1975_美国_HAR7505.pdf

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NATIONAL TRANSPORTATION SAFETY BOARD

WASHINGTON, D.C. 20594

HIGHWAY ACCIDENT REPORT

FRANCISCO FLORES TRUCK/PICKUP TRUCK WITH CAMPER AND TRAILER COLLISION, U.S. ROUTE 395, BISHOP, CALIFORNIA JUNE 29, 1974

REPORT NUMBER: NTSB-HAR-75-5

UNITED STATES GOVERNMENT

〔图〕STORAATIONALBOARDSAFET\n 〔图〕。.\n 〔图〕插图 〔图〕插图 〔图〕插图

ERRAIUM NATIONAL TRANSPORTATION SAFETY BOARD

〔图〕STORTAHATIONALSAFENBOARD\n WASHINGTON, D.C. 20594

HIGHWAY ACCIDENT REPORT - NTSB-HAR-75-5. Francisco Flores Truck/Pickup Truck with Camper and Trailer Collision, U.S. Route 395, Bishop California, June 29, 1974.

Please make the following change to subject report: Page 24, Change 4th paragraph to read:

"It is noted that the Manual of Uniform Traffic Control Devices does not address the class of suggested signs, nor does it specifically address the issue of long grades and appropriate signing configurations. There is precedent in the Manual in other areas --e.g., approaches to weigh stations, railroad cross-ings, closely spaced interchanges. The State of Colorado has developed signing configurations advis-ing of the slope and extent of remaining grade. 5/"

October 9, 1975

UNITED STATES GOVERNMENT

TECHNICAL REPORT DOCUMENTATION PAGE

TABLE OF CONTENTS

FOREWORD

The accident described in this report has been designated a major accident by the National Transportation Safety Board under the criteria established in the Safety Board's regulations.

This report is based on facts obtained from an investigation performed by the Safety Board. Cooperation in the investigation was received from the Bishop Area California Highway Patrol and California Department of Transportation, the Federal Highway Administration (Bureau of Motor Carrier Safety), and American Steel Foundries, Trailmobile, and White Freightliner.

The conclusions, the determination of probable cause, and the recommendations herein are those of the Safety Board.

SS-H-37

NATIONAL TRANSPORTATION SAFETY BOARD

WASHINGTON, D.C. 20594

HIGHWAY ACCIDENT REPORT

Adopted: August 14, 1975

FRANCISCO FLORES TRUCK/PICKUP TRUCK WITH CAMPER AND TRAILER COLLISION, U.S. ROUTE 395, BISHOP, CALIFORNIA, JUNE 29, 1974

SYNOPSIS

At approximately 12:50 a.m., P.d.t., on June 29, 1974, a tractor-semitrailer loaded with lumber was descending Sherwin Pass, a steep, long grade, at a speed of 35 mph, when the unit suddenly began to accelerate.

The tractor-semitrailer remained on the roadway for an additional 2 to 2½ miles of downhill travel. As it was in the process of negotiating a curve, the semitrailer separated from the tractor and overturned in front of a pickup/camper/trailer traveling in the opposite direction. All seven occupants of the pickup/camper were killed. The tractor-semitrailer driver received minor injuries.

The National Transportation Safety Board determines that the probable cause of this accident was the loss of braking which permitted the tractor-semitrailer to run away. The loss of braking effectiveness was caused by the improper adjustment of the brakes by the driver, the selection of too high a gear for descent, and a weight overload of the truck.

FACTS

The Accident

At about 12:50 a.m., P.d.t., on June 29, 1974, a tractor-semitrailer owned by Francisco Flores (hereinafter referred to as the truck) was southbound on Route 395, descending Sherwin Pass, a steep, long grade. The truck, en route from Susanville, California, to Los Angeles, California, was loaded with approximately 54,000 pounds of 2 x 10-inch finished lumber. The driver stopped at a parking area near the summit of Sherwin Pass, in order to adjust his brakes. (See Figure 1.)

The driver reported that he began his descent in seventh gear in reaction to the "Hill, Trucks Use Low Géars" and "Speed Limit 35 mph, Trucks" signs posted at the truck parking area. After the truck traveled 3 miles downhill from the truck parking area and began to descend a 2½-mile straight section of grade, other traffic caught up with it. Witnesses in these vehicles followed behind at a speed of 35 mph until the truck suddenly began to accelerate, at a point near the end of the straight section of grade.

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〔图〕SUMMIT120TruckSCALE IN MILES:Parking Area2Area where Automobileand other Tractor-SemitrailerNcaught up with Truckinvolve

Figure - 1

PLAN VIEW MAP OF SHERWIN PASS KEY POINTS IN ACCIDENT SEQUENCE

〔图〕102SCALE IN MILES2e AutomobileTractor-Semitrailerwith Truckn accidenSPEED LIMIT35 M.P.H.Truck SignArea where Truckbegan 〔图〕*ACCIDENT SITEBeginning of Straightand Level Roadway\n

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The truck managed to remain on the roadway for approximately 2 to 2½ miles of additional downhill travel, but forced at least one vehicle, traveling in the opposite direction, off the roadway. As the truck, traveling at an estimated speed of 110-125 mph, was in the process of negotiating a curve to the right, the semitrailer separated from the tractor and overturned in front of a pickup/camper/trailer (hereinafter termed the pickup), traveling in the opposite direction.

