Truck and Trailer Underride Collisions

John C. Glennon, Jr., BSAT

[ Prepared for the Indiana Continuing Legal Education Forum, May 2001]

Truck underride collisions can be deadly to passenger-vehicle occupants, even at low speeds. An underride occurs when a passenger vehicle collides with a straight truck or the trailer of a truck-tractor semi-trailer combination (truck) and runs under the truck, sometimes sheering the roof off the vehicle and killing the occupants. If the vehicle occupants are more fortunate, the impact will occur near one of the truck axles, preventing the vehicle from going completely under the truck. Two types of truck underride collisions will be discussed, side underride and rear underride.

A side underride collision usually happens at night or during low sun-angle conditions. The collision usually occurs when a truck driver either is attempting to cross or turn onto a street or highway, is attempting a U-turn, or is trying to back across traffic. Although truck drivers commonly assume that on-coming drivers can see them crossing the road, these on-coming drivers often do not see a truck crossing their path until its too late.

The main factor related to the driver.s ability to see a crossing truck is target conspicuity, or how well an object stands out from its background. Target conspicuity relies largely on contrast characteristics such as color, movement, brightness, shape and size as follows: (1) Color - An object with colors that closely resemble the colors in it.s background will be more difficult to see (less conspicuous) than one that has colors that are very different from it.s background. The addition of multiple colors to an object will also make it blend in better with the background. Additionally, dark colors are more difficult to see and other colors such as blaze orange and lime green are very easy to see; (2) Movement - An object that is sitting still or moving slowly will have little contrast against its background. Faster moving objects will begin to create contrast; (3) Brightness - An object.s brightness or reflectivity can affect contrast. A surface with poor reflectivity will have little contrast against a background with poor reflectivity, but a surface that reflects light well will stand out against a poorly reflective background; (4) Shape - The shape of an object can affect contrast, as well as the shape of markings or patterns on the object. Markings or patterns placed on an object, particularly ones that are random and inconsistent, can also affect contrast by erasing the object.s silhouette; (5) Size - A small object is less conspicuous than a larger object under most circumtances.

The best example to demonstrate the lack of contrast is hunters who choose clothing with non-bright colors that closely match the environment in which they plan to hunt. The most common clothing used also generally has very detailed patterns made up of randomly place shapes resembling objects found in the hunting environment such as leaves, sticks, tree bark, grass, etc. The hunter will also limit his movement allowing him to further reduce his contrast. These concealment tactics work so well that, during rifle season, deer hunters are required to wear a certain amount of blaze orange to prevent them from being misidentified as game. To illustrate a point made earlier, blaze orange clothing containing black randomly placed splotches cannot be used to meet the blaze orange hunting requirements in some states. These states have recognized that the additional color and the randomly placed pattern significantly reduces the effectiveness of the blaze orange.

Problems with target conspicuity and target contrast play a major roll in many traffic accident scenarios. However, this concept is generally not understood. Most commonly with trucks, the misconception is that their large size by itself makes them conspicuous. However, size is only one of the factors that create contrast between the target (truck) and it.s background. The assumption that this one factor in itself will make the truck conspicuous and in turn allow approaching drivers to see the truck is a fatal mistake.

Generally underride accidents occur at night. Most standard equipment such as box trailers will have enough contrasting capabilities to allow them to be seen during daylight hours. However, certain circumstances can cause underrides to occur during daylight hours. A low sun angle, for obvious reasons, will make a truck difficult to see. Trucks with protruding loads such as I-beams, pipe, or lumber are at high risk for daytime underride. These protruding loads usually are relatively small targets, have poor reflective qualities, are darkly colored and are a color that does not highly contrast with its background. For the same reasons, dump trailers with long tongues and other equipment such as pole trailers are also at risk for daytime underride.

At night the size of the trailer by itself will not make it conspicuous since other contrast problems will make the trailer virtually invisible. Without any close-in lighting reflecting off the painted surface of the trailer, it will appear black against the black background. Then an approaching driver will have to rely on the side marker lights for his only cue to the presence of the trailer. However, even with the legal placement of marker lights on the sides of the trailer, approaching drivers will still often not perceive the trailer as an obstruction blocking their path. Marker lights are small and can be spaced as far as 26 feet apart on the side of a trailer, not providing on-coming drivers with enough information to determine that what they are looking at is a trailer. Marker lights can be misleading, and without an external light source, trailers are often not identified until the headlights of oncoming vehicles directly illuminate them. When their headlights illuminate the trailer, on-coming drivers will only be 100 to 200 feet away, and unable to stop at higher speeds.

Additional factors can contribute to cause side underride collisions. The most obvious factor being inoperative or dirty lights on the trailer.s side. Ambient lighting intensity and placement can also contribute to cause the collision; for example, bright lights and/or bright sign-boards from a fuel station or truck stop near the side of the road. As an approaching driver.s eyes adapt to the bright lights, a "blackhole" in the nearby unlit road is created. This makes it very difficult for a driver to see any trailer, whether it is lighted or not. When operating near these brightly lighted areas, it is not uncommon for truck drivers to assume their trucks are more visible. However, it is more likely that the trucks are less visible than they would be in an area not adjacent to very bright lights. A similar situation can occur when a driver leaves a very bright area and enters an area with little or no light. Under this circumstance, called dark adaptation, drivers will have difficulty seeing dark objects, such as a semi trailer, until their eyes adapt to the change in light intensity.

