National 

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Safety Board

Washington, D.C.

National Transportation

Safety Board

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NTSB

Permit No. G-200

Highway Accident Report

Motorcoach Median 

Crossover and Collision 

With Sport Utility Vehicle 

Hewitt, Texas 

February 14, 2003

NTSB/HAR-05/02

PB2005-916202 National Transportation Safety Board

Notation 7727A 490 LíEnfant Plaza, S.W.

Adopted July 12, 2005 Washington, D.C. 20594
National Transportation Safety Board. 2005. Motorcoach Median Crossover and Collision With

Sport Utility Vehicle, Hewitt, Texas, February 14, 2003. Highway Accident Report NTSB/HAR-05/02.

Washington, DC.

Abstract: 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 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.

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; the effect on vehicle stability of differing front and rear tire tread depths; and the need to better

identify areas with a high risk of wet weather accidents and implement the necessary roadway

improvements. As a result of this investigation, 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. 

The National Transportation Safety Board is an independent Federal agency dedicated to promoting aviation, railroad, highway, marine,

pipeline, and hazardous materials safety. Established in 1967, the agency is mandated by Congress through the Independent Safety Board

Act of 1974 to investigate transportation accidents, determine the probable causes of the accidents, issue safety recommendations, study

transportation safety issues, and evaluate the safety effectiveness of government agencies involved in transportation. The Safety Board

makes public its actions and decisions through accident reports, safety studies, special investigation reports, safety recommendations, and

statistical reviews.

Recent publications are available in their entirety on the Web at <http://www.ntsb.gov>. Other information about available publications also

may be obtained from the Web site or by contacting:

National Transportation Safety Board

Public Inquiries Section, RE-51

490 LíEnfant Plaza, S.W.

Washington, D.C. 20594

(800) 877-6799 or (202) 314-6551

Safety Board publications may be purchased, by individual copy or by subscription, from the National Technical Information Service. To

purchase this publication, order report number PB2005-916202 from:

National Technical Information Service

5285 Port Royal Road

Springfield, Virginia 22161

(800) 553-6847 or (703) 605-6000

The Independent Safety Board Act, as codified at 49 U.S.C. Section 1154(b), precludes the admission into evidence or use of Board reports

related to an incident or accident in a civil action for damages resulting from a matter mentioned in the report. 
iii Highway Accident Report

Contents

Acronyms and Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Meteorological Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Operational Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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

Recommendation History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Exclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Accident Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Sight Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Roadway/Tire Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

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

Rut Depths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Roadway Friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Tire Friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Commercial Vehicle TiresóSpecial Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Texas Wet Weather Accident Reduction Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

FMCSA Exemptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Survival Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Motorcoach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

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

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Probable Cause . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Contents iv Highway Accident Report

Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Appendixes

A: Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

B: Recommendation History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

C: Tire Friction Testing Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
v Highway Accident Report

Acronyms and Abbreviations

AASHTO American Association of State Highway and Transportation 

Officials

ABS antilock braking system

ASTM American Society for Testing and Materials

CFR Code of Federal Regulations

CVSA Commercial Vehicle Safety Alliance

ECU electronic control unit

FHWA Federal Highway Administration

FMCSA Federal Motor Carrier Safety Administration

FMCSRs Federal Motor Carrier Safety Regulations

I-35 Interstate 35

MCMIS Motor Carrier Management Information System

MTMC Military Traffic Management Command

NCHRP National Cooperative Highway Research Program

NHTSA National Highway Traffic Safety Administration

PMIS Pavement Management Information System

psi pounds per square inch

SafeStat Motor Carrier Safety Status Measurement System

SAFETEA Safe, Accountable, Flexible, and Efficient Transportation Equity 

Act of 2003

SDM sensing diagnostic module

Suburban 2002 Chevrolet Suburban sport utility vehicle

Texas DPS Texas Department of Public Safety

TxDOT Texas Department of Transportation

WWARP Wet Weather Accident Reduction Program
vi Highway Accident Report

Executive Summary

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 Texasís decision to set a speed limit on Interstate 35, in the vicinity of

the accident, that did not take into account the roadwayís 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 motorcoachís 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:

ï Sight distance and speed as they relate to roadway design;

ï Roadway and tire friction interaction, particularly between commercial vehicle

tires and wet pavement;
vii Highway Accident Report

ï Effect on vehicle stability of differing front and rear tire tread depths; and

ï 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.

Executive Summary

1 Highway Accident Report

Factual Information

Accident Narrative

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 (I-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. (See figures 1 and

2.) 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 bus driver told investigators that he reported for duty at 7:15 a.m. on the day

of the accident and conducted a pretrip inspection of the accident bus, which he found to

be in good mechanical condition. He departed Central Texas Trails in Waco, Texas, about

7:55 a.m., arriving at the Memorial Baptist Church in Temple, Texas, about 8:50 a.m. (See

figure 3.) The driver loaded passengers onto the bus and departed for Dallas, Texas, about

9:31 a.m., where the church group was scheduled to attend a 1:00 p.m. concert. The driver

stated that he was driving about 65 mph (the posted speed limit was 70 mph during the

day) on I-35 when the intensity of the rain increased and he reduced his speed to about 60

mph. The driver and several passengers stated that visibility was about 1/2 mile due to the

rain and fog. The driver said that traffic was light and proceeding normally and that he

drove primarily in the right lane.
Factual Information 2 Highway Accident Report

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

2002 SUV 

 Motorcoach

 tire marks

Impact 

area 

 Motorcoach

 tire furrows

0 30 60 120 180

FEET

1996 Motorcoach 

 Unidentified

 tire marks

N
Factual Information 3 Highway Accident Report

Figure 2. Motorcoach postaccident position.

Figure 3. Accident location.
Factual Information 4 Highway Accident Report

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 ìreally hard.î 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 motorcoachís skid marks.

Injuries

Table 1. Injuries.*

Motorcoach 

passengers

Motorcoach 

driver

Suburban 

passengers

Suburban 

driver

Pickup 

truck driver Total

Fatal 5 0 1 1 0 7

Serious 8 1 1 0 0 10

Minor 21 0 0 0 0 21

None 0 0 0 0 1 1

Total 34 1 2 1 1 39

*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.
Factual Information 5 Highway Accident Report

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.

Figure 4. Seating diagram.

3

1

2

4

5

6

7

8

9

10

11

12

13

14

Lavatory

SAMPLE

F= FEMALE M= MALE #= AGE

INJURY LEVEL

AGE

GENDER

F - 14 : M

Source: NTSB

N

M

S

F

=

=

=

=

None

Minor

Serious

Fatal

*Passenger was readmitted to the 

hospital for treatment of injuries from a 

previous unrelated motor vehicle 

accident.

International Civil Aviation Organization

INJURY LEGEND

(EJECTED) M - 58 : S

F - 43 : M

F - 46 : M

(EJECTED) F - 83 : S

F - 67 : S

(EJECTED) F - 71 : F

M - 70 : M

(EJECTED) F - 81 : S

(EJECTED) F - 86 : M

(EJECTED) F - 67 : M*

M - 74 : M

(EJECTED) F - 58 : S

M - 54 : M

(EJECTED) F - 75 : M

(EJECTED) F - 41 : M

M - 59 : M

M - 64 : M

F - 61 : S

F - 73 : F

M - 82 : M

F - 66 : M

M - 66 : S (EJECTED)

F - 65 : S

F - 64 : S

F - 78 : F (EJECTED)

F - 70 : M

F - 71 : M (EJECTED)

F - 57 : M

M - 77 : M (EJECTED)

F - 54 : M

M - 42 : M

F - 56 : M

F - 73 : F (EJECTED)

M - 76 : F (EJECTED)

M - 73 : M
Factual Information 6 Highway Accident Report

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.

Damage to the motorcoachís 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 manufacturerís service

manual did not provide information regarding headrest padding.

1 According to the coroner, autopsies were not performed out of respect for the religious beliefs of the

families of the deceased.

Figure 5. Drainage ditch filled with water beneath motorcoach.
Factual Information 7 Highway Accident Report

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 Suburbanís 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 motoristís 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 Sheriffís

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.

Figure 6. Interior of headrest without padding.
Factual Information 8 Highway Accident Report

Figure 7. Suburban front seating area damage.

Figure 8. Suburban rear passenger seating area.
Factual Information 9 Highway Accident Report

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 driverís medical examination certificate was valid for 1 year, rather than

the standard 2 years, because of the driverís chronic hypertension, according to the

ìMedical Examination Report for Commercial Driver Fitness Determination,î 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 driverís blood and urine were tested by the Texas DPS and the Civil

Aeromedical Institute and found to be negative for alcohol and illicit or performanceimpairing 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 driverís 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.

2 The driver was prescribed 300 mg Tiazac® (generic name: Diltiazem) daily for hypertension (high

blood pressure) and 500 mg Nabumetone (generic) daily for osteoarthritis, a degenerative joint disease.

Date Time Activity Sleep 

February 11, 2003 12:00 a.m. ñ 2:00 a.m. Went to bed 

February 12, 2003 10:30 a.m. ñ 11:00 a.m. Awoke 8.5 ñ 11 hours 

11:00 p.m. Went to bed 

February 13, 2003 8:00 a.m. Awoke 9 hours 

10:00 p.m. Went to bed 

February 14, 2003 5:45 a.m. Awoke 7.75 hours 

 7:15 a.m. Reported for duty 

7:55 a.m. ñ 8:50 a.m. Driving 

8:50 a.m. ñ 9:31 a.m. Loading passengers 

9:31 a.m. ñ 9:59 a.m. Driving 

9:59 a.m. Accident 
Factual Information 10 Highway Accident Report

Vehicle and Wreckage Information

Motorcoach

The 1996 Dina Viaggio 1000 motorcoach was equipped with a Detroit Diesel

Corporation Series 60 engine and an Allison Transmission HT-755 5-speed automatic

transmission. The motorcoach had an odometer reading of 126,419 miles, which reflects

the number of miles traveled with that odometer, and a hubometer3

 reading of 216,928

miles, which reflects the mileage traveled by the vehicle.4

 The engine was equipped with a

model 765 Jacobs engine brake; the switch was in the off position during the postaccident

inspection, and the bus driver said that the engine brake was not on at the time of the

accident. The engine was also equipped with an electronic control module to control

engine timing and conduct diagnostics; its recording capability was not activated. The bus

had hydraulic power steering; inspection of this system revealed no defects. The

suspension was also examined for defects, and, except for a missing bump stop5 on the

left-front suspension, none were found.

An examination of the brake system revealed some mild heat checking6

 on the

inside surface of the drive axle brake drums. All brake pads were in good condition and

free of excessive wear and anomalies, except for the left drive axle brake drum, which had

axle grease contamination between the brake shoe and drum. All drums were round, and

their diameters were within manufacturerís specifications. All brakes were found to be

within adjustment limits. Testing determined that the pneumatic brake system delivered

air pressure to all brake chambers as designed.

The motorcoach was equipped with a Bendix MC-30 Antilock Braking System

(ABS) module for the drive (second) and tag (third) axles,7 which was nonfunctional at the

time of the accident. The steer axle was not equipped with an ABS module, and the

motorcoach was not manufactured with ABS sensors on the drive axle.8 The ABS module,

when functioning and fully connected, could monitor wheel speed on four wheels and had

two modulator valves; it had replaced an earlier model that could monitor wheel speed and

modulate braking on the tag axle only. During a braking event, weight is shifted off a

lightly loaded tag axle, preventing excessive tire wear.

3 The hubometer is generally used by maintenance personnel to provide reference mileage for service

of the axles, tires, and wheels.

4 Maintenance records did not indicate why the mileage difference exists; one possible explanation is

that the speedometer/odometer was replaced.

5 The bump stop acts as a cushion to prevent contact between the upper control arm and the spring

assembly in the event that the shock absorber is fully compressed.

6 Heat checking results in small cracks and possible bluing of the brake surface and is indicative of

friction, resulting in heat amassing during braking. Mild heat checking is a normal condition for heavy

vehicle brakes.

7 The front axle is the steer axle, the second axle is the drive axle, and the third axle is the tag axle. The

tag axle is an undriven third axle used to distribute the weight at the rear of a motorcoach.

8 ABS was not required for this motorcoach because it was manufactured before March 1, 1998, the

effective date for all motorcoaches to be equipped with ABS (49 CFR 393.55).
Factual Information 11 Highway Accident Report

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

entries10 were recorded relating to wiring system shorts in the vehicleís 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

9 A fault code identifies an anomaly in the ABS or in a system with which the ABS interfaces.

10 The other entries were: comments, low battery voltage, tests when the vehicle is powered up, and a

fault that occurred when the vehicle was on its right side after the accident.

11 The nonfunctioning ABS, which could not have been detected by the driver, could have been

detected during system maintenance.

Tire Inflation pressure (psi) Minimum tread depth (inches)

Actual Recommended Actual Required

Left steer axle 73 115 14/32 4/32

Right steer axle 94 115 15/32 4/32

Outside left drive axle 92 95 3/32 2/32

Inside left drive axle 87 95 2/32 2/32

Outside right drive axle 94 95 6/32 2/32

Inside right drive axle 90 95 5/32 2/32

Left tag axle 92 95 8/32 2/32

Right tag axle 92 95 5/32 2/32

12 Title 49 CFR 393.75.
Factual Information 12 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

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

13 The CVSA is a nonprofit organization dedicated to improving commercial vehicle safety that

comprises Federal, State, and Provincial government agencies and representatives from private industry in

the United States, Canada, and Mexico. The CVSA establishes and maintains commercial vehicle safety

operational standards and practices, inspection procedures, out-of-service criteria, and enforcement practices

and penalties that provide for uniformity, compatibility, and reciprocity among CVSA member jurisdictions

and industry partners.

Figure 9. Left drive axle tire treads.

14 Conversation with Stephen Keppler, Director of Policy and Programs, CVSA, March 1, 2005.
Factual Information 13 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.

Chevrolet Suburban

The 2002 Chevrolet Suburban was examined by Safety Board investigators for

mechanical factors that may have contributed to the Suburban driverís inability to avoid

the accident. The inflation pressures for tires not damaged in the collision were near the

manufacturerís 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 vehicleís

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 driverís seatbelt was buckled. The SDM further indicated that 5

Figure 10. Motorcoach damage.

15 The recommended pressure was 35 psi; the right-front tire was inflated to 34.5 psi and the right-rear

tire, to 31.5 psi. The other two tires were deflated due to accident damage.
Factual Information 14 Highway Accident Report

seconds16 before the airbagsí 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 Suburbanís 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

motorcoachís roof panel against the right side of the Suburban. As a result of the contact,

the left-rear corner of the Suburbanís 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.

16 The SDM records and saves data only once per second. Therefore, even though the SDM indicated

the speed 5 seconds before the accident, the speed may have been recorded anywhere between 5 and 6

seconds before the crash.