The front of the pickup was crushed rearward to the cab. The pickup burst into flame at impact; three occupants remained in the cab and were burned. Four occupants, presumably in the rear integral camper section of the pickup, were ejected and assailed by flying lumber. All seven occupants were killed. The truckdriver remained in the tractor and received minor injuries.

Accident Site

Sherwin Pass downgrade, where the accident occurred, was an approximately 10-mile section of U. S. Route 395, at a 6-percent grade for most of its length along the southern slope of Sherwin Hill. (See Figure 1.) The summit was located approximately 21 miles to the north of Bishop, California. The roadway was relatively curved and followed the rolling terrain from the summit to the beginning of the designated truck parking area for southbound traffic -- a distance of about 1 mile.

A "Truck Parking Area" sign identified this area, and two other signs -- "Hill, Trucks Use Low Gears" and "Speed Limit 35 mph, Truck," -- were posted in this area.

At the end of the truck parking area, the roadway changed from relatively level to a 6-percent grade. For the next 3 miles, Sherwin Pass was primarily a 2-lane, 2-way, relatively curved section of highway.

After this 3-mile section, the highway was straight for about 2/2 miles. The grade over this straight section was at 6 percent for the first mile, and fluctuated between 4.9 percent and 6 percent over the remaining portion. A "Speed Limit 35 mph, Trucks" sign was posted 1.9 miles from the beginning of the straight section, in the vicinity of the point at which the truck began to accelerate.

After the 2₂-mile section of straight roadway, the roadway was relatively curved, and the downgrade varied between 3.3 and 6 percent for the next two-thirds of a mile. After this, there was an approximately 1-mile section of straight roadway at a grade of 6 percent, leading to the curve where the accident occurred. This curve was to the downhill driver's right, 1,250 feet in length, with a radius of 2,000 feet and a maximum superelevation of 0.07. According to the California Department of Transportation, the design speed for the curve was 75 mph. (See Figure 2.)

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〔图〕以\n

Figure 2. Photo taken at end of straight section and beginning of curve where accident occurred.

There were two 13-foot-wide traffic lanes separated by a dashed white/yellow centerline, which was badly worn. There were 2-foot wide paved shoulder areas on each side of the roadway, separated from the traffic lanes by white edge lines, which were in good condition. The downhill side of the roadway had a 4-inch-high asphalt slip curb bounding the outside edge of the paved shoulder. The terrain beyond the shoulder area for downhill traffic was relatively flat but unimproved, with numerous depressions and loose rocks.

There were no signs along Sherwin Pass to advise of the steepness or length of the grade ahead. The roadway geometry appeared to have been blended with the natural features of the surrounding terrain with a minimum of excavation required. As a result, off-road escape avenues were limited since the terrain was at almost the same grade as the roadway and would offer no restraining feature. There were no purposely constructed escape areas or auxiliary roads that could serve as an escape road.

According to the California Department of Transportation, from 1969 to 1973, there were 93 accidents on Sherwin Pass, 4 of which resulted in fatalities. The accident rate for this period was 1.69 accidents per million vehicle miles as compared to the statewide average of 2.61 for 2-lane roads. There were 4 accidents, including this accident, in which brake failure was alleged.

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Environmental Factors

On the night of the accident, the weather was clear with no precipitation or fog; the road was dry. The moon was full and high above the horizon. Night tests, taken after the accident and under similar conditions, indicated that the moon did not significantly aid in the observation of items beyond the range of headlights.

Vehicles

Tractor -- The tractor was a 1966 White Freightliner, 3-axle cabover-engine with sleeper compartment. It was equipped with a turbo-charged 250 Cummins diesel engine, a Fuller 10-speed Model RT 910 transmission, and SQHD Timken rear axles with a 4.625-rear axle ratio. The tractor had standard air brakes with Type 30 chambers on the rear axles; the front wheels were not equipped with brakes. The frame had been lengthened, extending the vehicle wheelbase, prior to its purchase by the driver. The fifth wheel and slide were manufactured by American Steel Foundries and Prior, respectively.

Exterior damage to the front of the vehicle primarily involved steering, suspension, and frame components as a result of forces imposed to the right front corner through the bumper and front wheel area. (See Figure 3.)

〔图〕插图

Figure 3. Front view of tractor. Note attitude of vehicle and damage in area of right front wheel.

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There was no significant intrusion of the occupant compartment and no occupant contact points were noted inside the cab. No evidence of contact with the pickup was observed.

The transmission tower casting, attached to the vehicle's left side rail, was split in half. Corresponding damage was noted to the bottom right side of the slot in the cab floor, which passes over the top of the tower and shift lever when the cab is lowered into the frame.

Minor flange buckling of the side rails was evident just to the front of the fifth-wheel slide brackets. Several straight bolts used to fasten the front of the rear tandem axle suspension to the frame were determined to have been missing before the accident. Rust had formed along the inside edges of these holes. These bolts secure the suspension, the frame, and a crossmember. The two rear frame crossmembers were loose at the upper mounting bolts, allowing the frame to move.

