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Trailer Underride: Conspicuity, Human Factors
and Rear Bumpers
Expanded Edition, 1998, Part I published in 1995 by the
Institute of Police Technology and Management University of North Florida 4567
St. Johns Bluff Road, S. Jacksonville, Florida 32224-2645 (904) 620-4786
http://www.iptm.org/ Page numbers have been omitted from the following table because much of the text has been edited out in this electronic version. Also, throughout the text you'll see references to figures and diagrams. These are available only in the published version. TABLE OF CONTENTS PART I
Driver Expectation Some Definitions PART II
Does the New FMVSS Mandate an Adequate Underride Guard? PART I (This part is unchanged from the 1995 edition. What follows here is condensed from the published version.) It was a dark and foggy night . . . . A tractor-trailer rumbled along a lonely stretch of State Road 29. The rig burst through patches of mist hanging over the highway and rolled toward the outskirts of a town. The cargo, destined for a nearby factory, rested within the walls of the dirty, white trailer. Perhaps because visibility was reduced the truck driver drove by his destination and out of the small community; perhaps his mind was on other things. When he realized his mistake, the only thing to do was to turn around and head back. Meanwhile, somewhere out there, two motorists left their respective homes and headed toward town for work. The lead driver, a male in a Bronco, didn't know the woman behind him in an Oldsmobile station wagon. Each driver was aware of the others vehicle only in the sense that any driver is conscious of other traffic on the road. The trucker, upset with himself for missing his exit, cursed under his breath as he looked for a good place to turn his rig around. Well, no sense taking all night, why not just turn around at the next available place? It so happened that the first convenient spot was an intersection with a little county road, nestled between a couple of knolls. Impossible to make a U-turn, the truck driver knew he could shave a few minutes off his already overdue trip if he (1) stopped along the edge of the highway, (2) pulled onto the shoulder, (3) dropped into reverse gear, (4) cranked the rig across the highway into one of the legs of the county road, and (5) headed back to town. The trucker had pulled slightly onto the right shoulder and was ready to make his next move when he caught the glimmer of headlights somewhere in the distance. He figured he could back across the roadway before the oncoming traffic reached the intersection. What he didn't figure was the amount of time it takes to make such a maneuver. As the Bronco approached, the trailer bisected the highway at a 45-degree angle. Behind the Bronco, the Oldsmobile driver wondered about the thick patch of fog up ahead. Suddenly the Bronco smashed into that "fog." The Olds driver, stopping just in time, witnessed a case of trailer underride. DRIVER EXPECTATION Everyone knows a 48-foot-long white semitrailer should be easy to see. Considering that it is 13 feet high and nearly the size of a small house and the trailer had all the proper lights required by the Department of Transportation (DOT), how could the Bronco driver miss seeing it? The operative word is see. We will never know what that driver saw because he didn't live to tell his story. Another reason is that crosswise trailers are difficult, if not impossible, to identify or perceive because the headlights of the tractor appear to be on a vehicle that is traveling where its supposed to be in the opposing lane. The closer the oncoming motorist gets to the tractor, the brighter is the glare from the tractors lights and the more difficult it is for the motorist to see through the light wall. (See Photo 1.) Federal Motor Carrier Safety Regulation �393.14 requires one amber reflector and marker light at the center of the trailers side and similar devices on the side at the front. Rearward reflectors and markers need to be red. Although the trailer in this case had all these devices and the tractors headlights pointed away from the traffic, another problem confronted the motorist. WHAT ABOUT BACKLIGHTING? One may wonder about the backlighting presented by the truck stop from which the trucker recently left. Typically, people think that ambient lights on the other side of an angled tractor-trailer would help highlight the truck as it sits across the highway. It just doesn't work that way. Backlighting turns a tractor-trailer into a silhouette. A silhouette, by definition, is an outline that appears dark against a light background. A black silhouette at night on a dark road where there is no traffic does not register in the minds of motorists. Its like a black hole. As it happened, the motorist drove straight into and under the semi-trailer. The car and most of the lone occupant came out the other side. And there were no pre-collision skidmarks. IS TRAILER UNDERRIDE REALLY A PROBLEM? How much of a problem is trailer underride? Accident data collected from April 1979 to May 1980 was published in Results of the NASS Truck Underride Special Study. According to that study, 244 truck accidents with underride potential