Figure 11. Suburban damage.
Factual Information 15 Highway Accident Report

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 curve17

consisting of a 1.965 percent foreslope (uphill) and a ñ3.826 percent downslope

(downhill) is located south of the accident site. The crest of the curve occurs about 158

feet before the motorcoachís first skid mark.

17 A crest vertical curve is a hill that provides a smooth transition from one roadway grade to another.

The total length of the crest vertical curve was approximately 900 feet.

Figure 12. Accident location. (Source: Texas DPS)
Factual Information 16 Highway Accident Report

I-35 was first constructed in 191718 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 inch22 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.

18 Highway 2, as the road was originally called, was renamed U.S. 81 in 1932. In 1959, the road was

designated I-35.

19 Microsurfacing is a short-term improvement, generally lasting 5 to 6 years, in which a fine aggregate

and cement mixture less than 0.25-inch thick is applied.

20 P.G. Roe, A.R. Parry, and H.E. Viner, High and Low Speed Skidding Resistance: The Influence of

Texture Depth, TRL Report 367 (United Kingdom: Transportation Research Laboratory, 1998) 1.

21 Kenneth D. Hankins, Richard B. Morgan, Bashar Ashkar, and Paul R. Tutt, The Degree of Influence

of Certain Factors Pertaining to the Vehicle and the Pavement on Traffic Accidents Under Wet Conditions,

Research Study No. 1-8-69-133, Research Report 133-3F (Washington, DC: FHWA: September 1970) viii.

22 Galloway and others, Tentative Pavement and Geometric Design Criteria for Minimizing

Hydroplaningî FHWA-RD-75-11 (Washington, DC: FHWA, February 1975).

23 Peter Elsenaar, J. Reichert, and Raymond Sauterey, Pavement Characteristics and Skid Resistance,

U.S. Transportation Research Record No. 622 (Washington, DC: Transportation Research Board, 1976).

Pavement Surface Friction 

Characteristics

Pavement texture depth consists of 

microtexture, the component for 

surface friction, and macrotexture, the 

component for removing water to obtain 

better surface friction.

The cross slope of a lane is the slope 

from the center of the lane (referred to as 

the crown) to the outer edge of the lane 

that allows water to run off the roadway.

Ruts (or wheel paths) are depressions 

formed over time in pavement by traffic 

repeatedly traveling over the same path, 

compressing and wearing away the 

pavement. Water can collect in ruts, 

diminishing a roadway’s surface friction.

The coefficient of friction represents 

the frictional properties of the pavement. 

This number is used to evaluate the skid 

resistance of the pavement relative to 

other pavements and/or to evaluate the 

change in skid resistance of the 

pavement over time. The higher the 

coefficient of friction, the less likely a 

vehicle is to skid.

The skid (or friction) number is equivalent 

to the coefficient of friction times 100.
Factual Information 17 Highway Accident Report

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 sideswipe accidents. The National

Highway Traffic Safety Administrationís (NHTSAís) 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 Texasís land area.

24 Office of the State Climatologist, Department of Atmospheric Sciences, Texas A&M University.

Accident year Median crossover 

collision 

Rear-end 

collision 

Sideswipe 

collision 

Other 

 Dry Wet Dry Wet Dry Wet Dry Wet 

1996 2 0 12 1 6 0 37 10 

1997 3 0 12 3 7 0 39 9 

1998 5 1 18 1 6 1 51 8 

1999 3 0 18 2 5 0 47 6 

2000 4 2 10 0 6 1 53 18 

2001 10 2 45 6 16 6 42 8 

2002 7 0 36 5 15 5 17 12 

Total 34 5 151 18 61 13 286 71 
Factual Information 18 Highway Accident Report

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. TxDOTís 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 ìall maintenance and repair required

to

insure the highways are kept in their designed and constructed or updated condition.î25

The maintenance requirements further specified that no ruts be more than 0.5-inch deep.

TxDOTís 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

(ìvery goodî) to F (ìvery poorî). 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

25 Special Specification, Item 7219, ìTotal Maintenance and Operation of Highway,î Contract Nos.

6044-92-001 and 6045-71-001, Article 5óScope of Work, 5-18.

26 Texas Department of Transportation, Construction Division, Materials and Pavements Section,

Managing Texas Pavements, Basic Concepts and Data Interpretation for TxDOTís Pavement Management

Information System (PMIS) (Austin, Texas: TxDOT, 2002) 9.

27 The American Association of State Highway and Transportation Officials (AASHTO) recommends

that a barrier be considered for sites with a median width of less than 30 feet and an average daily traffic

count greater than 30,000, as was the case at the accident location. AASHTO, Roadside Design Guide 2002

(Washington, DC: AASHTO, 2002).

Fiscal year Distress Ride Condition Skid 

2000 n/a A n/a 12 

2001 A A A n/a 

2002 A A A 13 

2003 A B A n/a 

Note: TxDOT did not provide PMIS structural strength data. The variation between skid numbers 

could be due to a variation in the position of the skid trailer during testing, seasonal and weather 

variation, measurement error, slight speed differences, or temperature. 
Factual Information 19 Highway Accident Report

of the accident site between January and May 2005 and plans to expand I-35 to six lanes or

more to accommodate growing traffic demands beginning in 2010. According to TxDOT,

the complete reconstruction of I-35, including geometric changes to its vertical profile and

the flattening some of the crest vertical curves, would be considered after 2010.

Meteorological Information

Waco Regional Airport, approximately 10 miles north-northwest of the accident

site, reported visibility of 10 miles in thunderstorms at 9:51 a.m. on the day of the

accident. At 10:25 a.m., visibility was 2 miles in thunderstorms with heavy rain and mist.

Rain had begun at 10:02 a.m., with 0.12 inch falling in 23 minutes. At 10:37 a.m.,

visibility was 4 miles in thunderstorms, heavy rain, and mist, with a total of 0.20 inch of

precipitation in the last hour.

At 10:15 a.m., McGregor Executive Airport, approximately 7 miles west of the

accident site, reported 2 miles visibility and 0.19 inch of precipitation in the previous 19

minutes. At 10:36 a.m., McGregor Executive Airport reported 3 miles visibility with

thunderstorms beginning at 10:25 a.m. and 0.39 inch precipitation having fallen since

9:36 a.m.

The National Weather Service Weather Surveillance Radaró1988 Doppler,

located at the Dallas-Fort Worth Forecast Office, approximately 67 miles north of the

accident site, showed a large band of echoes28 moving northeast at 45 knots across the

area of the accident site about 9:59 a.m. At that time, the rainfall ranged from ìvery

heavyî to ìintenseî with an estimated rainfall rate of 1.10 to 2.49 inches per hour for the

12 minutes that the heavy echoes were over the accident site. The total precipitation

accumulation in the hour preceding 9:59 a.m. in the immediate vicinity of the accident

site was 0.25 to 0.50 inch.

Operational Information

Central Texas Trails

The motorcoach was owned by Central Texas Trails of Waco, Texas, an interstate,

for-hire passenger carrier. The majority of Central Texas Trailsís business was charter

service, although the company had four scheduled intrastate runs from Waco to Dallas,

San Antonio, Tyler, and Lampasas. Central Texas Trails also regularly served as a

subcontractor to another company in providing shuttle service for Fort Hood military base

in Texas and for the news media during Presidential visits to Crawford, Texas.

28 The echoes are interpreted as reflecting the type and rate of precipitation.
Factual Information 20 Highway Accident Report

Central Texas Trails had 9 full-time drivers and 19 part-time drivers. The company

participated in a drug and alcohol testing program as required by Federal regulations.29

Records indicated that one Central Texas Trails employee failed a controlled substance

test in 2002 and was terminated. Records further indicated that Central Texas Trails held

driver meetings three times a year in 2001 and 2002 to discuss safety and operational

issues. The accident driver attended all meetings.

The fleet consisted of 19 motorcoaches that were maintained by a full-time mechanic

and three assistants. According to company records, Central Texas Trails conducted

maintenance every 12,000 miles or when a driver reported a problem. The company also

maintained a maintenance file for each motorcoach as required by the Federal Motor Carrier

Safety Administration (FMCSA).30 In 2002, Central Texas Trails reported traveling 75,000

miles in interstate operation and 1,062,000 miles in intrastate operation.

The FMCSAís Motor Carrier Management Information System (MCMIS)31

reported that in the 24 months preceding this accident, Central Texas Trails had been

involved in two tow-away accidents with no injuries or fatalities.32 The MCMIS report

also indicated 24 driver and vehicle roadside inspections with no out-of-service violations

in the preceding 24 months.

Central Texas Trails had received a satisfactory rating in its most recent Federal

compliance review on September 13, 1993. The FMCSA conducted a postaccident

compliance review that also resulted in a satisfactory rating. In the postaccident review,

the FMCSA inspector noted that Central Texas Trails had no recordable crashes because

FMCSA policy33 excludes intrastate accidents from the rating process. The FMCSA

inspector wrote that if the three accidents in the 24 months preceding the review (the two

tow-away accidents reported in MCMIS and this accident) had been included in the

compliance review, Central Texas Trails would have received an unsatisfactory rating in

the accident factor and a conditional rating overall.

Before January 26, 1996, Central Texas Trails was an approved passenger carrier

for the Military Traffic Management Command (MTMC).34 Two inspections conducted

for MTMC on October 4, 1994, and November 21, 1995, revealed ìless than adequate

safety management controls to ensure continued compliance in support of safe operationsî

and specifically cited lack of a preemployment drug test, missing charter information,

falsified records of duty status, and a high percentage of out-of-service vehicles. Central

29 Title 49 CFR 382.

30 Title 49 CFR 396.3.

31 The MCMIS contains information on the safety fitness of commercial motor carriers subject to the

FMCSRs.

32 The MCMIS does not specify whether crashes are interstate or intrastate. Both crashes were

identified by Central Texas Trails as occurring during intrastate operation.

33 FMCSA memorandum ìExclusion of Intrastate Violations from the Safety Rating Process,î April 19,

2002, and FHWA memorandum ìCollection of Intrastate Noncompliance Information During Safety and

Compliance Reviews,î October 2, 1991.

34 Now the Surface Deployment and Distribution Command.
Factual Information 21 Highway Accident Report

Texas Trails was disqualified and removed from MTMCís list of approved carriers.

However, this action did not prevent Central Texas Trails from operating or from being

qualified under FMCSA regulations.35

Federal Motor Carrier Safety Administration

The Motor Carrier Safety Act of 1984 directed the U.S. Secretary of

Transportation to establish procedures to determine the safety fitness of commercial motor

vehicle owners and operators engaging in interstate and foreign commerce. Subsequently,

the Federal Highway Administration (FHWA)36 established safety fitness standards and

developed a method for determining whether a carrier has adequate management controls

to ensure acceptable compliance with the safety requirements. As a result of the Motor

Carrier Safety Act of 1990 and a 1997 rulemaking, the FHWA modified the original

methodology.

Six factors (see table 6) provide the basis for determining a carrierís safety rating,

that is, the degree to which the carrier is in compliance with the FMCSRs and therefore

meets the Federal safety fitness standard. Each factor is rated satisfactory, conditional, or

unsatisfactory, except the accident factor, which is rated either satisfactory or

unsatisfactory. A satisfactory rating means the carrier has not violated any acute

regulations37 or shown a pattern of noncompliance with critical regulations38 for that

factor. A conditional factor means the carrier has violated one acute regulation or has a

pattern of noncompliance with critical regulations. An unsatisfactory rating means the

carrier has violated two or more acute regulations or has a pattern of noncompliance with

two or more critical regulations. Factor 6, the accident factor, is based on the number of

recordable accidents39 in relation to the carrierís annual mileage. An accident rate below

1.5 per million miles is considered satisfactory;40 anything higher is rated as unsatisfactory

for the accident factor.

35 In 2004, the Safety Board issued Safety Recommendations H-04-20 and -21 to the FMCSA and

MTMC to share information regarding motor carrier ratings. (For further information, see National

Transportation Safety Board, Motorcoach Run-off-the-Road and Overturn, Victor, New York, June 23, 2002,

Highway Accident Report NTSB/HAR-04/03 [Washington, DC: NTSB, 2004]). Because the agencies plan

to share compliance audits, both recommendations were classified ìClosedóAcceptable Action.î (For

further information, see appendix B.)

36 Before the FMCSA was established on January 1, 2000, the Office of Motor Carriers, which was part

of the FHWA, had responsibility for motor carrier safety.

37 Acute violations of the FMCSRs or hazardous materials regulations demand immediate corrective

action regardless of the motor carrierís overall safety posture (for example, requiring or permitting the

operation of a vehicle declared out of service before repairs are made) (49 CFR 385, Appendix B II (b)).

38 Critical violations indicate deficiencies in the motor carrierís management controls (for instance,

requiring or permitting a driver to drive after having been on duty for 15 hours) (49 CFR 385, Appendix B II (b)).

39 The FMCSA defines a recordable accident as an occurrence involving a commercial motor vehicle

operating on a public road in interstate or intrastate commerce that results in a fatality, bodily injury, or a

vehicle incurring disabling damage that requires it to be towed from the scene (49 CFR 390.5).

40 For a carrier operating within 100-air-mile radius, then, according to the FMCSRs, an acceptable

accident rate is 1.7 per million miles.
Factual Information 22 Highway Accident Report

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 carrierís overall safety

rating (see table 7). Each of the six factors receives equal weight.

Table 7. Motor carrier safety rating table.

In 1991, the FHWAís Office of Motor Carriers (FMCSAís 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 carrierís overall safety rating.

On April 19, 2002, the FMCSA issued a memorandum modifying the 1991 policy

allowing intrastate data to affect a motor carrierís overall safety rating, stating that the

FMCSAís ìsafety 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.î Further,

Factor Applicable FMCSRs or other criterion

Factor 1óGeneral Parts 387 and 390

Factor 2óDriver Parts 382, 383, and 391

Factor 3óOperational Parts 392 and 395

Factor 4óVehicle Parts 393 and 396

Factor 5óHazardous Materials Parts 171, 177, 180, and 397

Factor 6óAccident Recordable preventable rate 

Number of 

unsatisfactory ratings

Number of 

conditional ratings

Resultant

safety rating

0 2 or less Satisfactory

0 more than 2 Conditional

1 2 or less Conditional

1 more than 2 Unsatisfactory

2 or more 0 Unsatisfactory
Factual Information 23 Highway Accident Report

Any violation discovered on a trip in intrastate commerce, except for violations

that fall under FMCSA jurisdiction as mentioned above,[41] shall not be included

in the Federal rating process. All intrastate violations discovered should continue

to be recorded on the CR [compliance review] to identify what area(s) of the

carrierís operation may require further review by our State counterparts. In

addition to violations in intrastate trips, recordable crashes or out-of-service

vehicle inspections discovered to have occurred during an intrastate trip, or on a

trip between two points in a foreign country, must be left out of the calculation of

the carrierís safety rating.

