Underride collisions are likely the most horrific and most deadly types of crashes. These reportedly represent one quarter of the fatalities from
truck-involved collisions. Crash Forensics can analyze and reconstruct underride collision. Accounting for all causative factors of the
crash. Below is a discussion outlining the problems causing these crashes. Link to our truck expert.
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.
Side Underride Collisions
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.
Target Conspicuity and Target Contrast
Problems with target conspicuity and target contrast play a major roll in many traffic crash 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 crashes 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 Underride Factors
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 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.
Side Underride Collisions
