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Underride Accidents

Marc Green

Side underrides occur when the driver collides with the side of a trailer that is blocking the roadway during a turn or while backing up. The driver's car chassis often goes beneath the trailer and the car's roof is sheared away. Unlike the trailer rear, the side has no guard to prevent the car underriding the larger trailer.

Here, I explain how to perform a more realistic examination of the factors that contribute to an underride accident. The avoidance task requires the driver to first sense the trailer's contrast, the difference in brightness between the trailer and its background. Next, the driver must notice and identify the contrast as being a trailer blocking the roadway. The driver then assesses the situation and gains an awareness that a collision is imminent if he takes no action. Next, he selects a response and finally, if time permits, he responds.1

Driver ability to successfully perform these steps depends on the constraints imposed by the physical, situational factors and by innate human limitations and predispositions. The following discussion describes how these two sets of factors operate when a driver faces a potential underride collision. I also discuss how scene photographs can mislead jurors.

Situational Variables

Tractor Headlamp Illumination

In many cases, the tractor-trailer is stopped with its headlamps pointing in the oncoming driver's direction. The headlamp beams shine in the driver's eyes and cause loss of contrast sensitivity. This occurs through two distinct visual mechanisms:

Light Adaptation. Viewers see contrast best when the eye is adapted to the prevailing ambient light conditions. A driver traveling a dark roadway would be adapted to a relatively low light level. His adaptation level, however, undergoes changes as he approaches the bright trailer headlamps. This bright light causes the driver to light adapt his fovea, where contrast vision is best. When the driver looks back down the road he is then mis-adapted for the dark area directly ahead. His ability to see the trailer is reduced.

Glare. A glare source is defined as a light that is significantly brighter than the prevailing light adaptation level. Glare effects fall into two main categories. "Discomfort glare" is the unpleasant visual sensation caused by a bright light. As the driver approaches the glaring headlamps and their brightness becomes very high, he may experience visual discomfort that causes him to look way from the light source. A glare source originating from the tractor in the left oncoming traffic lane may cause a driver to look somewhat rightward rather than directly straight down the roadway. Objects in the road ahead are then seen in peripheral vision, which has lower contrast sensitivity.

The second type of glare is called "veiling," because light from the glare source enters the eye and scatters around the ocular media. The viewer is effectively looking through a veil to see the world, and loses contrast sensitivity.

The amount of glare depends on several factors. Glare increases as the amount of light entering the eye grows. The closer the driver gets to the tractor, the more light entering the eye. Further, glare increases as the angle between the light source and the sightline decreases. Glare is likely severe on a narrow two-lane road, where the tractor's left headlamp may lie as little as 6 feet or less to the driver's left. Lastly, as people become older, the eye clouds, causing more internal scatter and more veiling. I discuss this more fully below.

Several other factors affect glare. One is height of the headlamps. Tractors create large glare effects because they have their headlamps mounted relatively high, putting them close to eye height for a normal car driver. The higher mounted headlamps project farther so they affect the driver at a greater distance down the road.2

Glare also increases when the viewer looks through scratched/dirty surfaces such as windshields and even eyeglasses. These surfaces scatter light more and increase veiling. Further, the scratches become visible and mask the roadway scene ahead. Rain also lowers visibility distances by a half or more.

Trailer Angle

The trailer's angle across the roadway is also an important variable. Visibility and conspicuity of the retroreflective tape, side marker lamps and the trailer itself decline as the angle departs much from 90o, when the trailer is perpendicular to the roadway.

Retroreflective Tape. The effectiveness of a given retroreflective tape strip depends on several factors, including "entrance angle," the angle between the headlamp beam and the retroreflective tape surface, and "observation angle," the angle between the headlamp beam origin, the retroreflective surface and the driver's eye (Figure 1).

Figure 1 Entrance and observation angles.

If the trailer angles across the roadway, then the increased entrance angle reduces the percentage of incident light that is reflected back in the beam direction and to the driver's eyes. Diamond retroreflective material3 is frequently used for tractor trailer markings. It is clear from the table that tape effectiveness falls significant when entrance angle is higher.

Moreover, the effectiveness also falls with increased "observation angle," the angle formed by the eye, headlamps and retroreflective surface. Drivers sitting higher above their headlamp beams, such as those in a large truck, have higher observation angles.

The lowered reflectance and brightness of angled retroreflective tape also affect distance perception. Even if the driver sees the retroreflective tape, he must interpret its meaning and judge its distance. When objects are very small and approach the resolution limit of the eye, viewers confuse, size, distance and brightness, and they judge the apparent distance based on their brightness. Dirt and tape wear will further lower brightness and increase apparent distance.

Side Markers Lamps. People see objects by sensing the images that they project on the eye. This image size depends on the object's orientation. Look at your hand in front of your face. Now turn the hand at an angle and notice that the profile it presents to the eye shrinks. The same effect shrinks the effective size of side marker lamps on angled trailers. For example, if the trailer angle is 45o, then a 3 inch lamp effectively shrinks in size to 2.1 inches wide. At 60o, it is effectively 1.5 inches wide. Moreover, if the lamp output is directional, then it will aim away from the driver's eye.