In an August 1, 2002, letter to the FMCSA, the CVSA expressed concern about

excluding intrastate violations from the rating process and requested that the

interpretation be rescinded. The FMCSA responded on September 3, 2002, that the

policy change could not be rescinded for legal reasons.42 The FMCSA did note that

intrastate accident and safety-related data are used to assign Motor Carrier Safety Status

Measurement System (SafeStat) scores, which identify and prioritize motor carriers for

on-site compliance reviews and roadside inspections.

The Safe, Accountable, Flexible, and Efficient Transportation Equity Act of 2005

(SAFETEA), which authorizes Federal surface transportation programs for highways,

highway safety, and transit for a 6-year period, was passed by Congress on July 29, 2005.

Section 4114 requires the FMCSA to include accident and inspection records of owners

and operators for both interstate and intrastate trips when determining the safety fitness of

motor carriers.

Tests and Research

I-35 Pavement Friction

To determine the water flow and the coefficient of friction43 that the motorcoach

may have experienced at the time of the accident, pavement friction testing was

conducted. At the request of Safety Board investigators, TxDOT flooded the pavement

surface where the accident occurred using a hose near the crown of the road. The water

flowed down the left and right wheel paths of the right lane. The water did not drain off

the pavement, and the ruts became flooded.

41 These violations relate to drug and alcohol use and testing, commercial driverís license, financial

responsibility, and hazardous materials.

42 With the passage of the Transportation Equity Act of the 21st Century, which allows the FMCSA to

place carriers with an unsatisfactory rating out of service, the FMCSAís rating program has changed from an

informational tool to an enforcement tool. Because of this, FMCSA counsel concluded that the agency

cannot rate carriers on the basis of accidents and inspections falling outside of the agencyís jurisdiction. The

FMCSA does not have jurisdiction over wholly intrastate operations.

43 A coefficient of friction represents the frictional properties of the pavement. This number is used to

evaluate the skid resistance of the pavement relative to other pavements and/or to evaluate the change in skid

resistance of the pavement over time. The higher a friction number, the less likely a vehicle is to skid.
Factual Information 24 Highway Accident Report

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 Facility45

under the direction of Safety Board investigators. The tires tested were the left- and rightfront 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.

44 This widely used testing method utilizes a measurement representing the steady-state friction force

on a locked test wheel (a full-scale automobile tire) as it is dragged over a wetted pavement surface under

constant load and at a constant speed while its major plane is parallel to its direction of motion and

perpendicular to the pavement. The water depth on the wet pavement is estimated to be 0.02 inch. The

ASTM standard specifies 40 mph, but 50 mph was used in Safety Board testing to more closely replicate the

accident speed.

45 In 2005, the facility where the testing took place was sold by General Dynamics and is now Calspan

Corporation.

Smooth tire (0/32-inch tread depth) Treaded tire (12/32-inch tread depth) 

Right 

lane, 

right 

wheel 

path 

Right 

lane, left 

wheel 

path 

Left 

lane, 

right 

wheel 

path 

Left 

lane, left 

wheel 

path 

Right 

lane, 

right 

wheel 

path 

Right 

lane, left 

wheel 

path 

Left 

lane, 

right 

wheel 

path 

Left 

lane, left 

wheel 

path 

Range of 

friction 

numbers for 

2 miles 

surrounding 

accident 

location 

0.12 to 

0.36 

0.14 to 

0.37 

0.29 to 

0.53 

0.41 to 

0.61 

0.42 to 

0.56 

0.44 to 

0.51 

0.59 to 

0.72 

0.60 to 

0.65 

Average 

friction 

number at 

accident 

location 

0.16 0.20 0.36 0.47 0.48 0.48 0.64 0.63 
Factual Information 25 Highway Accident Report

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.î46

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

46 AASHTO, A Policy on Geometric Design of Highways and Streets, Fourth Edition (Washington, DC:

AASHTO, 2002) 110.

47 The driver eye height of 7.5 feet from the ground (that is, the position of the driverís eye while seated

in the motorcoach) was measured by investigators on scene.

48 A 2.0-foot object height is used when calculating available sight distances. From A Policy on

Geometric Design of Highways and Streets, 127.

Tire Average 

tread 

depth* 

(inches) 

Water 

depth 

(inches) 

Longitudinal 

sliding 

coefficient 

of friction 

Lateral 

maximum 

coefficient 

of friction 

Left front 15/32 0.02 - - 

 0.11 0.27 0.46 

 0.19 - - 

Right front 15/32 0.02 0.30 0.61 

 0.11 0.29 0.54 

 0.19 0.28 0.46 

Left outer drive 4.56/32 0.02 - - 

 0.11 0.12 0.23 

 0.19 - - 

Left inner drive 4.06/32 0.02 0.15 0.30 

 0.11 0.12 0.24 

 0.19 0.10 0.16 

Right outer drive 7.18/32 0.02 - - 

 0.11 0.12 0.26 

 0.19 - - 

Right inner drive 7.93/32 0.02 0.16 0.31 

 0.11 0.13 0.24 

 0.19 0.12 0.20 

*The average tread depth measured by General Dynamics varied slightly from that 

measured by investigators on scene, owing to the smallness of the measurements and to 

differences in measuring technique (General Dynamics measured the tread depth of the 

entire tire and averaged the numbers; Safety Board investigators used the CVSAís 

technique of measuring the minimum tread depth). 
Factual Information 26 Highway Accident Report

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.

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. AASHTOís 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).

Figure 13. Sight distance to approximate end of traffic queue available to motorcoach 

driver in right lane.
Factual Information 27 Highway Accident Report

Simulation

Safety Board staff conducted a simulation49 of the accident to determine the

dynamics of the motorcoach, its interaction with the Suburban, the driversí potential

steering input, and the braking and speed of the vehicles. The simulation used data

collected on scene, including the vehiclesí tire marks, the points of impact between the

vehicles and their final rest positions, data from the Suburbanís SDM, and a modeling of

the tire forces based on tire testing at the accident scene and at General Dynamics.

The tires on the bus were modeled as commonly available commercial tires, with a

dry slide friction value50 of 0.48. The tiresí frictional characteristics were modified to

produce friction levels similar to those determined during the tire testing. The surface

friction values were modeled based on the on-scene pavement friction testing conducted

in both the right and left lanes.

The simulation indicated that the motorcoach was probably traveling about 61.2

mph as it reached the crest of the hill, and the driver had to provide a slight right steering

input to maintain a straight course on the roadway.51 In the simulation, when the driver

began to apply the brakes, the motorcoach had traveled 256 feet before the skid marks

began, and the simulation indicated the driver might have applied 15 pounds of brake

force. About 1.4 seconds later, the driver probably began to steer to the left such that the

steering wheel was at 180 degrees in about 0.7 second. The simulation further showed that

this steering was held for about 0.45 second, followed by the driver steering right at a rate

of 180 degrees per half second until the wheel was steered right about 450 degrees (1.25

turns), just before impact with the Suburban. The simulation also indicated that just after

the driver began to turn the wheel back to the right (about 0.15 second), he quite likely

increased the brake pressure to 35 pounds as he was traveling across the left lane and into

the median.

In the simulation (as in the accident), the motorcoach struck the left-front corner of

the Suburban, causing the Suburban to rotate about 100 degrees. As the vehicles became

disengaged in the simulation, the motorcoach began to roll onto its right side, probably

striking the top left-rear corner of the Suburban. The Suburban spun out from under the

motorcoach and then probably traveled off the roadway and up the concrete wall adjacent

to the roadway. After disengaging from the Suburban, the simulation indicated that the

motorcoach rolled onto its right side and slid about 68.7 feet. The Suburban then most

likely traveled back down the wall, turned, and slid into the motorcoachís roof. The

49 Software used for the simulation included the Human Vehicle Environment (HVE) system,

Simulation Model Non-linear (SIMON), Engineering Dynamics Corporation Vehicle Simulation Model

(EDVSM), Engineering Dynamics Corporation General Analysis Tool (EDGEN), Engineering Dynamics

Corporation Simulation of Automobile Collisions (EDSMAC4), and AutoCad.

50 Slide friction is the friction value at which longitudinal slip occurs (that is, a tire stops rotating and

slides on a surface, leaving behind a skid mark or tire mark) during a test at a given load and speed on a

particular surface. The tires used in the model had a slide value of 0.48 at 40 mph and 6,625 pounds.

51 The right steer was needed to counteract the tendency for the drive axle tires to slip as the bus slowed

going up the hill. In the simulation, the bus had to be slowed to follow the physical evidence and could have

slowed due to the rain becoming harder, increased water in the ruts, a perceived slipping of the tires, or in

reaction to the traffic ahead.
Factual Information 28 Highway Accident Report

simulation indicated that at the initial point of collision, the motorcoach was probably

traveling about 40 mph in a northwesterly direction, and the Suburban was probably

traveling 33 mph in a southerly direction.52 The total change in velocity for the

motorcoach was about 16 mph and for the Suburban, about 57.8 mph, according to the

simulation. The Suburbanís longitudinal change in velocity was 51.3 mph in the

simulation, which is fairly close to the change in velocity measured by the Suburbanís

SDM (48.8 mph). The SDM can only record 0.15 second of deceleration data

postaccident, and a review of the deceleration data from the SDM indicates that the

Suburban may not have reached its maximum change in velocity before the SDM stopped

recording. The simulation showed that the maximum deceleration of the Suburban was

46.8 g, and the maximum deceleration of the motorcoach was 7.6 g.

The simulation was rerun to determine the effect of the tire tread depth. In this

case, the drive and tag axle tires were given the same tread depth as the right-front tire

(15/32 inch), and the driverís steering and braking inputs remained the same. The

motorcoachís rear tires never locked during braking and steering (in the original

simulation they did lock), which allowed the tires to maintain a higher level of friction,

approaching the peak lateral and longitudinal values for the tires. The motorcoach crossed

the left lane and turned right on the shoulder and into the median until it traveled parallel

to the roadways in the median. The bus did not travel into the southbound lanes.

Other Pavement Information

National. Historically, the States have resisted the FHWAís efforts to set a

minimum frictional quality standard for commercial vehicle and passenger car tires. Each

State is unique in terms of frictional standards for highway functional classifications,

climatic conditions, liability considerations, and available aggregate material used in

pavements; thus, the minimum frictional quality standards for passenger car tires on wet

pavement can vary widely among the States. Most States have minimum frictional quality

standards for passenger car tires on wet pavement and no standards for commercial vehicle

tires on wet pavement. Although the FHWA does not set a minimum frictional quality

standard for interstate highways, each State must have standards for pavement design and

construction with specific provisions for high skid resistance values. States must also have

programs for correcting locations with low skid resistance and high or potentially high

accident rates.53 The FHWA issued a Technical Advisory, Skid Accident Reduction

Program,

54 on December 23, 1980, which provided guidance to the States for conducting

wet weather accident location studies in order to ìidentify locations with high incidence of

wet weather accidents, determine corrective measures, and take appropriate actions in a

timely and systematic manner.î55 While the advisory states that several methods can be

used to conduct wet weather accident location studies, only one method is presented.

52 The SDM indicated the Suburban was traveling about 40 mph 1 second before the collision, so, with

continued braking, the Suburban likely was traveling about 33 mph at the time of the collision.

53 Title 23 CFR 1204.4.

54 U.S. Department of Transportation, Federal Highway Administration, Skid Accident Reduction

Program, Technical Advisory T5040.17 (Washington, DC: FHWA, 1980).

55 Technical Advisory T5040.17.
Factual Information 29 Highway Accident Report

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 ìGuide for

Pavement Frictionî (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 roadwayís

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

56 The NCHRP conducts research in acute problem areas that affect highway planning, design,

construction, operation, and maintenance nationwide. This program, which was established in 1962, is

administered by the Transportation Research Board and sponsored by member departments of AASHTO

(that is, individual State departments of transportation) in cooperation with the FHWA.

57 For further information, see <www.tfhrc.gov/pavement/truck/tech2.htm>.

Range of friction number Tentative guidelines

Friction number (treaded) of 30 or lower

OR

Friction number (smooth) of 1 through 15

Friction may be a factor contributing to wet 

weather accidents.

Friction number (treaded) higher than 30 AND

Friction number (smooth) of 16 through 25

OR

Friction number (treaded) of 31 through 35 AND

Friction number (smooth) higher than 25

Uncertain whether friction is a factor contributing 

to wet weather accidents.

Friction number (treaded) higher than 35 AND

Friction number (smooth) higher than 25

Friction may not be a factor contributing to wet 

weather accidents.

58 Joliet, Illinois, highway accident investigation, January 26, 2001, NTSB docket number

HWY-01-FH-012.
Factual Information 30 Highway Accident Report

The States also set their own thresholds for permissible rut depths.59 For example,

Indiana considers ruts above 0.25 inch to be ìsevere.î Arizona classifies ruts less than

0.25 inch as ìlow,î ruts between 0.25 and 0.50 inch as ìmedium,î and ruts over 0.50 inch

as ìhigh.î Washington State considers ruts less than 0.25 inch to be ìvery good,î ruts

between 0.25 and 0.33 inch to be ìgood,î ruts between 0.33 and 0.50 inch to be ìpoor,î

and ruts greater than 0.50 inch to be ìvery poor.î60

Recommendation History

The Safety Board has issued numerous recommendations on tire tread depth,

roadway friction requirements, and motorcoach crashworthiness, as well as a

recommendation on the use of military motor carrier inspections. For more information on

these recommendations, see appendix B.

59 The FHWA has no defined severity levels for rut depths. For further information, see

<www.tfhrc.gov/pavement/ltpp/reports/03031/01.htm>.

60 For further information, see <www.fhwa.dot.gov/infrastructure/asstmgmt/pms08.htm>.
31 Highway Accident Report

Analysis

This analysis first identifies the factors and conditions that the Safety Board was

able to exclude as neither causing nor contributing to the accident. It then provides a brief

overview of the accident events, followed by a discussion of major safety issues, to

include sight distance and speed as they relate to roadway design; roadway and tire

friction interaction, particularly between commercial vehicle tires and wet pavement; the

effect on vehicle stability of differing front and rear tire tread depths; and the need to

better identify areas with a high risk of wet weather accidents and implement the

necessary roadway improvements.

Exclusions

The bus driver stated he slept normally the 3 days before the accident, receiving 7

to 9 hours of sleep each night. The driver stated that he snored; however, when the other

symptoms of sleep apnea were described, he denied experiencing such symptoms. At the

time of the accident, the driver had been on duty for 2 hours, 45 minutes, during which he

had driven for about 1 hour, 25 minutes, with 28 of those minutes immediately before the

accident.