The right side horizontal trunnion pin for the fifth-wheel assembly could not be found after the crash. Two horizontal trunnion pins are used to provide a pivot point and thereby vertical articulation between the fifth-wheel plate and the slide. (See Figures 4 and 5.) These pins have a calculated shear strength of 84,000 pounds single shear. Each pin is secured by a cotter pin with a calculated shear strength of 12,000 pounds double shear. Both horizontal trunnion pin holes were polished. The left side trunnion pin was bent slightly.

Four vertical set pins are used to set the fore-and-aft relationship of the fifth-wheel slide assembly on the tractor chassis. (See Figure 6.) The right rear vertical set pin could not be found after the crash.

Both fifth-wheel slides were distorted. The fifth-wheel and the left slide were found attached together and to the trailer at its final position. The left slide had been pried open to an attitude of 40° along the full length of the plate. (See Figure 4.) The right slide was found detached but near the tractor at final position. The right slide had been pried open to an attitude of 40° at the front of the slide. (See Figure 7.) While there was some distortion at the rear of the slide, the top and bottom of the slide were relatively parallel.

All components associated with the steering system appeared to be normal except for accident-induced damage.

The tractor rear tandem axle brake adjustments (slack adjuster travel) were:

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〔图〕FIFTH-WHEEL PLATEVNOZOH137NId NOINNN1\n

Figure 4. Fifth-wheel assembly. View of left horizontal trunnion pin.

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〔图〕插图

Figure 5. Fifth-wheel assembly, as attached to semitrailer postcrash. Note missing right horizontal trunnion pin and left slide plate.

〔图〕Note missing right rear vertical set pin.e 6. Right rear of tractor.\n

Figure 6. Right rear of tractor. Note missing right rear vertical set pin.

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〔图〕FRONTREAR\n

Figure 7. Right side, fifth-wheel slide. Note character of damage to front and rear.

The maximum stroke capability of Type 30 brake chambers is $2\frac{1}{2}$ inches. The cam bushings, anchor pins, hoses, and slack adjusters were in good mechanical condition. The tractor air system was checked by an air test and appeared to be operating normally with no air loss detected.

The engine valve covers for cylinders 1, 2, 5, and 6 were removed. No damage was noted to the valve train that would be produced from over-speeding the engine. Rocker arms, valve stems, and push rods were in place and undamaged.

The transmission was found in neutral at final rest. No unusual damage was noted with respect to internal transmission components.

Semitrailer -- The semitrailer was a 2-axle, 40-foot flatbed, 1972 Trailmobile, owned by Heavy Haul Truck Service, Incorporated. It also had standard air brake components with Type 30 chambers. (See Figure 8.)

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〔图〕插图

Figure 8. Semitrailer of truck involved in accident (partially reloaded after accident).

Exterior damage was limited to patterns of scraping marks. A tire smudge was noted along the bottom of the trailer bed, 6 inches in from the left side and 33 inches to the rear of the kingpin, suggesting that the bottom of the trailer was in contact with one of the tractor tires at upset. The trailer brake adjustments (slack adjuster travel) were:

The trailer air system was checked by an air test and no air loss was detected. Visual observation indicated that the braking system had been severely overheated.

The shipper advised that the trailer was loaded with 36,253 board feet of kiln-dried white fir lumber, weighing 1,500 pounds per 1,000 board feet for a total payload weight of 54,379 pounds.

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According to the weight limitations set forth in section 35551 of the California Motor Vehicle Code, the truck was 6,720 pounds over the legal limit. The total weight of the tractor (15,000 pounds) with its modifications (1,000 pounds estimated), the semitrailer (9,620 pounds), and the cargo (54,379 pounds) was determined to be 80,000 pounds; the California legal limit was 73,280 pounds.

Pickup/Camper/Trailer -- This unit consisted of a 1964 GMC pickup truck with an Alaskan camper shell attached to the rear bed of the pickup, pulling a utility trailer made from a Toyota pickup truck body.

Exterior damage involved extensive rearward deformation to the front of the pickup as a result of a head-on collision with the lumber from the truck. Fire had consumed all flammable material on the pickup and trailer. (See Figures 9 and 10.) The camper section was separated and projected from the pickup truck body during the collision. The collision was unsurvivable from an energy management standpoint. Physical evidence and the circumstances of the accident indicated no necessity for attempting a detailed mechanical inspection.

〔图〕插图

Figure 9. Front view of pickup.

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〔图〕插图

Figure 10. Rear view of pickup. (Trailer is in bed of pickup.)

The Truckdriver

The 25-year-old driver was the owner of the tractor unit. He worked under an owner-operator lease arrangement with Heavy Haul Truck Service, Inc., located in Santa Fe Springs, California. Heavy Haul provided the semitrailer authority(s) to operate, and hauling assignments. He purchased the tractor unit secondhand and had operated it for 11st months prior to the accident.

The driver held a valid Class 1 California chauffer's license with a medical examiner's certificate, issued on March 25, 1973. He obtained his first driver's license when he was 20 years old. His known traffic record included violations for following too close and not keeping a daily log readily available, both received when he was driving a truck. There was no official record of any traffic accidents.