were analyzed. The study reported: "Investigation determined that 56 of these were underrides with passenger compartment intrusion (PCI). Of the entire set these PCI accidents were most highly associated with severe injury (25% of the non-truck occupants were severely injured). . . NASS provides a national estimate of underride accidents for the study period. NASS estimates there were approximately 9,000 truck underride accidents with 5,000 involving PCI. Of the latter almost all (97%) involved some injury." SEMITRAILER CONSPICUITY Can anything help -- other than training and education -- to stem the statistics of side trailer underride? Most vision experts have long argued that the currently mandated side marker lamps are ineffective or inadequate to forestall this type of accident. Generally, trailers angled across roadways severely limit the amount of reflected or direct light that reaches oncoming drivers. In their paper "Underride Accidents: Headlights, Glare, and Nighttime Visibility," presented many years ago at Safety Conference IV in Montreal, Messrs. Douglas R. Brown, Jeffrey C. Bookwalter (both of Systems Engineering Associates, Worthington, Ohio) and Dennis Guenther (Department of Mechanical Engineering, Ohio State University) conclude: Headlight glare plays a major role . . . because the glare increases perception time." They add: "The side marker lights are not adequate warning devices because they are not associated with a trailer, not because they cannot be seen at a distance." MARKER LAMP EFFECTIVENESS As noted in some of the cases, even when headlight glare is not a factor, side marker lamps are deficient in trailer identification or recognition. Before a motorist can react to a hazard, the hazard must be perceived for what it is. Consider nighttime vision acuity and tractor-trailer conspicuity. The field of optometry has done about all it can to improve eyesight; therefore, responsibility lies with making trailers more visible. Bigger lamps? More candlepower? Additional lighting? Perhaps. Advances are being made toward enhancing trailer conspicuity. The article "Trailer Underride: The Almost Always Fatal Collision" (LAW and ORDER, May 1988) mentions that headlights, on dim, "probably allow the driver to see about 200 feet away." A vehicle at 55 miles an hour will cover 200 feet in about 2.5 seconds. It may take the entire 2.5 seconds to see, identify, and perceive a hazard. There is no time left even for reaction, let alone the time required to come to a stop. Moreover, further vision studies suggest that 175 feet is a more likely distance (with headlights on dim) for drivers to distinguish an object on a dark road. According to a University of Michigan study, a driver using high beams at night can see a pedestrian clad in white from about 300 feet. If the pedestrian wears dark clothing and the driver uses low-beam headlights, visibility drops to less than 100 feet. As discussed in "Trailer Underride Revisited" (LAW and ORDER, August 1992), the 3M Company addresses conspicuity in their booklet "Conspicuous Problems . . . Conspicuous Solutions." They define conspicuity as "the degree of observability of an object. The ease by which an object can be perceived." According to 3M, "Drivers recognize objects by five visual cues of conspicuity: Detection, Estimate of Distance, Determination of Length, Assessment of Shape, and Definition of Objects Relative Position." MOTORISTS JUDGEMENT OF TIME AND DISTANCE There is a situation in which side underride accidents may occur where the motorist, rather than the truck driver, bears the greater responsibility in avoiding a collision. The accidents often occur at night but are common enough also in broad daylight. Picture a tractor-trailer enroute north on a four-lane undivided highway. The trucker wants to turn left at an intersection. A car is approaching from the north at 55 miles per hour. The trucker begins his turn when the car is one-tenth of a mile (528 feet) from the intersection. Can he complete his turn before the car gets there? The rig will travel 84 feet from the moment the turn is begun until the trailer is clear. This includes the distance the rear of the trailer will still have to cover to reach the intersection and then angle across the two 12-foot lanes. The trucker will not stop before turning. At an average speed of 10 miles per hour through the intersection, it will require just under six seconds to complete the maneuver. Since at 55 miles per hour (80.85 feet per second) the approaching car will cover only 485 feet in a full six seconds, the rig will clear the intersection in time. However, if the semi had been loaded to capacity and the trucker had stopped before starting his turn, he might have needed some 11 seconds to accelerate and get his rig in the clear. The car, approaching at a steady 55 miles per hour, would have had to be at least 890 feet away to avoid a collision. As motorists drive down certain avenues and highways at night, they are confronted with all kinds of distractions by way of lights. There are overhead street lights, neon sign lights, traffic lights, clearance lights on trucks, oncoming headlights and fog lights, taillights and brake lights, and billboard lights. There are red