Blood and urine specimens collected from the driver after the accident were

negative for alcohol and illicit or performance-impairing drugs. The specimens were

positive for Diltiazem (the generic name for Tiazac®), a prescribed medication for

hypertension that has been approved by the Federal Aviation Administration for use by

civilian pilots. The driver was overweight and was being treated for hypertension and

osteoarthritis, neither of which, when under control, as in this case, would affect his

driving performance. The driver held a valid medical certificate. The Safety Board

concludes that the driver was neither fatigued nor under the influence of alcohol or

performance-impairing drugs, and, although severely overweight and suffering from

hypertension, had no medical conditions that contributed to the accident.

The first responders arrived about 12 minutes after the accident. All of the injured

were removed from the bus and transported within 44 minutes of the accident. The Safety

Board concludes that the emergency response was timely.

Based on measurements taken at the scene, the bus driver had a maximum of 767

feet to see the stopped vehicles ahead. The heavy rain may have reduced visibility even

further, but a precise distance is unknown.61 The simulation indicated that the driver may

have begun braking 2.5 seconds before he reached about 35 pounds of brake force, at

61 With rainfall as heavy as was recorded at the time of the accident, visibility may have been less than

1/4 mile and could have approached 0 feet.
Analysis 32 Highway Accident Report

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 ìfor 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.î63 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 driverís 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.

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 axleís ABS been

operational, it would have had minimal effect on the dynamics of this accident because the

majority of the motorcoachís braking capacity occurred at the drive and steer axles due to

the size of those brakes and the weight distribution of the motorcoach.

62 G. Johansson and K. Rumar, ìDriversí Brake Reaction Time,î Human Factors, Vol. 13, No. 1 (Santa

Monica, CA: Human Factors Society, 1971) 23-27.

63 Daniel B. Fambro, Kay Fitzpatrick, Rodger J. Koppa, and Dale L. Picha, ìDriver PerceptionóBrake

Response in Stopping Distance Situations,î Transportation Research Board, 1998 Annual Meeting

(Washington, DC: Transportation Research Board, 1998).

Figure 14. Time and distance available to bus driver.

Distance to end of queue

Traffic sighted 0

767

Begin brake application 2.3

562

15 lb. Brake application 2.8

517

Begin left steer 3.7

438

180 degree left steer 4.4

378

Tire marks begin 4.7

350

Begin right steer 4.8

339

Begin harder braking 5

328

35 lb. Brake application 5.5

Feet 288

Seconds
Analysis 33 Highway Accident Report

Accident Discussion

At the time of the accident, it was raining heavily, and the driver reported he had

reduced the motorcoachís speed from 65 mph to about 60 mph. As the motorcoach

approached the crest of a hill on northbound I-35, the driver stated that he noticed traffic

ahead beginning to slow. He began to move from the right to the left lane to avoid the

longer queue of traffic and applied his brakes. The driver stated that, at the same time,

another vehicle also moved to the left ahead of him, requiring the motorcoach driver to

provide more rapid steering input, followed by harder braking. As the motorcoach began

to move to the left lane, the rear wheels of the bus began sliding and the motorcoach began

rotating counterclockwise because of the reduced friction resulting from the low friction

values of the highway and also the low tread depths on the drive axle tires. The simulation

indicated that the bus driver rapidly turned the steering wheel to the right, probably to

correct for the sliding, but that the available friction between the drive axle tires and the

roadway was reduced such that the drive axle tires were unable to follow the steering

maneuver and continued to slide forward and to the right, causing the motorcoach to rotate

counterclockwise. The skid marks on the roadway and the path of the motorcoach as it

crossed the median indicate that the steering maneuver was unsuccessful. After crossing

the median, the motorcoachís right-front corner struck the left-front corner of a

southbound Suburban.

The north- and southbound travel lanes were separated by 40 feet (shoulders and

median), and the average daily traffic count in 2001 was 63,000 vehicles. AASHTOís

Roadside Design Guide64 recommends that the need for a median barrier be evaluated for

median widths (distance between travel lanes) of 30 feet or greater and average daily

traffic volumes of 30,000 vehicles or greater. At this location, a median barrier would be

optional, based on AASHTO guidelines. TxDOT installed a median barrier in January

2004, over a year after the accident. Although median barriers are generally designed to

prevent passenger vehicles from crossing a median, they can also help reduce the forward

motion of larger vehicles upon impact. The Safety Board concludes that although a

median barrier would not have prevented the motorcoach from leaving the travel lanes, it

may have either redirected the motorcoach or slowed it significantly before entering the

southbound lanes, possibly reducing the severity of the collision with the Suburban.

Sight Distance

Traffic had backed up to and stopped at a location on the downslope of a hill on

I-35. Because of the hill, the stopped traffic was obscured from the sight of approaching

motorists. Using the AASHTO formula,65 the sight distance available to the motorcoach

approaching the crest vertical curve was 732 feet. Measurements taken during the

on-scene investigation indicated that the sight distance available to the bus driver was 767

64 2002 Roadside Design Guide, 6-2.

65 A Policy on Geometric Design of Highways and Streets, 271.
Analysis 34 Highway Accident Report

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

66 The AASHTO formula and this measurement did not take into account weather, which may have

reduced the driverís visibility even further.

Figure 15. Available sight distance for a motorcoach.

Figure 16. Available sight distance for a passenger car.

900 feet

732 feet

Available sight distance

7.5-foot

driver eye

height

2.0-foot

object

height

Beginning of

crest vertical

curve

End of

crest vertical

curve

900 feet

580 feet

Available sight distance

3.5-foot

driver eye

height

2.0-foot

object

height

Beginning of

crest vertical

curve

End of

crest vertical

curve

Figure 15A.

Figure 15B.
Analysis 35 Highway Accident Report

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-35ís 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.

Vehicle type Speed (mph) Friction test

Calculated 

available sight 

distance (feet)*

Required 

stopping sight 

distance (feet)

Motorcoach 50 Smooth 732 607

Motorcoach 60 Smooth 732 900

Motorcoach 70 Smooth 732 1,293

Motorcoach 50 Treaded 732 399

Motorcoach 60 Treaded 732 572

Motorcoach 70 Treaded 732 798

Passenger car 50 Smooth 580 481

Passenger car 60 Smooth 580 696

Passenger car 70 Smooth 580 962

Passenger car 50 Treaded 580 335

Passenger car 60 Treaded 580 466

Passenger car 70 Treaded 580 631

* Passenger cars have a reduced sight distance because the driver height is 4 feet lower. Calculations 

were based on the AASHTO formula for available sight distance.
Analysis 36 Highway Accident Report

The Safety Board believes that TxDOT should 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. The Safety Board believes that the FHWA should issue guidance to its 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. The Safety

Board also believes that once these locations have been identified, the FHWA should assist

the States in developing and implementing a plan for repaving or other roadway

improvements. The Safety Board will inform AASHTO of the inadequate stopping sight

distances that could occur when poor vertical geometries exist at locations with low

coefficients of friction and speeds greater than the design speed and ask that AASHTO

encourage its members to rectify similar situations that may exist throughout the country.

The use of design speed was initiated in the 1930s, yet because design and

operational criteria have changed over the years, many locations exist nationwide where

the stopping distance may exceed the available sight distance. Members of the AASHTO

Committee on Design have expressed concern that roadway operating speeds exceeding

design speeds could lead to increased liability for the engineer or agency.67 When these

roadways were designed, a safety factor was often incorporated; thus, motorists may feel

comfortable traveling at speeds greater than the roadwayís design speed in good weather.68

However, in poor weather, such as existed during this accident, it may be prudent to alert

drivers of the condition. NCHRP Report 504 recommends that when a ìsafety concern

exists at a location, appropriate warning or informational signs should be installed to warn

or inform drivers of the condition.î69

While drivers generally reduce their speed in rain by about 6 mph,70 this may not be

enough to compensate for the roadway geometry and wet pavement conditions. Before this

accident, the bus driver was already traveling about 10 mph below the posted speed limit of

70 mph. As can be seen in table 11 earlier in this section, had the motorcoach been traveling

at 50 mph, the required stopping distance for the motorcoach would have been less than the

available sight distance, providing the driver with adequate time to react to the stopped

traffic ahead. The Safety Board concludes that although the speed limit on I-35 was 70 mph

and the design speed was 60 mph, the driver would have had to have been traveling 50 mph

or less to avoid the collision or at least to have reduced its severity. Further, the Safety Board

concludes that despite the safety factors that are incorporated into roadway design, roadways

with speed limits exceeding their design speed can constitute a hazard in wet weather.

67 Kay Fitzpatrick, Paul Carlson, Marcus A. Brewer, Mark D. Wooldridge, and Shaw-Pin Miaou,

Design Speed, Operating Speed, and Posted Speed Practices, NCHRP Report 504 (Washington, DC:

Transportation Research Board, 2003) 16.

68 Fitzpatrick and others, 80-81.

69 Fitzpatrick and others, 84.

70 Lin Zhang and Panos D. Prevedouros, ìMotorist Perceptions on the Impact of Rainy Conditions on

Driver Behavior and Accident Risk,î Transportation Research Board, 2005 Annual Meeting (Washington,

DC: Transportation Research Board, 2005).
Analysis 37 Highway Accident Report

Variable speed limit signs are one method of alerting drivers to reduce their speed

in response to a potentially hazardous condition during poor weather. These signs display

the posted speed limit during good weather, and, when weather conditions deteriorate, the

posted speed limit is decreased. Over 1,300 Environmental Sensor Stations have already

been deployed nationwide to provide transportation managers with information on current

conditions, including atmospheric, pavement, subsurface, and water level conditions.71

These data can be integrated with National Weather Service data to maximize the benefits

from information posted on variable message signs. Road weather data may be used in

automated motorist warning systems (such as variable speed limit signs) or sent to traffic

management centers so that signs can be changed manually.

The Florida Department of Transportation, for example, installed a motorist

warning system consisting of a flashing light mounted on a speed limit sign at an exit

ramp where 69 percent of crashes occurred on wet pavement. Vehicle speeds and speed

variances, as well as accidents, were reduced.72 The New Jersey Turnpike Authority uses

weather data to determine when to reduce speeds in inclement weather using variable

speed limit signs, which have reduced inclement weather accidents.73 The Washington

State Department of Transportation uses speed management through the Snoqualmie Pass

to inform motorists of reduced speeds due to inclement weather, including heavy rain

and/or standing water on the roadway, thus increasing safety.74

Redesigning the roadway to eliminate locations where stopping distance exceeds

available sight distance would be ideal; however, the Safety Board understands that

roadway redesign can be costly. Variable speed limit signs based on current weather

conditions could warn drivers to reduce their speed, thereby reducing their stopping

distance below the available sight distance in adverse weather conditions. The Safety

Board concludes that when redesigning a roadway for a higher design speed is not

feasible, variable speed limit signs can be used as a countermeasure to reduce speeds and

increase safety. The Safety Board believes that TxDOT should install variable speed limit

signs or implement alternate countermeasures at locations where wet weather can produce

stopping distances that exceed the available sight distance. The Safety Board also believes

that the FHWA should 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. The Safety Board will inform

AASHTO of the circumstances of this accident and of the benefits of using variable speed

limit signs in wet weather at locations where the operating speed exceeds the design speed

and where the stopping distance exceeds the available sight distance.

71 Paul Pisano, Brandy Hicks, Rudy Persaud, Lynette Goodwin, and Andy Stern, An Overview of

Federal Highway Administration Road Weather Management Program Activities, 83rd Annual American

Meteorological Society Meeting (Long Beach, CA: AMS, 2003). For further information, see

<ams.confex.com/ams/pdfpapers/54831.pdf>.

72 Lynette Goodwin and Paul Pisano, Best Practices for Road Weather Management (Washington, DC:

FHWA, July 24, 2002) 8.

73 Goodwin and Pisano, 26.

74 Goodwin and Pisano, 50-51.
Analysis 38 Highway Accident Report

Roadway/Tire Interaction

Investigators found a number of factors that contributed to the loss of available

friction between the tires and the road in this accident, including low macrotexture

pavement depths, rutting, and the low tread depths of the vehicleís drive axle tires. The

right laneís cross slope, which testing indicated was not steep enough to properly drain the

water off the pavement, exacerbated these problems. The sections that follow will discuss

factors affecting roadway and tire interaction in more detail.

Macrotexture Pavement Depth

The macrotexture, a roadwayís surface roughness, affects how water is removed

(drains) from the surface of the roadway. Macrotexture can have a great impact on surface

friction; the higher the macrotexture, the greater the friction, particularly in wet weather.

However, higher macrotexture depths can also increase road noise.

Currently, no national standards exist regarding acceptable or minimum

macrotexture pavement depth; decisions concerning such matters are left to the States.

Research conducted in Great Britain concluded that the effect of texture depth on loss of

friction was found to be greatest below depths of 0.028 inch.75 TxDOT conducted research

in 1970 that suggests a 0.035-inch macrotexture pavement depth be used for design

purposes.76 Other research recommended minimum pavement texture depths of 0.040

inch77or 0.015 to 0.031 inch.78 In the vicinity of the accident, the average macrotexture

depths in the left and right lanes were 0.024 to 0.027 inch and 0.017 to 0.018 inch,

respectively, well below the minimum levels recommended by TxDOT and others. As

could be seen during the on-scene testing, water, when applied to the roadway, flowed

down the left and right wheel paths and did not drain off the pavement. The Safety Board

concludes that the low macrotexture depth at the accident site prevented proper drainage

of the water from the roadway, allowing excessive water to remain on the road surface,

which, in turn, contributed to its minimal frictional qualities.

Rut Depths

While the low macrotexture pavement depths and the right laneís cross slope

prevented water from being removed from the roadway surface at the accident site, the

rutting found in the roadway contributed to the further accumulation of water during

the rainstorm at the time of the accident. During a heavy rain, such ruts could become

ìfull,î resulting in water depths greater than the average tread depths of the motorcoachís

75 P.G. Roe, A.R. Parry, and H.E. Viner, High and Low Speed Skidding Resistance: The Influence of

Texture Depth, TRL Report 367 (United Kingdom: Transportation Research Laboratory, 1998) 1.

76 Kenneth D. Hankins, Richard B. Morgan, Bashar Ashkar, and Paul R. Tutt, The Degree of Influence

of Certain Factors Pertaining to the Vehicle and the Pavement on Traffic Accidents Under Wet Conditions,

Research Study No. 1-8-69-133, Research Report 133-3F (Washington, DC: FHWA: September 1970) viii.

77 Galloway and others, Tentative Pavement and Geometric Design Criteria for Minimizing

Hydroplaningî FHWA-RD-75-11 (Washington, DC: FHWA, February 1975).

78 Peter Elsenaar, J. Reichert, and Raymond Sauterey, Pavement Characteristics and Skid Resistance,

U.S. Transportation Research Record No. 622 (Washington, DC: Transportation Research Board, 1976).
Analysis 39 Highway Accident Report

drive axles. When water depth exceeds tire tread depth, water cannot be channeled out

from beneath the tires, reducing the friction available between the tire and the roadway

surface.