The driver reported approximately 2 years of experience in driving trucks of various sizes, during which he traveled about 150,000 miles. A friend taught him to drive trucks, and after he discovered that he could not obtain a commercial license on his own, he enrolled in a truck driver's training school. Within approximately 2 months, he had completed the school, obtained his license, bought his tractor unit, and began to

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operate on an interstate basis for the first time. He had operated on this basis for about 10 months and had accumulated 100,000 miles of driving experience before the accident.

An inspection of the driver's schedule of activities from June 24 to 28, 1974, indicated violations of the following Federal Motor Carrier Safety Regulations: (1) 6/26/74, false daily log -- slept in tractor while log shows off-duty; (2) 6/27/74, false daily log -- log shows less time than what it took to make trip; (3) 6/28/74, driver was hauling interstate rather than authorized intrastate; and (4) 6/28/74, interstate placards from previous trip were still mounted on doors of tractor. At the time of the accident, the driver had been on duty for 15 3/4 hours.

This was the first time he had driven over this particular route. He had made a prior trip as a passenger in another truck. He had made a number of trips from Los Angeles to Phoenix, Arizona, a route described as similar in character to the road upon which the accident occurred.

The driver was in good health and without any physical disabilities. Blood tests did not indicate the pressence either of alcohol or of barbituates. He claimed to have been wearing his seat belt, but noted he was lying on the floor after the accident.

Pickup Driver and Passengers

The 60-year-old driver was the owner of the pickup. He and his party of one adult, three teenagers, and two children were enroute from Burbank, California, to Carson City, Nevada. All seven people were killed instantly in the collision.

Physical evidence and the circumstances of the accident indicated that the driver of the pickup did not contribute to the preimpact sequence of events. An autopsy was performed; no significant findings that could be associated with precrash events were noted.

Sequence of Events

The driver of the truck involved in the accident, a driver in a second truck from the same company who had accompanied him until 45 miles before the accident, and witnesses in two vehicles that had caught up with and were following behind related the following details regarding the accident.

The two truckdrivers had departed from Susanville, California, at about 3. p.m. on July 28, 1974. The second truckdriver began to have mechanical problems and stopped in Lee Vining, approximately 45 miles north of the accident scene, and remained behind. The driver involved in the accident continued south at about 10:30 p.m. He noted that when

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coming down the grade into Lee Vining, his brakes were not very strong, but he did not make any adjustments because he did not know there were any more steep grades ahead. When he arrived at the summit area of Sherwin Pass, he saw "truck warning/speed signs indicating downgrade, 35 mph."

He therefore stopped at the truck parking area near the summit and attempted to adjust his brakes before beginning the descent (brakes were found to be out of adjustment after the crash). The driver characterized an adjustment procedure, where using a 9/16-inch wrench (7/16-inch is required) he turned the adjustment screw until the linings were just in contact with the drum. This he would determine by alternately turning the screw some amount, then hitting the drum with the wrench, until a sound was produced that is unique to the situation where the brake linings are in contact with the drum. He would then back off on the screw to permit the linings to be close to contact but not dragging with the brakes off. He would then yank on the brake push rod, but was vague as to the reason.

He stated that he had started down the grade in seventh gear and had to keep a steady, light application of the brakes to keep the truck at the posted speed limit of 35 mph for trucks. (Air pressure reading at the summit was 120 pounds, according to the driver.) He drove approximately 3 miles of grade in this manner before reaching the 2½-mile-long straight section of grade.

The first vehicle to catch up with and follow behind the truck was an automobile with a driver and passenger. They both stated the truck was "meticulously traveling at a speed of 35 mph," and they had followed behind the unit for 3 to 4 minutes (this would be a distance of 1 3/4 to 2 1/4 miles). A second following vehicle, a tractor-semitrailer unit, also noted that he caught up with the lead truck and automobile when they were a short distance from the beginning of a 2₂-mile-long straight section of grade. He also confirmed the fact that the lead truck was operating at a speed of 35 mph.

The driver of the truck involved in the accident stated that his road speed/engine revolutions per minute (r/min) kept increasing as he traveled down the pass, until it required more and more braking pressure. He began the descent using the foot pedal for brake application (controls both tractor and semitrailer brakes). As more and more pressure was required, he began using the hand trolley valve, finally exerting as much pressure as he could on each and had pulled the trolley valve until it had bottomed out (no more distance to pull). He continued to apply pressure and maintain the vehicle on the road until reaching the curve before the accident.

After having trailed the truck for 3 to 4 minutes, the witnesses in the automobile next noted that the truck suddenly began to accelerate, in a manner described by the driver of the automobile "as though if whatever

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was holding him back seemed to let go" or "like you were in low gear and suddenly went to neutral." The driver of the following tractor-semi-trailer also noted "all of a sudden he just started pulling away." The driver of the truck involved in the accident emphatically denied any attempt to shift the transmission. Both following drivers confirmed the sudden acceleration occurred as the truck was approximately 2 to $2\frac{1}{2}$ miles down from the beginning of the $2\frac{1}{2}$-mile long straight section of grade (or 5 to $5\frac{1}{2}$ miles from the truck parking area where the driver stated he had adjusted the brakes).

