approximate end of the traffic queue. (See figure 13.) This sight distance, as measured, did not take into account limitations in visibility due to the hard rain falling at the time of the accident.

〔图〕Vehicle located atapproximate endof traffic queue\n

Figure 13. Sight distance to approximate end of traffic queue available to motorcoach driver in right lane.

The stopping sight distance, according to AASHTO, is the sum of the distance traversed from the instant the driver detects an object to the instant the brakes are applied and the distance required to stop once braking begins. The formula for calculating stopping sight distance is based on passenger car operation and does not explicitly consider truck (or motorcoach) operation. AASHTO's guidelines suggest a 3.5-foot driver eye height, a 2.0-foot object height, a 2.5-second brake reaction time, and a 0.34 coefficient of friction, resulting in a stopping sight distance of 570 feet at 60 mph (the design speed of I-35) and 730 feet at 70 mph (the daytime speed limit on I-35).

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The guidelines currently used by AASHTO for skid-resistant pavement design date from 1976. These guidelines have not been updated to reflect changes in vehicle or tire characteristics, data collection, traffic flow, construction techniques, materials, and other factors that affect pavement surface characteristics. To update the guidelines, the National Cooperative Highway Research Program (NCHRP)$^{56}$ is developing a “Guide for Pavement Friction” (project 1-43) that identifies technologies, processes, and practices suited for designing and constructing pavements with acceptable frictional characteristics. The project, which will be considered for adoption by AASHTO, is expected to be completed in October 2005.

The FHWA initiated the Truck Pavement Interaction research program in the 1980s to develop advanced methods and technologies that would allow a better understanding of the interactions between heavy vehicles, climate, and pavements.$^{57}$ The research primarily focuses on pavement performance and damage mitigation, not on the interaction between truck tires and pavement.

States. Each State has its own method of determining whether a roadway's friction is a factor in wet weather accidents or has fallen to such a level that the roadway needs to be repaved. For instance, the Illinois Department of Transportation has developed categorical rating guidelines that provide tentative guidance regarding the accident potential of a range of friction values (see table 10).

Table 10. Illinois Department of Transportation categorical rating guidelines.$^{58}$

®

Analysis

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which time the drive wheels likely locked (the tires would have started sliding) and began to skid. (See figure 14.) Given the speed of the motorcoach, the distance it would have traveled during perception and braking, and the sight distance available, the driver had about 2.0 to 2.3 seconds to detect the stopped traffic, decide what to do, and begin to brake. AASHTO utilizes a 2.5-second perception reaction time for all stopping sight distance calculations in its A Policy on Geometric Design of Highways and Streets, stating that "for approximately 90 percent of the drivers (in the Johansson and Rumar study)$^{62}$ a reaction time of 2.5 seconds was found to be adequate."$^{63}$ Thus, the driver reacted appropriately (his reaction was not delayed), especially given that the driver did not expect to stop on an interstate. Even had the rain not reduced the driver's visibility, he still would not have had enough time to react to the traffic ahead and safely stop the motorcoach. The Safety Board concludes that, based on the sight distance available to the driver and the time at which the driver began to apply the brakes and steer to avoid traffic ahead, his reaction was not delayed or impaired.

〔图〕15 Ib.Brake application35 Ib. Brake applicationBegin brake application180 degree left steerBegin harder brakingTire mark

Figure 14. Time and distance available to bus driver.

The ABS installed on the motorcoach was connected to the tag axle tires primarily to prevent flat spotting (worn spots) on the tag axle tires. The ABS was not initially, or at the time of the accident, connected to the drive axle. Had the tag axle's ABS been operational, it would have had minimal effect on the dynamics of this accident because the majority of the motorcoach's braking capacity occurred at the drive and steer axles due to the size of those brakes and the weight distribution of the motorcoach.

Analysis

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feet.$^{66}$ The AASHTO formula is based on an object height of 2 feet, whereas on-scene testing used a minimum object height of 4 feet to represent a passenger car as the object being sighted by the bus driver, thus accounting for a longer available sight distance. Figure 15 shows the longest available sight distance calculated by the AASHTO formula in the right lane for the motorcoach and a passenger car, taking into account the vertical curve preceding the accident site. Investigators calculated the distance required to the stop motorcoach, using the coefficients of friction from the pavement friction tests (which

〔图〕Available sight distanceBeginning ofEnd of732 feecrest verticalcrest verticacurvecurve7.5-footdriver eye2.0-fooheightobj

Figure 15A. Available sight distance for a motorcoach.