Although the ruts near the accident site were deeper than the average tread depth

of the drive axle tires, they were below TxDOTís recommended resurfacing threshold of

0.50 inch or deeper. The rutting in the right wheel path of the right lane was between

0.25 and 0.35 inch, equal to or higher than the average tread depth for the right tires

(0.22 and 0.25 inch). The rutting in the left wheel path of the right lane was between

0.40 and 0.42 inch, again higher than the average tread depths of the left tires (0.125 and

0.14 inch). However, because the majority of these ruts were between 0.25 and 0.50

inch deep, they met TxDOT criteria for a ìshallowî rutting problem, and resurfacing

was not required.

At locations where drainage is poor, it is possible that, in a heavy storm, the water

accumulating in the ruts could exceed the tread depth of many tires, not just tires with

minimal tread depths, leading to a reduction in available friction. Even the front tires on

the motorcoach, which were only 1 month old, had tread depths of less than 0.50 inch

(14/32 [0.44] and 15/32 [0.47] inch). For passenger cars, new tire tread depths can vary

from 8/32 to 11/32 inch. About 80 percent of passenger cars have at least one tire with a

tread depth of less than 8/32 [0.25] inch, and the average tread depth for all tires is 6.8/32

[0.21] to 7.0/32 [0.22] inch.79 Although the States have no uniform threshold for rut depths

before rehabilitation occurs, rut depths greater than 0.50 inch are generally unacceptable.

The Safety Board concludes that a rut-depth threshold of 0.50 inch is insufficient because

0.50 inch exceeds the tread depths of most vehicle tires, and standing water in the ruts can

lead to reduced tire-roadway friction situations in wet weather.

Differing friction conditions within a travel lane can also influence vehicle

handling. The average rut depth in the right lane, left wheel path, was 0.11 inch greater

than in the right wheel path. Thus, more water was probably standing or flowing

downgrade in the left wheel path than in the right wheel path, making it necessary for the

left tires to push more water out from between the tires and the pavement to maintain

traction. Furthermore, the tread depth for the right drive axle tires (minimum of 5/32 and

6/32 inch for the inner and outer tires, respectively) differed from the left drive axle tires

(minimum of 2/32 and 3/32 inch for the inner and outer tires, respectively). The reduced

tread depth further diminished the ability of the left tires to channel water away from the

tire and roadway interface and to continue to maintain traction on the road surface. This

caused a differential force between the left and right tires that resulted in the motorcoach

turning counterclockwise, requiring the driver to input a steering force to the right to

maintain his course.

79 Kristin Thiriez and Rajesh Subramanian, ìResearch Note: Tire Pressure Special Study Tread

Depth Analysis,î DOT HS 809 359 (Washington, DC: National Center for Statistical Analysis, NHTSA,

October 2001).
Analysis 40 Highway Accident Report

Research has shown that when the coefficients of friction in the wheelpaths differ,

this differential friction may cause a vehicle to spin out of control when braking.80 The

coefficient of friction was 0.16 for a smooth tire in the right wheelpath and 0.20 in the left

wheelpath, a 20-percent difference in friction. Research indicates that even a 17-percent

difference in friction resulted in rotations during braking that increased as speed increased,

up to 95 degrees rotation at 50 mph.81 In this accident, the driver attempted to compensate

for such rotation by steering, resulting in further loss of control of the motorcoach. The

Safety Board concludes that the greater rut depth in the left wheel path and the differential

friction between the wheelpaths resulted in a destabilizing counterclockwise rotational

force on the motorcoach.

Roadway Friction

When a roadway surface is flooded, as it was when this accident occurred, a

vehicle may hydroplane. Hydroplaning occurs when a tire is unable to push the water on

the surface out from the contact area between the tire and the roadway surface, resulting in

a reduction in friction and causing the tire to slide across the top of the water. As speed

increases, water cannot be pushed out from between the tire and the roadway quickly, thus

reducing the available friction. To determine whether the motorcoach could have

experienced this phenomenon, the speed at which hydroplaning would occur was

calculated for each tire.82 Calculations indicate that the lowest speed at which one of the

tires would hydroplane was about 71 mph for the front left tire, which had the lowest tire

pressure (73 psi). The remaining tires would not hydroplane until the speed reached 73 to

79 mph. The motorcoach driverís and witnessí statements, as well as the simulation,

indicate that the motorcoach was traveling about 60 mph; therefore, the Safety Board

concludes that the motorcoach did not hydroplane before losing control.

A study of the effect of water depth on braking coefficients indicates that ìa

progressive loss in braking force coefficientî can occur long before full hydroplaning

develops.83 In this accident, the drive axle tires likely locked up when the motorcoach was

rapidly steered, and the brakes were applied, resulting in the motorcoach beginning to

slide. The driverís attempt to regain control by rapid steering input was unsuccessful

because no traction was available to the tires. Probably to correct for the sliding, the bus

driver rapidly turned the steering wheel to the right, and because the available friction

between the drive axle tires and the roadway was exceeded to the extent that the drive axle

tires were unable to follow the steering maneuver, the motorcoachís rear continued to slide

forward and to the right. As stated earlier, the bus driverís reaction occurred in a timely

manner in relation to the distance from which he could see the stopped traffic, yet despite

this, he would not have been able to avoid hitting stopped traffic. The Safety Board

80 John C. Burns, ìDifferential FrictionóA Potential Skid Hazard,î 1976 Annual Meeting,

Transportation Research Board (Washington, DC: Transportation, Research Board, 1976).

81 Burns, 2.

82 Equation derived from: Don L. Ivey. Truck Tire HydroplaningóEmpirical Confirmation of Horneís

Thesis (College Station, Texas: Texas Transportation Institute, May 1971).

83 G.C. Staughton and T. Williams, Tyre Performance in Wet Surface Conditions, Transport and Road

Research Laboratory, Laboratory Report 355 (United Kingdom: Ministry of Transport, 1970).
Analysis 41 Highway Accident Report

concludes that the water accumulation on the roadway, likely due to the low macrotexture

depth, rutting, and inadequate cross slope, contributed to the loss of available friction

between the tire and the road.

Wet pavement friction is a measure of the force generated when a tire slides on a

wet pavement surface. Wet pavement friction decreases with increasing speed.84 Friction

numbers can be used to evaluate pavement friction and compare pavement with other

surfaces or the same pavement over time. Many factors influence the friction, including

surface design, age, traffic volume, seasonal changes, and speed. The level of friction

required depends upon the traffic characteristics, climate, and roadway geometry.

TxDOT uses a statistical evaluation to determine the degree to which the friction

of a roadway, derived from an ASTM smooth tire tested at 40 mph on wet pavement, is

below the mean. Postaccident testing at 50 mph in the vicinity of the accident site revealed

the average friction coefficients in the right lane, right and left wheel paths, of 0.16 and

0.20, respectively, and in the left lane, right and left wheel paths, of 0.36 and 0.47,

respectively. TxDOTís PMIS showed pavement friction coefficients in the right lane of

0.12 in 2000 and 0.13 in 2002. (These variations can be attributed to differences in the

location of the skid trailer.) All of the pavement friction values in the right lane were very

low; in fact, they were equivalent to performance on ice (see the discussion later in this

section regarding Texas roadway friction).

Most States set minimum friction quality standards for design and maintenance of

pavement that only take into consideration passenger cars, not commercial vehicles. Yet,

according to the NCHRP Report 400:

Truck tires tend to have lower wet friction coefficients than passenger car tires

because they are designed primarily for wear resistance. Olson et al.85 estimated

that truck tires have coefficients of friction that are about 70 percent of those of

passenger car tires.86

The guidelines currently used by AASHTO for skid-resistant pavement design

date from 1976 and are based on passenger car tire characteristics. The guidelines have not

been updated to reflect changes in vehicle or tire characteristics, data collection, traffic

flow, construction techniques and materials, or other factors that affect pavement surface

characteristics. NCHRP Project 1-43, ìGuide for Pavement Friction,î due to be published

in October 2005, is intended to update some of the information in AASHTOís 1976

guidelines.

84 NCHRP Synthesis of Highway Practice 291: Evaluation of Pavement Friction Characteristics.

(Washington, DC: Transportation Research Board, 2000) 5.

85 P.L. Olson, D.E. Cleveland, P.S. Fancher, L.P. Kostyniuk, and L.W. Schneider, Parameters Affecting

Stopping Sight Distance, NCHRP Report 270 (Washington, DC: Transportation Research Board, National

Research Council, June 1984).

86 Daniel B. Fambro, Kay Fitzpatrick, and Rodger J. Koppa, Determination of Stopping Sight

Distances, NCHRP Report 400 (Washington, DC: Transportation Research Board, National Research

Council, 1997) 17.
Analysis 42 Highway Accident Report

Each Stateís requirements for pavement quality are unique in terms of frictional

standards for highway functional classifications, climatic conditions, liability

considerations, and available aggregate materials; thus, the FHWA believes that

evaluating skid resistance nationally is not feasible, and it has set no Federal standards for

minimum frictional quality for wet pavement. The FHWA requires that States have

minimum frictional quality standards for passenger car tires on wet pavement but does not

require such standards for commercial vehicle tires. Although the Safety Board

understands the FHWAís reluctance to set pavement standards because of the State

variations in roadway materials and environments, research to determine minimal

acceptable pavement standards, taking into consideration tire frictional properties, could

help States understand the effects of inadequate pavement conditions and prevent

conditions such as those found at the accident site from developing.

The FHWA does not specify a minimum frictional quality standard for interstate

highways. A review of Texas PMIS skid data for fiscal years 1999 and 2000 revealed that 50

percent of the roads in the interstate highway system had coefficients of friction of 0.26 or

below; 50 percent of the U.S.-numbered highways in Texas, 0.31 or below; 50 percent of

State highway system roads, 0.35 or below; and 50 percent of the farm-to-market roads, 0.41

or below. Coefficients of friction on Texas interstate highways have values that are generally

0.10 to 0.15 units lower than on State highways and farm-to-market roadways because

noninterstate roadways are typically surfaced with high macrotexture seal coats.87 These

surfaces provide higher coefficients of friction than the hot-mix asphalt or Portland cement

concrete used to surface Texas interstate highways, but they are also noisier and less durable.

Coefficients of friction on icy surfaces can range from 0.12 to 0.25.88 TxDOT

PMIS data revealed that the mean coefficient of friction on all Texas roadways is 0.36 and

that 50 percent of the interstate highways in Texas have coefficients of friction 0.5 to 1.0

standard deviation below this mean. The accident location had even lower coefficients of

friction. As coefficients of friction decrease, the required stopping distance increases.

Though no nationwide recommended minimum coefficient of friction exists for

highways, those in use by other States can provide a basis for comparison. The

coefficients of friction on I-35 were close to those coefficients of friction found on icy

surfaces. TxDOT was aware of the low frictional qualities of I-35 as early as 2000, when

the PMIS data indicated a coefficient of friction of 0.12. Despite this, TxDOT had no

immediate plans to repave this roadway to increase its frictional qualities because many

interstate roadways in Texas have similar coefficients of friction and all of the other

87 However, as these seals lose aggregate over time, they can form a layer of asphalt on the surface that

is extremely slippery.

88 (a) J. Stannard Baker, Traffic Accident Investigation Manual (Evanston, Illinois: Northwestern

University Traffic Institute, 1975). (b) Francis Navin, Michael Macnabb, and Connie Nicoletti, Vehicle

Traction Experiments on Snow and Ice, SAE 960652 (Warrendale, PA: Society of Automotive Engineers,

1996). (c) A.H. Easton, ìSummary of Tests on Motor Vehicles Under Winter Conditions,î Highway Research

Board Proceedings, 1961 (Washington, DC: Highway Research Board, 1961) 565-581. (d) M. McBride,

ìSkid Tests on Nine Vehicles on an Ice Covered Surface,î Accident Reconstruction Journal, Vol. 4, No. 2

(Overland Park, KS: Criterion Press, 1992). (e) J. Hunter, Reconstructing Collisions Involving Ice and

Slippery Surfaces, SAE 930896 (Warrendale, PA: Society of Automotive Engineers, 1993).
Analysis 43 Highway Accident Report

factors (distress, ride, and condition) were rated at the highest level (A). The Safety Board

concludes that TxDOTís PMIS does not adequately identify roadways where hazardous

conditions exist due to low coefficients of friction and does not expeditiously prioritize

these locations for rehabilitation, increasing the risk for accidents such as the one that

occurred at this location. The Safety Board believes that TxDOT should change its PMIS

to increase its emphasis on roadways with low coefficients of friction in determining

maintenance priorities.

Tire Friction

The friction testing results for the drive axle tires were 0.10 on 0.19 inch of water

and 0.15 on 0.02 inch of water (equivalent to performance on ice), values significantly

lower than those of the front tires (0.28 to 0.30), primarily because the drive axle tires had

a much lower tread depth (2/32 versus 14/32 inch). As was demonstrated by the testing,

the tread depth can significantly affect the friction available in wet weather; available

friction is critical to the vehicleís ability to stop.

Calculations, based on the tire coefficients of friction derived from friction

testing89 and the configuration of the brakes, indicated that the drive axle brakes would

have locked up at a brake application pressure of about 22 psi. The front and tag axle tires

would have required a brake pressure of about 32 and 33 psi, respectively, before they

locked up on the same low friction surface of the right lane. This difference can lead to

instability of the vehicle during hard braking or emergency maneuvers. Had the drive axle

tires been the same tread depth as the front axle tires, they would not have locked up until

a brake application pressure of 47 psi was applied.90 The Boardís simulation indicated that

the driver likely applied the brakes at a pressure of 35 psi. The simulation further showed

that when tires with tread depths similar to those of the front tires (14/32 or 15/32 inch on

all the wheels) were used, the driver would likely have been able to maintain sufficient

control of the motorcoach to avoid crossing into the southbound lanes. While the

motorcoach would have left the roadway, the Suburban probably would not have been hit

and the motorcoach probably would not have rolled over.

Research and testing on passenger cars indicate that ìfriction forces at highway

speeds are reduced to half or less of the new tire value if the tire wear exceeds about 50

percent,î91 as it did on the drive axle tires. This research also indicated that the lateral

friction of tires decreases well before hydroplaning is expected to occur.92 Further, when

the worn tires are placed on the rear of passenger cars, the handling of the vehicle changes,

since the rear tires have more tendency to slide.

89 These tire coefficients of friction were measured on a roadway surface similar to the surface

measured on scene.

90 The drive axle wheels require more brake pressure to lock than other wheels because the drive axle

supports more of the motorcoachís weight than the other axles.

91 William Blythe and Terry D. Day, Single Vehicle Wet Road Loss of Control; Effects of Tire Tread

Depth and Placement, SAE 2002-01-0553 (Warrendale, PA: Society of Automotive Engineers: 2002).