The passenger in the automobile and the trailing truckdriver both noted symptoms of overheating brakes with respect to the lead truck. The passenger in the automobile noted a smell as from overheated brakes after the truck began to pull away: The trailing truckdriver noted "I could see from the car lights that was in front of me, I could see there was a little whisper of smoke off the brakes but nothin' excessive, it wouldn't seem like to me." (This presumably was before the truck began to pull away.) The trailing truckdriver and the witnesses from the automobile noted smoke coming from the wheels of the tractor after the accident.

The driver of the automobile followed the truck after it began to suddenly move away until the speedometer read 63 mph, then dropped back because that speed was, in his opinion, too high for that type of descent. He noted that he probably had to step on the gas a little to keep pace as the truck pulled away.

After the truck suddenly began to accelerate, the driver managed to keep it on the roadway for approximately 2 to $2\frac{1}{2}$ additional miles. The trailing truckdriver noted that he continued to maintain the speed of his vehicle at 35 mph after the unit involved in the accident began to pull away. After he had trailed behind for about 1 to $1\frac{1}{2}$ miles, he entered the l-mile section of straight roadway leading to the curve where the accident occurred. At this point, he saw "the explosion and fire" from the accident. The driver of the following automobile came to a stop about three-fourths of a mile behind the accident. After the accident, a driver who had been traveling in the opposite direction, reported that the truck involved in the accident had forced him off the highway approximately one-half mile before the accident.

The truckdriver involved in the accident reported that the warning light which serves to indicate high water temperature/low oil pressure/low air pressure came on as he approached the curve. He said that as he was about to "start the curve" he saw a vehicle approaching him that was over the yellow line on his side of the road. He also mentioned that he could not see the yellow line (centerline). He let go of the brake trolley valve, blew his horn, and twisted the steering wheel all the way to the right. He thought he had gone off the road since it felt like the front tire hit a soft spot.

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The driver thought the other vehicle had passed by, noting this in his outside rearview mirror. He, therefore, began to steer to his left with both hands in order to get back on the pavement, but could not regain control of the vehicle. The next thing he remembered was lying on the floor and hearing voices outside the cab.

Marks on the Roadway

The physical evidence associated with this accident was distributed along some 800 feet of the roadway. Figure ll serves to indicate the character of the roadway, the significant elements of the physical evidence, and the collision dynamics of the vehicles involved.

ANALYSIS

Truck Brake Adjustment

Brake adjustments are made to minimize the distance the brakeshoes and linings must travel to contact the drum, while insuring that the brake push rod travel from a "brakes off" to a "brakes on" position is held to a minimum (normally 1 to $1\frac{1}{2}$ inches). Since the maximum distance the brake push rod can be extended is $2\frac{1}{2}$ inches on these type 30 chambers, there are $1\frac{1}{2}$ to $1\frac{1}{2}$ inches of reserve stroke. This reserve is necessary to accommodate brake lining wear and the expansion resulting from the heat produced during braking. The adjustment made by the driver did not provide the necessary reserve stroke distance.

The adjustment procedure characterized by the driver has inherent dangers. There may be uneven lining wear or the brakeshoes may be at a cant when the adjustment is made. The quality of the sound produced during a test for lining contact may be influenced by an inconsistent strike against the drum. There is no direct measure of lining position or the extent of brake push rod travel. There is not a proper test for whether or not the linings are dragging.

There is the possibility that the driver did not in fact adjust the brakes since in describing how he adjusted brakes, he did not quote the proper wrench size. However, the brakes of the tractor owned by the driver, and those of the trailer owned by the leasing company, were all similarly out of adjustment. This would support the theory that one person made the adjustments and the driver would most likely have been that person.

The driver stated that he had left Lee Vining, a stop he had made before reaching the summit, at 10:30 p.m. This was independently confirmed by the truckdriver who had remained behind in Lee Vining. The accident happened at 12:50 a.m. The distance from Lee Vining to the summit is 45 miles and the speed limit for a majority of that distance is 55 mph. It would appear that there was sufficient time to travel that distance, make the adjustments, and begin the descent.

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〔图〕AA STONCFRANCISCO FLORES TRUCK/PICK-UP TRUCKWITH CAMPER AND TRAILER COLLISIONU.S. ROUTE 395. BISHOP, CALIFORNIAJUNE 29,1

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The driver exhibited very little, if any, knowledge or experience with respect to vehicle maintenance or repair. The Bureau of Motor Carrier Safety (BMCS) of the Federal Highway Administration does not require in its Federal Motor Carrier Safety Regulations that on-road adjustment of air-mechanical brakes is required in order for a driver to be qualified to drive in interstate or foreign commerce. BMCS believes that a driver should know how to detect potential or existing problems but not necessarily expect to correct them. If a driver does attempt to correct existing problems, then he should have some training or information available to guide him in his endeavor.

NTSB investigators questioned several truckdrivers concerning brake adjustment procedures. A majority of them indicated they knew how to adjust brakes, particularly trailer brakes. They could envision situations in which adjustments would be attempted. They described a variety of adjustment techniques, one of them being the same as that related by the driver involved in this accident.

Truck Overload/Driver Gear Selection for Descent

The 6,720-pound overload permitted about 14 percent more cargo to be carried for a 9 percent vehicle weight overload. This increased (1) the load imposed on the braking system; (2) the force accelerating the vehicle after loss of braking potential; and (3) the center of gravity height of the trailer, thereby decreasing its resistance to upset. Had the overload factor not been present, the point where the accident occurred might have changed and the relative positions of the involved vehicles would have been different. Due to the sparseness of opposing vehicles, other vehicles may not have been involved.