lights, green lights, amber lights . . . lights of about any color and hue you can think of. If a car in front of a motorist caught up in this light show suddenly swerves for whatever reason, the motorist may be inclined to swerve too, just in case there is something there to miss. Tractor-trailers, especially double-bottoms, are about the size of a house when viewed from the side. Yet without lights or a considerable amount of reflectorized material, these objects, when blocking the road, are nearly impossible to discern. Motorists have difficulty discriminating dark-appearing objects from grayish-black backgrounds. It isn't that the motorists cannot see the objects but rather that they need time and distance to identify and recognize the objects as hazards and lacking this, may not be able to stop their vehicle without having a collision. Nor are the motorists alone. Truckers at the wheel of a tractor sometimes run into such objects too. MARKER LAMPS Drivers also run into properly lighted trailers. One might question, however, just how properly lighted are some of these trailers. Their running lights may meet all the DOT specifications, but truck drivers doing routine checks of tires, hoses, lamps, etc. do not generally mess with those top marker lights; they are 13 feet from the ground. How often does anyone clean them? Over time, even lenses at the bed of the trailer -- just three and one-half feet from the pavement and easy to wipe off -- get dirty. What were once bright red lights become dim pink dots in the distance to a motorist approaching from the rear. SPEED DETERMINATION OF AN UNDERRIDING VEHICLE Speed is usually an issue, but it is difficult to determine impact speeds in underride cases. When a vehicle has had its hood, its A-pillars, and its top sheared off, an impact speed estimate is speculative at best. However, if instead of underriding the trailer, the vehicle has struck the drive wheels of the tractor or the wheels of the trailer, it might be possible to calculate energy losses based on crush. The energy consumed can then be translated into speed by using any number of computer programs or this may be done the old-fashioned way with complex equations and a hand-held calculator. Still, some subjectivity remains. The reason it is difficult, if not impossible, to calculate the speed of a vehicle that underrides a trailer is, according to the NASS study, that "a step damage pattern is believed to result. Here two areas of the vehicle front, one above and one below the beltline, are impacted as a result of one force acting on an overhanging structure. Unlike other CDC [Collision Deformation Classification] damage codes, this one [with step damage] uniquely identifies the underride." Part II of this treatise, which follows, was written in 1998 and, with the more recent information available, expands on a number of the points discussed in Part I, written earlier, and introduces several new ideas. As the information contained in Part I retains its relevance and usefulness, Part I remains unchanged in content from the 1995 edition. PART II [This is a condensed version of the published manuscript.] DOES THE NEW FMVSS MANDATE AN ADEQUATE UNDERRIDE GUARD? Effective January 26,1998, a new Federal Motor Vehicle Safety Standard governs new trailers and semitrailers at or above a Gross Vehicle Weight Rating (GVWR) of 10,000 pounds. This regulation mandates requirements for rear underride protection guards (a.k.a. ICC or DOT bumpers). Section 393.86, titled "Rear impact guards and rear end protection," states: "Each trailer and semitrailer with a gross vehicle weight rating of 4,536 kg (10,000 pounds) or more, and manufactured on or after January 26,1998, must be equipped with rear impact guard that meets the requirements of FMVSS No. 233 (49 CFR 571.223) in effect at the time the vehicle was manufactured." Up to that date, the lower horizontal member of the rear underride guard need not be lower than 30 inches (762 mm). From that date forward, however, the height (ground clearance) must be no higher than 22 inches (560 mm). Moreover, the guard must extend to within four inches (100 mm) of the sides of the trailer, with exceptions for rounded guard ends, an d must be as close to the rear of the vehicle as possible (within a 12-inch zone in front of or behind the rear of the trailer). As noted in the Federal Register of January 24, 1996 (Volume 61, Number 16), "Using rounded guard ends will diminish the hooking potential of the guards when the trailer is turning sharply. Guard ends that are rounded upward and attached to the vehicle may add strength to the horizontal member near the side extremity of the vehicle." Excluded from the new regulation are single unit trucks, truck tractors, pole trailers, low chassis and special purpose vehicles, and "wheels-back" vehicles. The latter being trailers and semitrailers, their rear axle is so far to the rear as to put the tires within 12 inches of the back of the trailer. Consumer safety groups -- and the general public -- are opposed to one or more of these exclusions, especially for the single unit trucks, such as some dump trucks. There is also a problem with the stipulated 22-inch clearance. The top of nearly all automobile bumpers is