92 Blythe and Day, 10.
Analysis 44 Highway Accident Report

Tire friction testing also indicated that the lateral friction, the stability of a tire

during turns and lateral maneuvers, was much lower for the rear tires (0.23 to 0.26)93 than

the front tires (0.46 to 0.54). Thus, when a maneuver such as braking or steering is

attempted, the worn rear tires, with reduced longitudinal and lateral friction, are unable to

maintain their grip on the road and will begin to slide rather than follow the front tires

through the intended maneuver. Research on passenger vehicles found that ìnormal lane

change maneuvers can lead to loss of control on a wet road if sufficient difference in tread

depth exists front to rear, with the better treaded tires on the front axle of a passenger

car.î94 Lower tread depths on the rear tires of passenger vehicles create an inherent safety

problem, and while they are likely to have the same effect on commercial vehicles, which

was confirmed in the simulation, the extent of this effect cannot be determined. In this

accident, when the driver tried to brake and then to abruptly turn the wheel back to the

right when the drive axle tires started to slip, little lateral friction was available for the

maneuver, and thus the motorcoach continued to rotate counterclockwise. Yet, with tires

that had a greater tread depth, the motorcoach probably would have responded to the rapid

right steering maneuver and would not have continued its counterclockwise rotation. The

Safety Board concludes that the minimum tread depths of the drive axle tires, including

the smaller tread depth on the left drive axle tires, particularly in combination with the

nearly new front tires, contributed to wheel lockup and the subsequent rotation of the

motorcoach.

The FMCSRs95 currently require that the tread depth for the front wheels of a

commercial vehicle be at least 4/32 inch and the tread depth for all other tires be at least

2/32 inch. The CVSA guidelines for placing a vehicle out of service include steer axle tire

treads of less than 2/32 inch in two adjacent grooves and any other tire treads less than

1/32 inch in two adjacent grooves. The effect of tread depth on commercial vehicle

handling has not been evaluated since these requirements were instituted over 30 years

ago, and no data are available to determine how these tread depth requirements were

determined. The Safety Board previously made a recommendation to NHTSA on this

matter as a result of its investigation of the November 16, 1980, motorcoach accident near

Luling, Texas,96 in which the motorcoach lost traction on the wet pavement and skidded

off the road:

H-81-33

Accelerate activity to establish rulemaking action for minimum frictional

quality standards for commercial vehicle tires.

93 At 60 mph and 0.11-inch water depth.

94 Blythe and Day, 14.

95 Title 49 CFR 393.75.

96 National Transportation Safety Board, East Side Church of Christ Bus Skid and Overturn U.S. Route

183 Near Luling, Texas, November 16, 1980, Highway Accident Report NTSB/HAR-81/04 (Washington,

DC: NTSB, 1981).
Analysis 45 Highway Accident Report

Yet nothing was done, and the recommendation was classified ìClosedó

Unacceptable Actionî 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 ìClosedó

Unacceptable Actionî on May 19, 1989 (see appendix B). Yet, this accident and recent

research98 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

NHTSAís 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 TiresóSpecial 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.

97 National Transportation Safety Board, Tractor-Semitrailer/Intercity Bus Head-on Collision Interstate

10, Beaumont, Texas, May 4, 1987, Highway Accident Report NTSB/HAR-88/01 (Washington, DC: NTSB,

1988).

98 Blythe and Day.

99 Tire manufacturers do disseminate this information to their maintenance facilities.
Analysis 46 Highway Accident Report

The minimum permissible tread depth on a commercial vehicle tire, which was

established in 1969, is 2/32 inch for any tire except the steer axle tires. As shown in the

testing and simulation, the permissible tread depth did not provide the motorcoach driver

with enough friction to maneuver the vehicle on wet pavement in an emergency situation,

thus leading to the accident. Even the right drive axle tires, with their 4/32-inch tread

depths, had coefficients of friction that were not much greater than those experienced by

the left drive axle tires with their 2/32-inch tread depths. The Safety Board issued Safety

Recommendation H-88-1 in 1988 asking that the FHWA prohibit the use of tires worn

below 4/32 inch on any axle, yet the minimums were not changed (see appendix B).

Because the interaction between the tires and the pavement can affect the amount of

friction available to vehicles, the Safety Board concludes that to prevent accidents similar

to this one, commercial vehicle tire-roadway interaction, which is not currently being

considered, must be taken into account when determining the necessary wet pavement

friction of the roadway and tire tread depth minimums. This is particularly true for

commercial vehicles, for which the tire coefficients of friction are already lower than for

passenger vehicles.

The FHWAís Truck Pavement Interaction research program focuses primarily on

pavement performance and damage mitigation. Research on the interaction of commercial

vehicle tires, the effects of tread depth, and pavement friction has not been conducted in

over 30 years, and this accident demonstrates that the current minimum requirements for

commercial vehicle tire tread depth are inadequate, especially when combined with the

minimal frictional qualities of Texasís interstate highway system. The Safety Board

believes that the FHWA should 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, the FHWA should 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.

Texas Wet Weather Accident Reduction Program

Texas established the WWARP to identify locations for improvement throughout

the State that are overrepresented in wet weather accidents. Currently, the Texas WWARP

threshold for determining hazardous roadway segments requires that five accidents occur

within a 1/10-mile (528-foot) segment on an interstate roadway. This threshold distance is

fairly short given the high speeds, traffic volumes, number and weight of trucks, and

limited number of intersecting roadways found on interstates. By limiting segments to

1/10 mile, TxDOT may fail to identify longer segments of roadway that have equally

hazardous roadway conditions in wet weather. The WWARP indicates that more than half

(14) of all the district offices in Texas had zero or one location identified as hazardous.

The urban district offices of Houston, San Antonio, Dallas, Austin, and Fort Worth,

however, had a disproportionately high number of wet weather accident sites identified
Analysis 47 Highway Accident Report

(410 of the total 467 reported high-accident sites in the State). Because of the threshold the

WWARP uses for identifying hazardous locations, the program is weighted toward

identifying locations in high population areas, virtually ignoring other locations that are

just as hazardous but in less densely populated areas.

Sixteen percent of the accidents that occurred on I-35 in the vicinity of the accident

from 1996 to 2002 were in wet weather. That segment of roadway was exposed to

precipitation only 2.35 percent of the period. Thus, although travelers had nearly a seven

times greater chance of being in an accident in wet weather than dry weather at the

accident location, the WWARP did not identify this area as a high-accident location. 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.

The data used for the WWARP are 3 years old. Because of the lag time in this data,

locations that have pavement friction issues in wet weather may not be identified quickly

enough, as pavement quality deteriorates over time. Further, this accident location had low

friction values, a ìshallowî rutting problem, and poor roadway geometry that may have

indicated an increased accident risk. The Safety Board concludes that Texasís WWARP

methodology is not sufficiently refined to measure wet weather accident risk, fails to

identify the greater risk of being involved in a wet weather accident on the segment of I-35

in the vicinity of the accident, and, generally, does not identify problems in a timely

manner.

In the 5 miles on either side of the accident site, fewer than three accidents per

1/10 mile occurred within a 3-year time frame (under both wet and dry pavement

conditions), on average. Yet this location had a higher percentage of wet weather

accidents than other similar U.S. highway segments (16 percent versus 13 percent).100

When accidents occur on an interstate, the results can be severe because of the high

speeds and high traffic volumes. In the vicinity of the accident, the poor roadway

conditions consisted of low macrotexture pavement depths, a rutting problem, and low

skid numbers. These roadway conditions can be an indicator of potential wet weather

problems, and, by not taking these conditions or roadway geometry into consideration,

Texasís WWARP may not identify highly dangerous locations. The Safety Board

believes that TxDOT should revise and validate its WWARP so that improvement

priorities are not disproportionately influenced by the number of accidents that occur but

also consider locations where surface conditions and geometry lead to very low friction

coefficients and dangerous conditions.

Each State has its own method for determining whether a roadwayís 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 rating guidelines (see

table 10), state it is ìuncertainî whether the friction in the right lane of I-35 would have

100 NHTSA estimates that, in 2003, approximately 13 percent of all accidents on the interstate highway

system occurred in wet weather. For further information, see NHTSA, Traffic Safety Facts 2003: A

Compilation of Motor Vehicle Crash Data From the Fatality Analysis Reporting System and the General

Estimates System (Washington DC: U.S. DOT/NHTSA, 2004) 47.
Analysis 48 Highway Accident Report

been flagged as potentially contributing to wet weather accidents, given the pavement

friction test numbers (see table 8). For the left lane, the Illinois guidelines indicate friction

ìmay not be a factorî contributing to wet weather accidents. The Illinois system for wet

weather pavement management is considerably different from the Texas system because it

does not rely on the number of accidents to evaluate and remedy locations that may have

more wet weather accidents, as Texasís WWARP does. Furthermore, Illinois drivers are

exposed to more wet weather (3.8 percent of 2003), yet experience the same percentage of

wet weather accidents (16 percent of all accidents occurred in wet weather in 2003) as

drivers in Texas.

While the FHWA provides limited guidance and one possible method of

identifying and reducing wet weather accidents,101 no additional guidance on best

practices has been developed by the FHWA since the technical advisory Skid Accident

Reduction Program was published in 1980. Thus, the States develop their own

methodologies, which may or may not be adequate. The Safety Board concludes that

Federal guidance on identifying and eliminating locations with wet weather accident

problems is limited; had more comprehensive guidance existed, Texas may have

implemented a more robust WWARP. The Safety Board believes that the FHWA should

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.

FMCSA Exemptions

Central Texas Trails met FMCSR requirements and received a satisfactory rating

after the accident, despite having had three reportable accidents (including the subject

accident) in the previous year. All three of these accidents occurred during intrastate

operation. Though the FMCSRs state that the accident rate is determined by both interstate

and intrastate operations (accidents and mileage), a 2002 FMCSA memorandum prohibits

inspectors from including intrastate crashes or mileage when determining out-of-service

violations during compliance reviews. Thus, according to the FMCSAís compliance

review, the accident rate for this carrier was zero (no interstate accidents in the previous 12

months). Had the three intrastate accidents and mileage for the previous 12 months been

included in the FMCSAís calculations, the accident rate would have been about 2.64 per

million miles traveled, resulting in an unsatisfactory rating in that area and an overall

conditional rating. While a conditional rating would not have prevented Central Texas

Trails from operating and may not have prevented this accident, it may have placed the

carrier under greater scrutiny by the FMCSA or State officials.

Although it is the FMCSAís policy to oversee interstate operations and it is the

Statesí responsibility to oversee intrastate operations, violations recorded in both types of

operation are directly relevant to the overall safety posture of a carrier and thus should be

considered. Regardless of whether a carrier is engaged in interstate or intrastate

101 Technical Advisory T5040.17.
Analysis 49 Highway Accident Report

operation, its vehicles and drivers are often operating on the same roads as the traveling

public and interacting with the same vehicles. Although it did not have an impact on this

accident, the exclusion of intrastate mileage and accidents from the FMCSAís

compliance reviews can lead to an inaccurate representation of a motor carrierís safety

posture. The Safety Board is encouraged that Congress has addressed this issue in the

SAFETEA legislation. 

Survival Aspects

Motorcoach

The passengers within the motorcoach likely sustained their injuries from being

thrown to the right into other passenger seats and into windows and other interior

components as the motorcoach rotated across the median and southbound lanes and

rolled over onto its right side. The passengers found under the motorcoach made contact

with the windows that shattered and broke away and were subsequently ejected during

the motorcoachís overturn. The motorcoachís interior passenger compartment was not

compromised by intrusion during the accident sequence and therefore adequate

survivable space was available. Given the available postaccident survivable space within

the passenger compartment, had passengers remained in their seating areas, their

exposure to injury-causing impacts and to ejection would have been reduced and they

may not have sustained such serious, or fatal, injuries. The headrests on the aisle seats,

which no longer had interior padding, created another possible injury-causing

mechanism inside the motorcoach for nonejected passengers. The Safety Board

concludes that the lack of motorcoach occupant protection systems to retain passengers

in their seating compartments during the accident sequence contributed to passengers

being partially or fully ejected from the motorcoach and to their serious and fatal injuries

due to contact with non-occupant-protected surfaces, other passengers, or surfaces

outside of the motorcoach. The Safety Board will inform the American Bus Association

and the United Motorcoach Association that the padding on the headrests on the aisle

seats can become worn, possibly causing injuries in an accident, and should be inspected

and replaced, as necessary.

The Safety Board has made recommendations to NHTSA in the past regarding the

importance of keeping motorcoach occupants within their seating compartment during

collisions to prevent serious injuries. (See appendix B.) NHTSA has informed the Safety

Board that, in cooperation with Transport Canada, it is conducting research on window

glazing material designed to keep occupants inside the motorcoach. The Safety Board

encourages NHTSA to continue its work on motorcoach occupant protection issues. The

Safety Board will also inform the American Bus Association and the United Motorcoach

Association of the circumstances of this accident and encourage them to reiterate to their

members the importance of providing occupant protection systems.
Analysis 50 Highway Accident Report

Suburban

There was little, if any, survivable space available for the driver and front seat

passenger in the Suburban. The crush damage was so significant that the airbags and

lap/shoulder belts could not have prevented the fatal injuries that were sustained. The

passenger in the rear seat was restrained but did not experience as much crush. The Safety

Board concludes that despite the peak deceleration experienced by the passengers of the

Suburban (about 46.8 g, as found during the simulation),102 the passenger in the rear seat

quite likely sustained serious, rather than fatal, injuries, because of the protection provided

by the lap/shoulder belt and the limited crush that occurred within his seating area.

102 The deceleration recorded by the SDM was about 20 g and averaged over 110 milliseconds. The

deceleration determined by the simulation is an instantaneous peak deceleration. Furthermore, the SDM

stops recording 150 milliseconds after the event, so the full deceleration experienced by the passengers may

not have been recorded if it extended beyond the recording capabilities of the SDM.
51 Highway Accident Report

Conclusions

Findings

1. The driver was neither fatigued nor under the influence of alcohol or performanceimpairing drugs, and, although severely overweight and suffering from hypertension,

the driver had no medical conditions that contributed to the accident.

2. The emergency response was timely.

3. 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.

4. Although a median barrier would not have prevented the motorcoach from leaving the

travel lanes, it may have either redirected the motorcoach or slowed it significantly

before entering the southbound lanes, possibly reducing the severity of the collision

with the Chevrolet Suburban.

5. The wet pavement at the accident site, combined with Interstate 35ís 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.

6. Although the speed limit on Interstate 35 was 70 mph and the design speed was 60

mph, the driver would have had to have been traveling 50 mph or less to avoid the

collision or at least to have reduced its severity.