In a similar vein, the gear selected for descent by the driver also increased the load imposed on the braking system. Selection of a lower gear would have increased engine braking ability. More importantly, however, the reduced road speed would have reduced the rate at which kinetic energy would have to have been dissipated by the braking system. For example, if the vehicle were in fourth gear rather than the reported seventh gear, the kinetic energy imposed on the braking system at any one time would have been 4 times less. This would have extended the ability of the braking system since less heat buildup would have occurred.

The signs at the truck parking area first advised truckdrivers to descend in the lower gear range and established the maximum truck speed limit of 35 mph. Weather conditions, the capabilities of the vehicle, the load, etc., are factors that must be accounted for by the driver in determining the specific gear selected. There is nothing in the Manual

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of Uniform Traffic Control Devices 1/ to suggest or require the provision of additional information regarding the character of the roadway to aid in the driver's decisionmaking process.

Several truckdrivers and specialists in the trucking industry were interviewed concerning the gear selection the driver should have made under the circumstances faced at the beginning of the descent. There was a consistent request for additional information -- How steep was the hill? How long was the hill? Were there a lot of curves? Answers to these questions were initially withheld because the driver involved in the accident was relatively unfamiliar with the roadway and one of the main reasons for signing is to aid the unfamiliar driver. The gear selection by those interviewed was consistently in the range of third to fifth gear.

After the additional information was provided, those interviewed indicated they would have selected either third or fourth gear for the descent. It was also reported that in some areas or States, the signing of hills was too consistent -- that is, all downgrades, irrespective of their character, were signed the same. This served to influence drivers to react to what they could see or remember about the character of the roadway rather than the information displayed by the signing.

Truck Brake Fade/Precrash Shift to Neutral

Since the diesel engine of this truck did not provide significant engine braking, $\frac{2}{1}$ the brakes had to act as the primary method of retardation for controlling road speed. As the driver began his descent, use of the brakes produced normally expected brake lining temperature increases which result in decreased lining-to-drum frictional coefficients and drum expansion. As this condition developed, a heavier application of the brakes and further extension of the brake push rods were required to maintain the same retarding force. Without relief, heat buildup continued until the brake linings began to overheat and smoke. Drum expansion would also continue. Driver and witness statements confirm that this condition developed.

Since the slack adjustors were considerably out of adjustment, it is probable that brake push rods became fully extended and could not maintain appropriate lining-to-drum contact. Since the brake adjustments ranged between 2 and 2 3/8 inches, it would be expected that loss of braking would occur progressively, wheel by wheel, thereby transferring an additional burden to those wheel braking systems still functioning.

1/ Manual on Uniform Traffic Control Devices for Streets and Highways, 1971 Edition, U.S. Government Printing Office, Washington, D.C. Developed with the cooperation of the American Association of State Highway Officials, and the National Joint Committee on Uniform Traffic Control Devices.

2/ There are devices that, when incorporated, increase engine braking capability. This tractor was not equipped with this type of device.

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The lack of significant smoking at the wheels, as observed by following witnesses, would support this hypothesis.

As the braking systems lose their effectiveness, road speed and therefore engine speed increase until the engine is damaged due to over-speeding. According to the manufacturer, any engine speed in excess of 2,800 r/min would have resulted in valve train damage.

Given the tire rolling radius and the total gear ratio at each gear position, it is possible to derive the engine speed that would be induced by the road speed of a vehicle going downhill. 3/ If the driver had been operating in seventh gear, the engine r/min was already at 2,600 r/min, had been so for at least the time the witnesses were following behind, and would have reached 2,800 r/min at a road speed of 38 mph. At the witnessed speed of 63 mph, the engine in seventh gear would have reached 4,680 r/min, considerably in excess of the critical speed of 2,800 r/min. Post-crash inspection of the engine revealed no evidence of valve train damage. This indicates that the transmission must have been shifted to neutral very close to the point where the truck began to accelerate.

Semitrailer Upset/Fifth-Wheel Separation

Speed calculations made with respect to witness observations indicate that in order for the truck to open a gap of approximately 1 mile in distance between it and the tractor-semitrailer following behind, it would have been traveling at a speed of between 110 and 125 mph as it approached the curve where the accident occurred. Additional calculations, employing SAE J-688 "Truck Ability Prediction Procedure," indicate that a truck of this type can attain speeds in this range if it is in neutral over the distance traveled during runaway. It was also determined that a road speed of about 125 mph would permit the generation of sufficient force to upset the semitrailer if it followed a path equivalent to the curvature of the roadway of the accident site. A road speed of about 135 mph would serve to upset the tractor and semitrailer in combination if both elements of the unit followed a path equivalent to the curvature of the roadway. 4/

The tire scuff marks produced before upset indicate that while the trailer was following a path equivalent to the curvature of the roadway, the tractor was following a shorter radius of curvature, thereby lowering the road speed at which the combination could be upset. Physical evidence also indicates that separation of the fifth-wheel assembly occurred as a result of the semitrailer upsetting, not vice versa. The outer

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piphery of the semitrailer tires was in contact with the pavement 155 feet before upset of the semitrailer while still following the curvature of the roadway. (See Figure 11.) The tractor was redirected to the left as the semitrailer rolled to the left and the bottom of the upsetting semitrailer came into contact with the left rear tires of the tractor. This occurred at a point just before the semitrailer wheels lost contact with the pavement. The tractor was redirected to the left by the upsetting semitrailer and not driver actions, as evidenced by the wheels on the left side of the tractor tracking before the tractor began to slide broadside. Only at this point did the scuff marks from the semitrailer tires change from following the curvature of the centerline, indicating that all influence of the trailer's remaining attached to the tractor was lost at this point.