less than 22 inches from the ground. On large automobiles such as the Lincoln Town Car and Oldsmobile Custom Cruiser, the bumper's top is right at 22 inches from the ground. The 22-inch ground clearance stipulation will certainly prove to be too high as the automotive industry designs and introduces hybrid fuel-efficient electric-gasoline cars. Along with being up to 50% lighter than today's automobiles, these cars will be even lower to the ground, with plastic bodies forthcoming. Stephen G. Hadley, Coordinator of the Underride Network (a worldwide non-profit volunteer organization dedicated to educating the public about and assisting/serving the victims of truck and trailer underride crashes), notes that "our government knew this when it performed its crash tests for the new, more substantial guard. And with a 10+ year turnaround for a new guard to filter through the fleet, these [lighter cars] might be the majority cars on the highway during this substandard guard's reign." Part of the underride problem is with law enforcement. In many municipalities, local police and sheriff departments get very little -- if any -- training regarding heavy truck enforcement. Many state police and highway patrol agencies have specialized personnel who investigate commercial vehicle accidents but even some state units know very little (or seem to care little) about underride guards. Substantial rear underride guards have been available for some time. One called the RP 92-94 (RP = Recommended Practice) will withstand more force than many underride guards seen on the highway today. To ascertain the energy-absorption capabilities of the RP 92-94, a staged crash was undertaken at the Law Enforcement & Security Training Division of the Texas Engineering Service (TEEX) at Texas A&M University. This staged event took place during CRASH '97, a seminar for accident reconstructionists. Staging personnel put a 4,280-lb 1992 Ford Explorer at nearly 30 mph (28.8 mph) into a 45,000-lb tractor-semitrailer outfitted with an RP 92-94 bumper. The 1986 Great Dane refrigerated trailer was specially loaded with one 7,450-lb concrete block, one 2,720-lb concrete block, and four 80-lb bags of cement. The bumper indeed collapsed, but only partially. It allowed the bullet vehicle to "ride down," thereby preventing intrusion or penetration into the Ford's occupant compartment -- view of Ford Explorer behind a semitrailer, post-crash). A second staged crash, utilizing a rear underride device known as the "Georgia bumper," was carried out a few days later. DUMP TRUCKS PRESENT A SPECIAL PROBLEM Another case involved a large dump truck the rear axle of which was well forward of the back of the bed. There never was any sort of rear underride guard on the truck at all. When a vehicle underrode the back of the bed, investigating officers did not consider the absence of the bumper as a causative circumstance. Dump trucks such as the one shown weigh about 18 tons empty. The dump bed (not counting the extension over the cab) is about 26 feet long and can carry over 42 tons of coal. The legal GVW limit in Kentucky is 90,000 pounds. If such trucks were designed and built to haul the legal limit, the bed would need to be only 16-3/4 feet long -- or ten feet shorter! Since the danger zone behind the rear tires is about six feet, a shorter bed would reduce the overhang considerably and thereby lessen the danger. Many trucking companies insinuate that it wasn't just inattention but a matter of the motorist falling asleep. Left out of the equation, however, is that many cars are underriding semitrailers that are either stopped or moving significantly slower than normal, and remarkably slower than other traffic. It is not a case of 90-mph cars running into the rear of 65-mph semis. What happens is that the tractor-semitrailers -- often loaded, which slows their acceleration rates even more -- merge too slowly into traffic. Such trailers get rear-ended when traveling 25-35 mph by unsuspecting motorists who may see the back of the trailer but appreciate neither their closing speed nor the fact that the rig is barely moving compared to other traffic. In a technical report for Transport Canada titled "The Perceptual Basis Of Heavy Vehicle Conspicuity And The Role Of Retroreflective Materials In Increasing Driver Decision Sight Distances," Brian Tansley and Will Petrusic of Carleton University's Psychology Department stated it this way: "The dynamic aspect of control of a moving vehicle involves the visual perception of the driver's own movement relative to the roadway and of the movement of other vehicles relative to his/her own." Precollision skidmarks are rare. Occasionally investigators will find some, but the marks are usually not very long. This obviously means that by the time the motorist realized that a semi posed a problem, he or she was far enough back to get on the brakes, but too close to skid to a stop. Nearly every underride case occurs at night. Once in a while, though, such instances happen in broad daylight. When the big rigs go through crossovers on four-lane highways, they can usually be seen and recognized from a considerable distance. In some cases there will be lots of skidmarks. Several years ago the 3M company produced