7. Despite the safety factors that are incorporated into roadway design, roadways with

speed limits exceeding their design speed can constitute a hazard in wet weather.

8. When redesigning a roadway for a higher design speed is not feasible, variable speed

limit signs can be used as a countermeasure to reduce speeds and increase safety.

9. The low macrotexture depth at the accident site prevented proper drainage of the

water from the roadway, allowing excessive water to remain on the road surface,

which, in turn, contributed to its minimal frictional qualities.

10. A rut-depth threshold of 0.50 inch is insufficient because 0.50 inch exceeds the tread

depths of most vehicle tires, and standing water in the ruts can lead to reduced

tire-roadway friction situations in wet weather.

11. The greater rut depth in the left wheel path and the differential friction between the

wheelpaths resulted in a destabilizing counterclockwise rotational force on the

motorcoach.
Conclusions 52 Highway Accident Report

12. The motorcoach did not hydroplane before losing control.

13. The water accumulation on the roadway, likely due to the low macrotexture depth,

rutting, and inadequate cross slope, contributed to the loss of available friction

between the tire and the road.

14. The Texas Department of Transportationís Pavement Management Information

System does not adequately identify roadways where hazardous conditions exist due

to low coefficients of friction and does not expeditiously prioritize these locations for

rehabilitation, increasing the risk for accidents such as the one that occurred at this

location.

15. The minimum tread depths of the drive axle tires, including the smaller tread depth on

the left drive axle tires, particularly in combination with the nearly new front tires,

contributed to wheel lockup and the subsequent rotation of the motorcoach.

16. 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.

17. To prevent accidents similar to this one, commercial vehicle tire-roadway interaction,

which is not currently being considered, must be taken into account when

determining the necessary wet pavement friction of the roadway and tread depth

minimums.

18. Texasís Wet Weather Accident Reduction Program methodology is not sufficiently

refined to measure wet weather accident risk, fails to identify the greater risk of being

involved in a wet weather accident on the segment of Interstate 35 in the vicinity of

the accident, and, generally, does not identify problems in a timely manner.

19. Federal guidance on identifying and eliminating locations with wet weather accident

problems is limited; had more comprehensive guidance existed, Texas may have

implemented a more robust Wet Weather Accident Reduction Program.

20. The lack of motorcoach occupant protection systems to retain passengers in their

seating compartments during the accident sequence contributed to passengers being

partially or fully ejected from the motorcoach and to their serious and fatal injuries

due to contact with non-occupant-protected surfaces, other passengers, or surfaces

outside of the motorcoach.

21. Despite the peak deceleration experienced by the passengers of the Chevrolet

Suburban (about 46.8 g, as found during the simulation), the passenger in the rear seat

quite likely sustained serious, rather than fatal, injuries, because of the protection

provided by the lap/shoulder belt and the limited crush that occurred within his

seating area.
Conclusions 53 Highway Accident Report

Probable Cause

The National Transportation Safety Board determines that the probable cause of

this accident was Texasís decision to set a speed limit on Interstate 35, in the vicinity of

the accident, that did not take into account the roadwayís 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 motorcoachís 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.
54 Highway Accident Report

Recommendations

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)
Recommendations 55 Highway Accident Report

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)

BY THE NATIONAL TRANSPORTATION SAFETY BOARD

MARK V. ROSENKER RICHARD F. HEALING

Acting Chairman Member

ELLEN ENGLEMAN CONNERS DEBORAH A. P. HERSMAN

Member Member

Adopted: July 12, 2005

57 Highway Accident Report

Appendix A

Investigation

The National Transportation Safety Board was notified of the Hewitt, Texas,

accident on February 14, 2003. The Safety Board dispatched an investigative team

consisting of members from the Washington, D.C.; Atlanta, Georgia; and Fort Worth,

Texas, offices. Groups were established to investigate human performance; motor carrier

operations; and highway, vehicle, and survival factors and to conduct on-scene

documentation.

Representatives of the Texas Department of Transportation, the Texas Department

of Public Safety, Central Texas Trails, Motor Coach Industries, and Bendix Commercial

Vehicle Systems, LLC, participated in the investigation.

No public hearing was held; no depositions were taken.
58 Highway Accident Report

Appendix B

Recommendation History

Tire Tread Depth

Luling, TexasóNovember 16, 1980

On November 16, 1980, the Safety Board investigated a motorcoach accident near

Luling, Texas, in which the motorcoach lost traction on the wet pavement and skidded off

the road.1

 The Safety Board determined that the probable cause of the accident was the

low wet cornering capability of the marginal, yet ìlegal,î rear bus tires and the low

frictional quality of the wet pavement, which combined to produce loss of rear tire traction

and vehicle control as the bus was being operated at or near the posted 55 mph speed limit.

As a result of that accident, the Safety Board recommended that the National

Highway Traffic Safety Administration (NHTSA):

H-81-33

Accelerate activity to establish rulemaking action for minimum frictional

quality standards for commercial vehicle tires.

This recommendation was classified ìClosedóUnacceptable Actionî on

August 21, 1986.

The Safety Board also recommended that the Texas Department of Public Safety

(Texas DPS):

H-81-41

Reevaluate tire tread depth inspection criteria that limits the tread depth

criteria to only one tire in each set of dual wheels while permitting that tire

to have less than 2/32 inch of tread depth in the shoulder grooves at the

same time.

On June 20, 1983, the Texas DPS responded that the inspection requirement of

2/32 inch applies to both tires on a dual set of wheels. A December 7, 1983, letter

indicated that the Texas DPS would adhere to the standards prescribed by the U.S.

1 National Transportation Safety Board, East Side Church of Christ Bus Skid and Overturn U.S. Route

183 Near Luling, Texas, November 16, 1980, Highway Accident Report NTSB/HAR-81/04 (Washington,

DC: NTSB, 1981).
Appendix B 59 Highway Accident Report

Standards Institute for Highway Traffic Safety but would continue to study the tread depth

criteria. On March 6, 1995, because of a lack of relevant information from the Texas DPS,

Safety Recommendation H-81-41 was classified ìClosedóUnacceptable Action.î

Beaumont, TexasóMay 4, 1987

On May 4, 1987, a tractor-semitrailer jackknifed on wet pavement near Beaumont,

Texas, crossed the median, and struck a motorcoach, resulting in six fatalities.2 The Safety

Board determined that tractor likely jackknifed or partially hydroplaned because the rear

tractor tires were in poor condition with minimal tread depth and low tire pressures, which

resulted in reduced cornering and braking forces. As a result of that accident, the Safety

Board recommended that the Federal Highway Administration (FHWA):

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.

H-88-2

Issue an On-Guard Bulletin to advise owners, operators, maintenance

personnel, and State commercial vehicle inspectors of the problems

associated with operating vehicles equipped with tires worn below 4/32

inch tread groove depths.

On August 25, 1988, the FHWA responded that, in late 1987, it had identified the

ìhydroplaning of heavy vehiclesî as a high-priority national problem area for research,

which the FHWA believed should provide a basis for determining minimum tread depth

standards. However, the FHWA stated that because of budget constraints, research on

hydroplaning could not begin until 1991. Because of the FHWAís inaction on this critical

safety issue, on May 19, 1989, the Safety Board classified Safety Recommendations

H-88-1 and -2 ìClosedóUnacceptable Action.î No further research was conducted in

1991.

The Safety Board also recommended that the United Bus Owners of America, the

American Bus Association, and the American Trucking Associations, Inc.:

H-88-5

Advise members of the circumstances of the May 4, 1987, accident in

Beaumont, Texas, and the potential vehicle handling problems that may be

encountered with using tires with marginal tread depth.

2 National Transportation Safety Board, Tractor-Semitrailer/Intercity Bus Head-on Collision Interstate

10, Beaumont, Texas, May 4, 1987, Highway Accident Report NTSB/HAR-88/01 (Washington, DC: NTSB,

1988).
Appendix B 60 Highway Accident Report

Both the American Trucking Associations, Inc., and the United Bus Owners of

America provided the information to their members; the recommendation was

consequently classified ìClosedóAcceptable Actionî for the American Trucking

Associations, Inc., on July 25, 1988, and for the United Bus Owners of America on

June 28, 1988. There is no record of a response from the American Bus Association.

Pavement Friction

The Safety Board has made numerous recommendations regarding pavement

surface conditions and pavement friction.

Bethesda, MarylandóOctober 11, 1975

On October 11, 1975, a motorcoach lost traction on a wet roadway near Bethesda,

Maryland.3

 The vehicle began to slide and then rolled over, injuring 26 occupants. As a

result of its accident investigation, the Safety Board recommended that the FHWA:

H-76-24

Establish minimum skid resistance values both for newly constructed and

for existing pavement surfaces. Such minimum values must provide an

acceptable margin of safety to accommodate all vehicle types under normal

as well as predictable emergency maneuvering, and should consider the

known varieties of commercial tire rubber compounds and the relationship

of design speed and highway geometrics. After the minimum skid

resistance values are determined, revise applicable highway design and

pavement maintenance manuals accordingly.

The FHWA responded that skid resistance is an extremely complex and technically

controversial subject, noting that States must individually evaluate pavement design and

construction and maintenance practices to ensure that skid resistance properties are

suitable for the traffic and must establish procedures to identify and correct skid-prone

locations. Further, the FHWA wrote that it could not correlate skid test results between

States; thus, evaluating skid resistance nationally is not feasible. Safety Recommendation

H-76-24 was ìClosedóSupersededî on October 7, 1976, by Safety Recommendations

H-80-23, -52, and -56, issued as a result of the Safety Boardís 1980 skid resistance study.4

3 National Transportation Safety Board, Special InvestigationóMetropolitan Coach Corporation,

Charter Bus Accident, Bethesda, Maryland, October 11, 1975, Highway Accident Report

NTSB/HAR-76/06 (Washington, DC: NTSB, 1976).

4 National Transportation Safety Board, State Highway Skid Resistance Programs, Safety

Effectiveness Evaluation NTSB/SEE-80/06 (Washington, DC: NTSB, 1980).
Appendix B 61 Highway Accident Report

Also as a result of the Bethesda accident investigation,5 the Safety Board

recommended that NHTSA:

H-76-25

Compare frictional coefficients obtained with a commercial vehicle tire to

that obtained with an ASTM E-274 skid-test tire and publish findings.

Also, determine whether there is a greater tendency for commercial truck

and bus tires than passenger car tires to lose traction on wet pavement.

H-76-26

Develop a Federal Motor Vehicle Safety Standard to require a minimum

frictional coefficient for all commercial motor vehicle tires.

NHTSA responded that it had completed the testing and found significant

differences in design and load ratings between commercial vehicle and passenger car tires;

further, although commercial tires displayed no greater tendency to lose traction on wet

pavement, a greater tendency to lose traction does exist on dry pavement or ice. As a

result, Safety Recommendation H-76-25 was classified ìClosedóAcceptable Responseî

on January 2, 1986.

NHTSA also responded that it concurred with Safety Recommendation H-76-26

and that, although it was addressing higher priority items, it would continue to collect data

on truck tire traction. Based on NHTSAís response, Safety Recommendation H-76-26 was

classified ìClosedóAcceptable Alternate Actionî on April 5, 2001. No standards were

developed.

Scipio, UtahóAugust 26, 1977

On August 26, 1977, near Scipio, Utah, a van and truck tractor-semitrailer collided

head-on during a moderate-to-heavy rainstorm, killing eight passengers in the van.6

 The

Safety Board recommended that NHTSA:

H-79-6

Examine the full potential effect of fluctuating and progressively lower

pavement frictional quality on vehicle performance.

On April 20, 1979, NHTSA responded that it had an ongoing effort to develop data

on commercial tire traction characteristics. On July 15, 1991, NHTSA wrote that it would

explore options for implementing a task forceís recommendation that a standardized

measurement procedure be developed, noting that full-scale testing could then be

5 NTSB/HAR-76/06.

6 National Transportation Safety Board, Osterkamp Trucking, Inc., Truck/Full Trailer and Dodge Van

Collision, U.S. 91, Near Scipio, Utah, August 26, 1977, Highway Accident Report NTSB/HAR-79/01

(Washington, DC: NTSB, 1979).
Appendix B 62 Highway Accident Report

conducted to establish a database of pertinent tire characteristics that could serve as input

to vehicle dynamics models and as benchmarks for vehicle component design. Based on

this response, Safety Recommendation H-79-6 was classified ìClosedóAcceptable

Actionî on July 15, 1991.

As a result of the same accident, the Safety Board recommended that the FHWA:

H-79-7

Evaluate the procedures used in the Safety Boardís investigation of this

accident [testing friction of both left and right wheel tracks] for possible

inclusion in FHWA guidelines for determining the frictional quality of

pavements during pavement inventory programs.

The FHWA responded on September 19, 1979, that it uses the ASTM E-274 test

procedure, which calls for the testing of the center of the left wheel track to provide a skid

number, and that testing both wheel tracks is impractical. On February 12, 1980, the

FHWA wrote that it had not found evidence that different friction levels between the left

and right wheel tracks causes a significant increase in accidents. The FHWA stated that it

would advise the States it favors having skid testers equipped to test both wheel tracks and

that such testing is advisable when investigating high-accident sites. Safety

Recommendation H-79-7 was classified ìClosedóAcceptable Actionî on April 4, 1980.

Skid Accident Prevention

State Highway Skid Resistance Programs Studyó1980

In 1980, the Safety Board concluded a nationwide study on improvements in skid

resistance standards.7

 The study identified the magnitude, location, and characteristics of

fatal accidents on wet pavement by analyzing national accident and weather data. The data

indicated that fatal accidents occur about four times more often on wet pavement than

might be expected. The data also identified States with a significantly higher than

expected percentage of fatal accidents on wet pavement. As a result of its study, the Safety

Board recommended that the FHWA:

H-80-21

In conjunction with the National Oceanic and Atmospheric Administration,

provide weather data to Federal, State, and local agencies, and promote and

use these data to reduce accidents on wet pavement.

7 National Transportation Safety Board, State Highway Skid Resistance Programs, Safety

Effectiveness Evaluation NTSB/SEE-80/06 (Washington, DC: NTSB, 1980).
Appendix B 63 Highway Accident Report

H-80-22

Promote further research into the relationship of wet-pavement accidents

(1) to low void ratios in pavement surface mixes, (2) to highway

construction materials, and (3) to artificial light conditions.

H-80-23

Test the use of the Wet Fatal Accident Index (WFAI) as an aid to identify

and evaluate State programs aimed at reducing accidents on wet pavement.

The FHWA responded on August 18, 1981, to all three safety recommendations.