The upsetting semitrailer would serve to place a heavy vertical load on the right side fifth-wheel assembly as it resisted semitrailer over-turn. Damage to the right side fifth-wheel slide indicated that the fifth-wheel horizontal trunnion pin was in place before upset. The forward part of the right side fifth-wheel slide opened to 40°. For this to occur, the right side horizontal trunnion pin must have been in place in order to permit a vertical load to be imposed on and open the slide before overturn. Since the rear edge of the right side fifth-wheel slide was less damaged and this slide remained near the tractor, the apparent sequence of failure involved loading and separation of the right side fifth-wheel slide, loss of the right side horizontal trunnion pin, and subsequent semitrailer overturn.

The character of damage to the rear edge of the right side fifth-wheel slide suggests that the rear vertical set pin for the slide was also in place. The bending of the bottom half of the slide near the rear corner indicates that there was resistance to rearward movement of the slide, probably as a result of the pin being in place.

After separation of the right side of the fifth-wheel assembly, the force of the overturning semitrailer was shifted to the left side, opening the left-side fifth-wheel slide and imposing sufficient force to produce redirection of the tractor to the left with counterclockwise rotation.

From a design standpoint, it might be argued that had the tractor remained attached to the semitrailer, this might have provided that necessary split second to avoid contact with the opposing vehicle. However, it can also be successfully argued that without the overload condition, the forces imposed on the fifth-wheel assembly would have been less. The looseness of the tractor frame may also have had some influence on the nature of the forces imposed on the fifth-wheel assembly. The attitude of the vehicle established by the driver as he attempted to negotiate the curve may have aggravated the condition. It was therefore determined that the fifth-wheel assembly functioned as well as could be expected given the extremely severe accident environment.

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Driver's Ability to Perceive Opposing Vehicle

An analysis was made using the relative closing speed between the truck and the opposing pickup in order to examine the truckdriver's claim that the opposing pickup was on his side of the road as he attempted to negotiate the curve and to determine if the pickup driver had any opportunity to avoid the accident. Speed calculations for the truck indicate that it had approached the curve at a speed of about 125 mph, or 184 feet per second. The character of the at-impact tiremarks for the pickup indicated there was dissipation of forward motion of the pickup before deflection to its right and rearward. Given the prevailing speed limit, the type of vehicle, and its uphill travel of about 1 mile before impact, a speed of 45 mph was assumed for the pickup velocity.

At these speeds, the distances between the vehicles as the truck-driver was about to enter the curve would have been 1,150 feet. With that type of separation, the tractor headlights would not provide illumination of the opposing vehicle or the section of the curve it was traveling on, and the driver would have had to look across the curve to see the opposing vehicle. (See Figure 11.) The driver had enough of a problem in attempting to negotiate the curve. He already had experienced difficulty in maintaining control of his vehicle in the straight section before the curve, where he had forced a vehicle off the road. If he had looked for the opposing vehicle, it was highly improbable that the driver could have distinguished its lateral placement.

The driver's comments regarding leaving the road, observing the opposing vehicle in his rearview mirror, and steering left to regain the roadway after the tractor passed by the opposing vehicle were also examined. Physical evidence indicates that neither the tractor nor the semitrailer left the road before upset. The feeling of leaving the road may have resulted when the semitrailer rolled and contacted the rear tire of the tractor. The tractor was redirected by the upsetting semitrailer and not driver actions. If the driver had looked in the rear view mirror, he would have observed the collision, not that he had safely passed by the pickup.

The pickup driver would not have had any significant input to advise him of the potential upset. The truck was on the appropriate side of the road and the time consumed from first side-scuffing of the unit's tire to upset would have been about 1 second. Therefore, the pickup driver had no possibility of avoiding the collision.

Traffic Control Features for Out-of-Control Vehicles

There were no traffic control features present that would adequately serve to induce or permit a more appropriate emergency action once loss of braking potential was experienced. The "Speed Limit 35 mph, Trucks"

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signs posted near the point where the truck began to accelerate would serve to indicate the need for trucks to remain in gear and maintain the posted limit. However, it would not serve to provide information that would permit the driver to evaluate the relative success of attempting to remain in gear and on the highway, until he could reach level ground.

Perhaps more positive information, advising of the relative slope, remaining length of grade, and character of the roadway geometry, would have served to alter driver actions after he began to experience loss of braking potential. This would not have served to prevent an accident but may have altered the course of events which occurred. Given this information, the driver may have selected another course of action -- e.g., running off the road or deliberately crashing the vehicle early in the sequence and therefore at a lower speed, permitting his engine to fail but retaining its re-tarding capability as long as possible. Although not considered solutions, in retrospect, they would have entailed less risk than the course of action taken.