a video titled "The Beauty of Conspicuity." It contained a section on Decision Sight Distance explaining that the Federal Highway Weight Administration (FHWA) commissioned a company called Biotechnology to study time/distance situations regarding underride accidents. The study results show that a driver "with average reflexes, eyesight, decisiveness and sobriety, driving an average car" [at 60 miles per hour] will take almost three seconds to see and recognize a hazard, almost seven seconds to decide on a course of action, and almost 4.5 seconds to complete an avoidance maneuver. This is a perception/reaction time (PRT) several times greater than that generally accepted by "vision experts." In side-underride cases, one party always argues that the trailer came equipped with all the necessary lights. The three obligatory lights along the side of a semitrailer are indeed all that are "required." However, they are merely three dots of light seemingly suspended four feet in the air. They do not necessarily connect to anything. Flatbed trailers are especially dangerous because motorists can see both over and under the trailer and those obligatory lights may appear to be way off in the distance. TRAILERS IN SOME COUNTRIES ALSO HAVE SIDEGUARDS In Japan and Europe, because of the immense amount of bicycle and motor scooter traffic, semitrailers have side underride guards. These may not prevent a car at 55-mph from going partially beneath the side of a semitrailer, but in the typical T-bone side-underride case, sideguards braced with lateral beams allow the car to "ride down" the collision. Sideguards are placed on tanker trucks as well. Sideguards are beneficial in situations on 4-lane highways where a tractor-semitrailer and an automobile are fairly parallel, one overtaking the other and the car is in one of the truck's "blind spots" when the trucker changes lanes. Without the presence of the side- guards, the car would become engulfed beneath the bed of the semitrailer. Sideguards, however, deflect the car away and only minor sideswipe damage results. This is obviously preferable to having the car demolished and its occupants perhaps injured or killed. Likewise, when the big rigs make right turns, they usually make a wide sweep, first to the left, then to the right. An unsuspecting motorist, thinking the rig is making a left turn, starts to pass on the right only to become swallowed up by the trailer. U.S. trailer manufacturers and trucking companies have long been aware of the protection and safety benefits that side rails provide but have successfully defeated any legislation mandating the implementation of the devices. In some recent crash tests, it was shown that side rails indeed prevent cars from underriding even at nearly 90 degrees. MORE ON REAR BUMPERS Heavy vehicle underride has become such a worldwide problem that a symposium called "SAE Heavy Vehicle Underride Protection TOPTEC" took place in April 1997 in Palm Springs, California. Byron Bloch, an independent consultant in auto safety design and crashworthiness, offered at his presentation that "The new NHTSA safety standard for a rear underride protection guard is certainly better than the obsolete and ineffective ICC-regulation rear bumper that began back in 1953." "But no," he said, referring to the new standard, "it's not good enough." Bloch explained that the vast majority of the obsolete ICC rear bumpers that you see hanging down at the rear of trucks and trailers are too high off the ground (typically in the 24 to 28-inch range), are too narrow across the rear, and are too weak." Many such bumpers are bent, twisted, rusted, and have no reflective tape. There is a device called a "dock lock." This is a mechanism affixed usually to a building or loading platform to which truckers can fasten their trailers after backing them up to the platform. It secures the trailer during the loading or unloading process, especially when forklifts run in and out of the trailer cross the rear door sill. Backing into those devices can damage and thereby weaken the underride guard. Simply backing into the loading docks themselves may incapacitate the ICC bumper. IN HOLLAND, AUSTRALIA AND ELSEWHERE [Much of this section has been omitted from this electronic version.] On the lighter side for a moment, George Bernard Shaw once wrote: "Britain and America are two countries separated by the same language." This becomes evident when one reads a British publication titled "The Law and Its Meaning" and consolidating the statutory instruments in the exact terms of what the regulations say. Regarding underrun bumpers, there are exemptions. The vehicles most commonly exempted in Britain are (transcribed verbatim): "Fire engines, Tractive units of artics, Tippers and gritters, Car or caravan transporters, Pole trailers, Concrete mixers and agitators, Bare chassis (for most practical purposes), and Inset tail-lifts at least a metre deep, integral with the body." The latter type includes, according to Police Constable Andrew Clay of West Sussex, England, "a brick lorry that has a small fork lift truck attached to the back on a tail lift that is employed only during loading/unloading." My personal favorites are tippers and gritters. I thought tippers might