The FHWA stated that it believed decisions on the source and use of weather data rested

with State officials. The FHWA further stated that it was investigating pavement surface

texture parameters to determine whether they could be used to better describe skid

resistance characteristics, noting that once the parameters of microtexture and

macrotexture were more definitively understood, they could be better correlated with wet

pavement accidents. The FHWA also stated that several other pavement surface texture

studies were ongoing. The FHWA further wrote that the States could develop their own

programs to identify high-accident locations, using the WFAI or a similar program. Based

on the FHWAís response, Safety Recommendations H-80-21 and -22 were classified

ìClosedóAcceptable Action,î and Safety Recommendation H-80-23 was classified

ìClosedóAcceptable Alternate Actionî on November 13, 1981.

In its 1980 skid resistance study,8 the Safety Board also evaluated the safety

effectiveness of a sample of State highway programs. As a result, the Safety Board

recommended that the FHWA:

H-80-52

Develop program objectives for comprehensive wet weather skid

resistance programs that can be used to both guide and evaluate State

programs.

The FHWA responded on April 26, 1982, that it had supplied a technical advisory to

the States, Skid Accident Reduction Program. Based on this action, Safety Recommendation

H-80-52 was classified ìClosedóAcceptable Actionî on January 19, 1983.

Also as a result of its skid resistance study,9

 the Safety Board recommended that

the FHWA:

8 NTSB/SEE-80/06.

9 NTSB/SEE-80/06.
Appendix B 64 Highway Accident Report

H-80-54

Issue a revised Federal-Aid Highway Program Manual (FHPM 6.2.4.3)

which promotes (1) full width surface treatments, (2) skid trailers with left

and right wheel locking capabilities, (3) skid testing at the posted speed

limit, as proposed in the FHWA NPRM ìSkid Resistance Pavement

Surface Design,î (4) evaluation of the skid resistance properties of all

newly developing surface treatments, and (5) increased Federal

participation on skid resistance projects.

H-80-55

To assure comprehensive, coordinated skid resistance programs, promote

further research to examine (1) the measurement of rutting and its effects

on wet pavement accidents; (2) more effective signing system to advise

motorists of safe speeds on slippery, rutted, or poorly drained wet surfaces

and on all new surfaces; (3) use of tire tread depths more representative on

those used by motorists to measure skid resistance; and (4) the effect on

skid resistance of immediately allowing heavy truck traffic on newly

constructed or newly overlayed surfaces.

H-80-56

Develop a program to enhance dissemination of and the sharing among

States of skid resistance information. Elements of the program should

include: (1) the compilation of an instructional text for a state-of-the-art

manual for Federal, State, local, and county agencies; (2) periodic regional

meetings to review skid resistance research and successful operating

programs; and (3) periodic publication of a description of State programs

and current research studies on skid resistance.

The FHWA responded to these recommendations on September 13, 1985, after

conducting a survey of the States on skid resistance. The FHWA stated (1) that it had

issued a memorandum to field offices advising action to eliminate partial width

resurfacing, (2) that it recommends both wheel paths be tested when evaluating highaccident sites, (3) that it believes skid testing in accordance with ASTM Standard E274 at

40 mph is a reliable tool for inventory purposes and identifying skid-prone locations, (4)

that current policy and guidance indicate the States should evaluate their pavements, and

(5) that since Federal-Aid participation is dictated by law, situations outside of FederalAid participation would require the FHWA to recommend to Congress that ìslippery

pavementsî be given priority over other safety improvement programs. Based on this

response, Safety Recommendation H-80-54 was classified ìClosedóAcceptable

Alternate Action,î and Safety Recommendation H-80-56 was classified ìClosedó

Supersededî by Safety Recommendation H-82-34, issued as a result of the Safety Boardís

1980 study on wet pavement accidents.10

10 National Transportation Safety Board, Fatal Highway Accidents on Wet Pavement, the Magnitude,

Location, and Characteristics, Highway Safety Study NTSB/HSS-80/01 (Washington, DC: NTSB, 1980).
Appendix B 65 Highway Accident Report

The FHWA also responded on September 13, 1985, that (1) the measurement of

rutting on a large-scale basis had just become possible and testing was underway, with

planned completion in March 1986; (2) the development of signing systems had reached

an impasse because of the cost and reliability of such systems; (3) truly representative tire

depths can change rapidly, and by using testing standards for both ribbed and smooth tires,

the pavementís macrotexture and microtexture can be measured; and (4) no evidence

exists that heavy traffic on fresh asphalt pavement causes skidding problems. On May 18,

1988, the FHWA wrote that a system had been developed and tested to inventory roadway

topography and the extent of water ponding and depth of flow at various rainfall rates. The

letter also stated that the FHWA was sponsoring a research study to evaluate a prototype

variable speed limit sign. Consequently, on May 18, 1988, Safety Recommendation H-80-

55 was classified ìClosedóAcceptable Action.î

Luling, TexasóNovember 16, 1980

Recommendations regarding pavement condition were also issued as a result of

the Luling, Texas, accident investigation.11 The Safety Board recommended that the

FHWA:

H-81-39

Evaluate Pennsylvania Department of Transportation policies for the

placement of ìSlippery When Wetî signs and the detection and correction

of potential wet pavement problem locations for national policy purposes.

On November 17, 1982, the FHWA responded that it had reviewed documents on

Pennsylvaniaís policies for improving sites with low skid resistance and that they

satisfactorily implement the FHWAís advisory on the elements of a skid accident

reduction program. Based on this action, Safety Recommendation H-81-39 was classified

ìClosedóAcceptable Actionî on January 27, 1983.

Notice of Proposed Rulemaking, ìSkid Resistant Pavement 

Surface Designî

On March 4, 1982, the FHWA announced the withdrawal of its Notice of Proposed

Rulemaking (NPRM) on ìSkid Resistant Pavement Surface Design.î This rulemaking,

initiated in November 1977, solicited comments on the broad issue of skid-resistant

pavement surfaces and wet pavement accident reduction. In April 1980, the scope of the

rulemaking was narrowed to a few areas with the potential to significantly reduce

accidents on wet pavement. In part, the rulemaking would have required aggregates from

acceptable prequalified sources, analysis of wet pavement data, and, as stated in the

NPRM, ìperiodic review of State highway agency practices relating to skid resistant

pavement surfacesî to ensure that the skid-resistant pavement surfaces were maintained.

11 NTSB/HAR-81/04.
Appendix B 66 Highway Accident Report

In response to the withdrawal, the Safety Board recommended in its 1980 study on wet

pavement accidents12 that the FHWA:

H-82-34

Conduct and publish a comprehensive review of each Stateís Skid Accident

Reduction Program to identify problem areas, to develop corrective

recommendations where necessary, and to disseminate more widely

innovative local practices of proven value and general applicability.

The Safety Board classified Safety Recommendation H-82-34 ìClosedó

Acceptable Actionî on July 28, 1988.

Motorcoach Occupant Protection Systems

The Safety Boardís 1999 bus crashworthiness study13 found that one of the primary

causes of preventable injury in motorcoach accidents was occupant motion out of the seat

during a collision. As a result of this study, the Safety Board recommended that NHTSA:

H-99-47

In 2 years, develop performance standards for motorcoach occupant

protection systems that account for frontal impact collisions, side impact

collisions, rear impact collisions, and rollovers.

H-99-48

Once pertinent standards have been developed for motorcoach occupant

protection systems, require newly manufactured motorcoaches to have an

occupant crash protection system that meets the newly developed

performance standards and retains passengers, including those in child

safety restraint systems, within the seating compartment throughout the

accident sequence for all accident scenarios.

On October 27, 2000, NHTSA responded that it had initiated a research plan in

conjunction with motorcoach manufacturers to support the motorcoach crashworthiness

recommendations, which were subsequently classified ìOpenóAcceptable Response,î on

April 18, 2001. On August 28, 2001, following the Boardís investigation of a motorcoach

accident in New Orleans, Louisiana,14 in which 10 of the 22 passengers who died had been

ejected from the bus, the Safety Board reiterated Safety Recommendations H-99-47 and

-48 to NHTSA. In its report on the New Orleans accident, the Safety Board noted that it

12 NTSB/HSS-80/01.

13 National Transportation Safety Board, Bus Crashworthiness Issues, Highway Special Investigation

Report NTSB/SIR-99/04 (Washington, DC: NTSB, 1999).

14 National Transportation Safety Board, Motorcoach Run-off-the-Road, New Orleans, Louisiana,

May 9, 1999, Highway Accident Report NTSB/HAR-01/01 (Washington, DC: NTSB, 2001).
Appendix B 67 Highway Accident Report

had identified occupant protection issues similar to those discussed in the Bus

Crashworthiness Issues report. On March 6, 2002, NHTSA advised the Safety Board that

it helped organize the Bus Manufacturers Council to create industry-wide standards to

enhance motorcoach safety. NHTSA also stated that it had sponsored a motorcoach public

safety meeting on April 30, 2002. However, by the time the Safety Boardís report on the

Victor, New York, accident15 had been adopted on June 22, 2004, little progress had been

made on these recommendations, and they were again reiterated.

In a June 3, 2004, meeting, NHTSA reported that it is currently focusing on roof

crush and window retention technology to help prevent ejections. In June 2004, Transport

Canada issued a report on motorcoach window retention and the forces generated by

occupants on window retention. In an October 5, 2004, letter, NHTSA stated it is building

upon the results of Transport Canadaís report and is developing a program to test

motorcoach windows and the surrounding structure to determine feasible occupant

protection improvements using advanced glazing materials and bonding techniques, as

well as evaluating the structureís role in window retention. NHTSA expects to complete

the research by September 2006. Therefore, on March 18, 2005, the Safety Board

classified Safety Recommendations H-99-47 and -48 ìOpenñAcceptable Action,î pending

development of industry-wide standards for occupant protection.

FMCSA and Surface Deployment and Distribution Command 

Inspections

On June 23, 2002, near Victor, New York, a motorcoach, operated by Arrow Line,

Inc. (Arrow), departed the roadway and crashed, killing 5 passengers and injuring 41.16

Arrow had undergone inspections by the Military Traffic Management Command

(MTMC)17 and serious violations and operational deficiencies were noted, yet this

information was not transmitted to the FMCSA, nor was it required to be. Consequently,

Arrow continued to operate with a satisfactory rating from the FMCSA. As a result of the

investigation, the Safety Board recommended that the FMCSA:

H-04-20

Utilize motor carrier safety information, including results of compliance

audit reports provided by the U.S. Department of Defense Surface

Deployment and Distribution Command, to determine whether further

review of a motor carrier is warranted.

15 National Transportation Safety Board, Motorcoach Run-off-the-Road and Overturn, Victor, New

York, June 23, 2002, Highway Accident Report NTSB/HAR-04/03 (Washington, DC: NTSB, 2004).

16 NTSB/HAR-04/03.

17 Now the Surface Deployment and Distribution Command (SDDC).
Appendix B 68 Highway Accident Report

The Safety Board also recommended that the SDDC:

H-04-21

Provide motor carrier information, including timely results of passenger

carrier inspection processes and ratings, to the Federal Motor Carrier

Safety Administration.

On October 13, 2004, the FMCSA replied that the SDDC now forwards to the

FMCSA copies of all compliance audits for carriers with the lowest ratings, which would

indicate serious safety violations. FMCSA headquarters staff review these compliance

audits and forward them to the appropriate field offices for investigation. The SDDC sent

a similar response on August 23, 2004, noting that it is coordinating with the FMCSA to

meet periodically to discuss inspection criteria and further promote communications

between the two agencies. Based on these actions, the Safety Board classified Safety

Recommendation H-04-20 ìClosedóAcceptable Actionî on April 1, 2005, and Safety

Recommendation H-04-21 ìClosedóAcceptable Actionî on February 15, 2005.
69 Highway Accident Report

Appendix C

Tire Friction Testing Results

Tire Minimum 

tread 

depth

Velocity 

(mph)

Water 

depth 

(inches)

Longitudinal 

coefficient of 

friction

Peak 

longitudinal 

friction

Average 

peak 

lateral 

friction

Cornering 

stiffness 

(lbs/deg)

Cornering 

stiffness 

coefficient*

Left 

front

14/32 60 0.02 - - - - -

60 0.11 0.27 0.52 0.46 888 0.161

60 0.19 - - - - -

Right 

front

15/32 40 0.02 0.53 0.68 0.74 888 0.181

40 0.11 0.25 0.54 0.70 914 0.166

40 0.19 - - 0.70 998 0.162

60 0.02 0.30 0.65 0.61 936 0.170

60 0.11 0.29 0.48 0.54 782 0.142

60 0.19 0.28 0.49 0.46 706 0.128

70 0.02 0.24 0.57 0.51 891 0.162

70 0.11 0.25 0.41 0.40 686 0.125

70 0.19 0.22 0.40 0.29 617 0.112

Left 

outer 

drive

3/32 40 0.02 0.26 0.64 - - -

40 0.11 0.25 0.54 - - -

40 0.19 0.25 0.55 - - -

60 0.02 - - - - -

60 0.11 0.12 0.21 0.23 542 0.100

60 0.19 - - - - -

70 0.02 - - - - -

70 0.11 - - - - -

70 0.19 - - - - -

Left 

inner 

drive

2/32 40 0.02 - - 0.54 1099 0.204

40 0.11 0.24 0.60 0.49 976 0.181
Appendix C 70 Highway Accident Report

Tire Minimum 

tread 

depth

Velocity 

(mph)

Water 

depth 

(inches)

Longitudinal 

coefficient of 

friction

Peak 

longitudinal 

friction

Average 

peak 

lateral 

friction

Cornering 

stiffness 

(lbs/deg)

Cornering 

stiffness 

coefficient*

40 0.19 0.28 0.57 0.48 961 0.178

60 0.02 0.15 0.37 0.31 734 0.136

60 0.11 0.12 0.27 0.24 489 0.091

60 0.19 0.10 0.24 0.20 494 0.091

70 0.02 0.12 0.26 0.20 571 0.106

70 0.11 0.09 0.17 0.12 280 0.052

70 0.19 0.10 0.14 0.09 217 0.040

Right 

outer 

drive

6/32 40 0.02 0.27 0.59 - - -

40 0.11 - - - - -

40 0.19 - - - - -

60 0.02 - - - - -

60 0.11 0.12 0.23 0.26 569 0.105

60 0.19 - - - - -

70 0.02 - - - - -

70 0.11 - - - - -

70 0.19 - - - - -

Right 

inner 

drive

5/32 40 0.02 0.29 0.66 0.55 1059 0.196

40 0.11 0.28 0.58 0.50 936 0.173

40 0.19 0.27 0.59 0.50 932 0.173

60 0.02 0.16 0.34 0.30 774 0.143

60 0.11 0.13 0.27 0.24 546 0.101

60 0.19 0.12 0.23 0.16 559 0.104

70 0.02 0.12 0.24 0.22 631 0.117

70 0.11 0.09 0.18 0.15 352 0.065

70 0.19 0.09 0.17 0.09 260 0.048

*The cornering stiffness coefficient was derived as the tire turned from 0 to 1 degree.