A very slow-moving truck can also contribute to cause a side underride collision. Obviously, a slow-moving truck will take more time to cross the road, making it a hazard for a longer period of time. Additionally, a slow-moving truck that creates less contrast than a faster moving truck, will be hard to perceive as an obstruction. At low speeds, the side-marker lights will appear to remain stationary. As a result, an approaching driver will most likely perceive these lights as distant taillights or roadside markings.

Another situation that contributes to cause a side underride collision occurs when a truck is making a left turn or backing across traffic. Typically, when a truck with a trailer enters a road making a left turn or when it backs across traffic, the tractor will be centered in the one travel lane while the trailer is blocking oncoming traffic in the opposing lane. With the tractor.s headlights pointed directly at oncoming traffic, the dark trailer will be less visible since the drivers. eyes will be adapting to the headlights. Also, oncoming drivers will generally perceive the truck only as an approaching vehicle in the opposing lane, not one blocking their path. If the headlights from the tractor are bright, the approaching driver will not only have light adaptation problems, but also may do as most people are taught and look away using the white pavement edgeline to navigate. If this happens, the approaching driver will most likely never see the trailer in his lane.

Rear underride collisions, when a passenger vehicle runs under the rear of a straight truck or semi-trailer, are also a major problem. In addition to the normal rear-end collision circumstances, an underride will typically result from a poorly-marked truck or trailer that is either parked on the side of the road, moving slowly as it enters the road, slowing to exit the road, or slowing for a railroad crossing. Additional contributors that can cause rear underride are: inoperative, dirty, or dim taillights; taillights placed very close together; a failure to properly use reflective triangles when parked or broken down on or near the road; and a failure to use emergency flashers when entering or exiting the highway at slow speeds.

Trucks often take a long time to accelerate or slow down when entering or exiting roads. Passenger-vehicle drivers on highways with speed limits ranging from 55 mph to 75 mph typically do not expect to meet a truck traveling at 10 mph. Not only is this lack of driver expectancy a problem, but also a driver has difficulty in accurately determining distance and closing speed at night. Closing speed is usually determined by a subconscious comparison of an object to its background and by the rate at which the object gets larger. At night, the background around an object is usually all black, which makes the size of a faraway object and the rate at which it gets larger very difficult to determine accurately. When viewing another vehicle, the distance between an approaching vehicle.s headlights or the distance between a followed vehicle.s taillights and how high they appear to be is what the brain uses to try to determine how far away that vehicle is. The rate at which the lights move farther apart is how closing speed is determined. One problem that drivers have in processing this information is that the distance between the lights and their mounting height is different on each type of vehicle. The most striking example of different light separation distances is the difference between a standard semi-trailer, which has taillights that are about 8 feet apart, and a bobtail tractor, which has taillights that are about 2 feet apart. If these two vehicles were the same distance from an observer at night, the observer would subconsciously assume that the bobtail tractor was farther away. This distance judgment problem becomes very apparent when a driver meets a vehicle with only one headlight or taillight that is illuminated and he struggles to determine how far away, and how quickly that vehicle is approaching.

Not uncommon is for drivers to be startled at night when they realize that the vehicle in front of them is much closer and moving much slower than they had perceived. When this dilemma happens, the primary factor that determines if a collision will occur is the difference in speed between the two vehicles. If the following vehicle is traveling much faster than the lead vehicle, the following vehicle driver may not have enough time to react once he realizes the vehicle in front of him is traveling very slowly.

As a result of trailer underride collisions, the U.S. Department of Transportation has mandated new conspicuity markings for trailers, which originally had to be installed on all trailers made after December 1, 1993. These markings are now required on all trailers. Most commonly, this requirement is met by using retroreflective tape which is designed to reflect light back to the source and is capable of reflecting light at great distances. The tape must be 2 inches tall and must have a red and white alternating pattern. The tape must be placed on the sides and rear of the trailer 15 to 60 inches from the ground. On the sides of the trailer, tapes must be evenly spaced covering at least 50% of the length of the trailer. On the rear of the trailer, tape must cover 100% of the width with four 12-inch white stripes marking the upper corners. These markings not only show the size and shape of the trailer, but also the highly reflective surface and red and white coloring creates contrast against the dark background. These markings have been standardized, so they will not only allow trailers to be seen by approaching drivers, but also that their pattern will be recognized as a trailer.

The retroreflective tape is very effective in making trailers visually stand out. However, a truck driver should never assume that the presence of this tape on a trailer will automatically guarantee that the trailer will be seen. If the tape is dirty, badly worn, or if the truck is at a steep angle to traffic, oncoming drivers may not be alerted to the presence of the trailer. The underride hazard is still present. Therefore, informing truck drivers of this hazard is very important. With this knowledge, drivers will know when to exercise caution and what situations to avoid.

About the Author

John C. Glennon, Jr., is a forensic automotive technologist who performs crash reconstruction and detailed vehicle testing for trucking companies, insurance companies and lawyers involved in investigating and litigating motor vehicle collisions. He has a B.S.A.T. degree in Automotive Technology, he is a triple-certified Master Automotive Technician, and he holds a Class A CDL in the state of Kansas.


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