be British for dump trucks, but no. Clay advises, "Tippers are usually eight-wheel twin steer 32 tonne rigid goods vehicles that take or remove earth or aggregate to or from building sites or landfill sites." "Gritters," Clay adds, "are usually a 24-tonne chassis to which a hopper is added to the back and then two spinning spreaders disperse grit onto the road surface from the rear of the vehicle . . . ." Something we Americans would call a salt spreader or snow-removal truck. Returning to a more serious vein, countries around the world are becoming concerned with the instances of underride. George Rechnitzer of Monash University's Accident Research Centre in Clayton, Victoria, Australia, states, "Crashes involving heavy vehicles and other road users are recognised internationally to be a significant contributor to the total number of people killed or seriously injured in road crashes." At the 15th International Technical Conference on the Enhanced Safety of Vehicles, Rechnitzer made a presentation on "Development and Testing of Energy Absorbing Rear Underrun Barriers for Heavy Vehicles." In his presentation Rechnitzer said, "Rear underrun crashes are a particularly severe crash type because the floor structure of most heavy vehicles is above bonnet height. Cars can run under this structure (e.g., the tray of a rigid truck) with the tray penetrating through the car's windscreen pillars and into the passenger compartment. The usual occupant protection features built in cars such as seat belts, airbags, crush zones are bypassed and ineffective in this crash type." In a comprehensive study titled "Truck Involved Crash Study: Report on Fatal and Injury Crashes of Cars Into the Rear of Trucks," (May 1991), Rechnitzer and Foong Cee Wai further discuss this and several other issues. As a partial solution, Rechnitzer adds, "An effective means of preventing underrun lies in adding a frame structure to the rear of the truck which is of sufficient structural strength and geometry to engage the front structure of a car . . . . "Most heavy vehicles," he goes on, "do have some sort of barrier already, but these are typically poorly designed and quite ineffective. Rear underrun crashes in Australia account for some 15 or so people killed every year, and some hundreds injured." Specializing in vehicle crashworthiness and occupational health and safety issues, Mr. Rechnitzer documented a "Truck Involved Crash Study" which included a detailed literature review and detailed investigations of over 52 crashes involving 45 fatalities, including crashes of trams and buses with cars. "The study has identified that design changes are both feasible and effective, and that the frontal, side, and rear design of trucks can be significantly improved to reduce the harm potential in crashes involving other road users." This is in line with European findings, according to Rechnitzer, which countered commonly held notions that the main problem was the mass of the truck -- a factor not readily amenable to change. RETROREFLECTIVE MATERIALS After undertaking pertinent research, another Canadian, Eric Hildebrand, along with Peter Fullarton, both from the University of New Brunswick, prepared a paper with the title "Effectiveness of heavy truck conspicuity treatments under different weather conditions" for Proceedings of the Canadian Multidisciplinary Road Safety Commission X, June 8-11,1997, Toronto, Ontario. That study revealed: "All retroreflective tape treatments which were tested provided significant increases in visibility thresholds relative to an untaped trailer under each weather condition except fog . . . . The presence of fog decreased the visibility thresholds by as much as 90 percent for these field tests. The rear of the trailer seems to exhibit more of a relative reduction in visibility threshold than the side as weather conditions deteriorate. Nevertheless, even under rain and snowy weather conditions, a substantial benefit is derived by using retroreflective tape." Hildebrand and Fullarton found that the rear "full white outline" tape configuration was found to be most effective under all weather conditions but that no one pattern/color was found to be more effective given specific weather conditions. "Full" or solid white tape was found most effective for the side of the trailer under all weather conditions except rain. The study noted that "The Canadian standard allows the substitution of solid white in favour of the alternating red/white tape, for either the rear or sides," and that "new Canadian legislation also permits the use of yellow and alternating yellow/white tape." REAR BUMPERS AGAIN AND LIGHTING RETROREFLECTIVE TAPE AND CONSPICUITY [The above sections have been omitted from this electronic version.] In closing -- accident investigators, reconstructionists and the motoring public must be aware of the possibility of slow-moving rigs on our highways and of the fact that heavy commercial vehicles occasionally block roads. They should also be cognizant that marker lamps do get dirty and that drivers might not always associate an occasional dot of light or two as being on something the size of a house that's sitting in their lane of travel.
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