There were no escape routes provided for runaway vehicles. An escape route is any existing or purposely constructed off-road feature that could be easily detected, approached at high speed and a shallow entry angle, and which would have some restraining quality that would be sufficient to stop and contain an out-of-control vehicle. The previous accident history, etc., may not serve to justify construction of this type of facility. However, when a particular design policy creates the potential for runaway vehicles, the provision of escape routes and appropriate signing should be an integral part of that policy. The presence of additional signing and adequate escape areas could have prevented this accident or drastically reduced its severity.

It is noted that the Manual of Uniform Traffic Control Devices does not address the class of suggested signs, nor does it specifically address the issue of long grades and appropriate signing configurations --e.g., approaches to weigh stations, railroad crossings, closely spaced interchanges. The State of Colorado has developed signing configurations advising of the slope and extent of remaining grade.5/

The American Association of State Highway Officials' Policy on Geometric Design of Rural Highways 6/ does not discuss escape routes in its section on grades.

CONCLUSIONS

1. The truckdriver adjusted the brakes before his descent and employed a technique that resulted in improper adjustment at all braking wheels for both the tractor and semitrailer.

5/ National Highway Traffic Safety Administration, Monarch Pass, Colorado, Schoolbus Crash, March 31, 1972; DOT HS 600 779.

6/A Policy on Geometric Design of Rural Highways, American Association of State Highway Officials, 1965 Edition, Washington, D.C.

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1. There is evidence that truckdrivers would attempt an on-road adjustment of externally adjustable braking systems and that a variety of techniques are employed, some with inherent dangers for producing improper adjustment of the brakes.

1. The truck was illegally overloaded, imposing an additional load on the improperly adjusted braking system, increasing acceleration of the vehicle after loss of braking capability, and increasing its ability to upset.

1. The truckdriver incorrectly selected a gear ratio for the descent that served to increase the load imposed on the improperly adjusted braking system.

1. Truckdrivers and specialists in the trucking industry consistently indicated that they would have selected a lower gear in reaction to the traffic control signs near the summit of Sherwin Pass, but the need for additional information related to the character of the roadway to aid in making their decision was also apparent.

1. The presence of more definitive signing and adequate escape areas could have prevented or reduced the severity of this accident.

1. The truckdriver could have taken more appropriate emergency actions. His relative inexperience with respect to driving and emergency actions, the lack of information concerning the character of the roadway ahead, and the lack of an existing escape route served to prevent a more appropriate course of action.

1. The truck speed reached a level where, given the physical characteristics of the vehicle and roadway, upset of the trailer at this location was unavoidable.

1. Physical evidence, including the character of damage to the fifth-wheel assembly, indicates the horizontal trunnion pin and right rear slide set pin were in place before upset.

1. The fifth-wheel assembly functioned as well as could be expected given the extremely severe accident environment.

1. The pickup driver had no possibility of avoiding the collision.

PROBABLE CAUSE

The National Transportation Safety Board determines that the probable cause of this accident was the loss of braking which permitted the tractor-semitrailer to run away. The loss of braking effectiveness was caused by the improper adjustment of the brakes by the driver, the selection of too high a gear for descent, and a weight overload of the truck.

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RECOMMENDATIONS

The National Transportation Safety Board recommends that the Federal Highway Administration:

1. Develop and disseminate throughout the motor carrier industry, an "On Guard" bulletin alerting drivers of commercial vehicles equipped with externally adjustable braking systems of:

a) The need to be familiar with company policies and practices with respect to on-road adjustment of brakes;

b) methods and techniques for detecting potential or existing problems in adjustment;

c) the scope of the problem in insuring proper brake adjustment;

d) methods and techniques for the proper on-road adjustment of braking systems currently and generally in use. (Recommendation H-75-17)

1. In cooperation with the motor carrier industry and driver organizations, determine what critical roadway characteristics information should be available, especially to drivers of commercial or other large vehicles, to assure their safe descent of long/steep highway grades. (Recommendation H-75-18)

1. Review the existing application of signing configurations for long/steep highway grades in each State in order to identify problem areas and to obtain information for developing more adequate signing standards for various highway grades. (Recommendation H-75-19)

1. Establish, in its adopted Manual of Uniform Traffic Control Devices for Streets and Highways, provisions which require the presentation of critical roadway characteristic information to drivers of commercial or other large vehicles at long/steep grades, and insure application of these provisions through the authority contained within Highway Safety Program Standard 13, Traffic Engineering Services. (Recommendation H-75-20)

1. Establish a design policy for long/steep grades that will insure the provision of escape routes when the character of the grade has a potential for contributing to the generation of runaway vehicles. (Recommendation H-75-21)

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BY THE NATIONAL TRANSPORTATION SAFETY BOARD

/s/ JOHN H. REEDChairman

/s/ $\frac{\text{FRANCIS H. McADAMS Member}$

/s/ $\frac{\text{LOUIS M. THAYER}}{\text{Member}}$

/s/ ISABEL A. BURGESS Member

William R. Haley, Member, did not participate in the adoption of this report.

August 14, 1975