〔图〕Available sight distanceEnd ofBeginning of580 feetcrest verticalcrest verticacurvecurve2.0-foo3.5-footobjecdriver eyehei

Figure 15B. Available sight distance for a passenger car.

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were on wet pavement) in the right lane. (See table 11.) Required stopping distances for the motorcoach ranged from 399 to 1,293 feet and for passenger cars from 335 to 962 feet, depending on speed and tire tread.

Table 11. Required stopping distance in the right lane.

As can be seen in table 11, the required motorcoach stopping distance in the right lane exceeds the available sight distance for smooth tires at speeds of 60 mph or greater. For a treaded tire, the required stopping distance is only exceeded at 70 mph or greater. The alignment of the roadway at the accident site, resulting in a vertical curve, combined with the low coefficients of friction on I-35 when the pavement is wet, created a situation in which vehicles may not have been able to avoid hitting traffic stopped on the roadway. I-35 was designed in 1955 with a 60-mph design speed. Since that time, the speed limit has been raised to 70 mph, yet the roadway geometry has remained the same, creating a potentially dangerous situation. The 85th percentile speed for traffic flow on I-35 in the vicinity of the accident was 74 mph, 14 mph over the original design speed. Thus, similar vehicles on the roadway may find it difficult to react in time to stop for traffic ahead, particularly in wet weather. The Safety Board concludes that the wet pavement at the accident site, combined with I-35's roadway geometry and speed limit exceeding the design speed, created a situation in which drivers may not have had enough time to react and come to an emergency stop on the interstate or to avoid a collision.

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Yet nothing was done, and the recommendation was classified “Closed—Unacceptable Action” on August 21, 1986 (see appendix B). In 1988, the Safety Board issued the following recommendation to the FHWA as a result of the May 4, 1987, accident in Beaumont, Texas.$^{97}$

H-88-1

Revise Sections 393.75(B) and (C) of the Federal Motor Carrier Safety Regulations to prohibit the use of tires worn below 4/32 inch on any axle of a commercial interstate vehicle.

Again, no action was taken and the recommendation was classified “Closed—Unacceptable Action” on May 19, 1989 (see appendix B). Yet, this accident and recent research $^{98}$ show that reduced tread depth can lead to reduced friction and, ultimately, loss of control. Because no requirement exists that the tires have similar tread depths, the Safety Board believes that NHTSA should conduct testing on the effects of differing tread depths for the steer and drive axle tires. The Safety Board also believes that once NHTSA’s testing is complete, the FMCSA should modify the tread depth requirements for each axle to reflect the results of the research. In addition, because the adverse effects of mounting worn tires on the rear axle of vehicles are not widely known,$^{99}$ it is important that both commercial vehicle owners and consumers are aware of this information. The Safety Board will inform the United Motorcoach Association, the American Bus Association, the American Trucking Associations, the Owner-Operator Independent Drivers Association, the American Automobile Association, and the National Safety Council of the adverse handling that can result when worn tires are placed on the rear axles of vehicles, particularly when the front tires are fairly new with good tread depths.

Commercial Vehicle Tires—Special Considerations

The coefficient of friction measured by friction testing consists of both the roadway friction (as measured by the pavement friction test) and the friction provided by the tires. As stated earlier, when a tire is unable to channel water from under the area in contact with the roadway surface, the friction available to that tire decreases. In this accident, the roadway had a low friction value, and the drive axle tires were at a minimum tread depth. Combined, these factors led to the drive axle tires having very little friction available to support braking or to follow steering maneuvers. In fact, the friction available to the drive axle tires was similar to that available on ice. Once these tires lost traction, the driver began to steer and increase the amount of braking but was unable to regain control of the motorcoach because the remaining tires also likely began to lose traction. The Safety Board concludes that the minimal friction available to the tires and on the roadway combined to cause the motorcoach to lose control when the driver attempted to maneuver to avoid stopped traffic.

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Highway Accident Report

Appendix C

Tire Friction Testing Results

Appendix C

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Highway Accident Report