The side marker lamps, like the reflective tape, have a very small size that causes difficulty in judging distance. As they shrink, they act increasingly as "point sources," whose distance is difficult to judge. They are also easily confused with other point sources such as reflectors on mailboxes, etc.

Trailer Type. The effect of angle depends partially on the type of trailer, tanker, box or flatbed. When light hits a trailer, the direction of scatter depends on the surface smoothness. At one extreme, a perfectly rough matte surface, called a "Lambertian surface," scatters light equally in all directions. In this case, the angle does not affect the amount of light reflected from the headlamp beam back to the driver. At the other extreme is a perfectly smooth "specular" mirror surface, which reflects light only in one direction according to the rule, "angle of reflectance equals angle of incidence." Real surfaces generally fall somewhere between Lambertian and specular. A smooth metal tanker, however, will do a good job of approximating a specular surface. Suppose that a trailer is sitting at a 45o angle in the roadway as shown in Figure 2. The headlamp beams strikes the trailer at a 45o angle. Most light then reflects at a 45o angle, which is a 90o angle to the driver's eye. The driver will see only a black hole where the trailer is located.

Figure 2 Reflection created by a purely specular tanker trailer.

Since the shiny metal was not perfectly specular, some amount of light might have reflected back in the beam direction toward the driver. However, the reflectance problem was compounded by the tanker's rounded side. Light hitting the upper curved part would also reflect upward while light hitting the lower part would reflect downward, so both miss the driver's eye. At most, a driver might see a vague, thin line of light, where the curved surface was perpendicular to the ground. This is likely a very unusual sight that would be difficult to identify.

A white box truck is less specular, so more of the light scatters in directions back toward the driver. The amount depends on smoothness of the finish, and cleanliness of the surface - dirty trailers are more matte, which is good, but reflect back less light overall, which is bad. Lastly, dark colors reflect less light than white. It is irresponsible to paint a trailer any color other than white. Painting trailers with a large colored corporate logo is also a bad idea.

Finally, flatbed trailers are most difficult to see because they reflect no light back to the driver. This expectation has been confirmed in studies on retroreflective tape effectiveness. The ability of retroreflective tape to reduce accidents is greatest for flatbed trucks because the trailer itself presents no information to the oncoming driver4. If the flatbed is carrying a load, the cargo may reflect some light.

These are all general principles, but trailer visibility can be determined scientifically with more precision. The technique requires a re-creation with an exemplar tractor-trailer and light measurements using a "luminance photometer," a specialized device that measures the amount of light reaching a viewer's eye. Very briefly, the investigator reads the amount of light coming from the background and from the trailer. Next he calculates the contrast, the difference between the trailer and the background. Lastly, he compares the trailer contrast to data showing the amount of contrast that a viewer would require. If the required contrast exceeds the available contrast, then the trailer is not visible. Procedural details are described in Green et al., (2008).

Even if the trailer is visible, however, this does not mean that the viewer is likely to see it. Seeing depends on many factors, including expectation, identification and situational assessment. Drivers do not expect to see objects blocking the road; if seen, the distance and meaning are uncertain. For example, the top of a box trailer might easily be interpreted as the horizon in dim light. I discuss this more fully below.

Driver Variables


Glare Susceptibility. Older drivers are less likely to avoid an underride collision. I have already mentioned their higher glare susceptibility. The International Commission on Illumination method for calculating glare shows that veiling doubles by about age 62 and trebles by about age 74. Older viewers are also slower to recover from glare and to readapt to dim light.

Reaction Time. There is a common myth that age does not affect driver reaction time. It is true that many studies find no slowing of driver perception-reaction time with age5. However, these studies are performed in very simple situations where there is little uncertainty or complexity. Many other studies6 show that as uncertainty and complexity grow, performance falls for all viewers but it falls faster for the elderly. An older driver confronted with the vague outline of an uncertain shape at an uncertain distance is likely to respond more slowly than a younger driver. The effect will be especially great when confronted with an "avoidance-avoidance" conflict. (See below.)

Other Visual Losses. A full discussion of visual losses with age would be require many textbooks, so I'll just add a few more. First, older viewers have poorer contrast sensitivity, especially in the low illumination conditions of nighttime driving. Second, they have poorer ability to notice objects in the visual periphery. Third, they are poorer at judging motion. All of these can affect their ability to sense, identify and avoid trailers blocking the roadway.

Reaction Time

Many who analyze underride accidents (mis)apply the "standard" 1.5 seconds reaction time. However, reaction times can rise dramatically when a trailer is blocking the entire roadway. First, the
1.5 seconds is not universally applicable. It is derived from studies performed in daylight conditions, not low visibility nighttime viewing. Further, drivers may face an "avoidance-avoidance conflict."

A person deciding upon response alternatives weighs the possible outcomes of the various responses. Some outcomes are good" and produce a tendency to approach. Other outcomes are "bad" and produce avoidance. If choosing between a good and bad (approach-avoidance) outcome, the decision is easy and reaction time is fast. If choosing between two good outcomes (approach-approach), then there is some conflict and reaction time is slower. The worst situation, however, is the avoidance-avoidance conflict when both alternatives are bad. The reaction time becomes very long because the viewer vacillates.

A driver who finds himself in imminent collision with a trailer blocking the entire roadway may be in an avoidance-avoidance conflict. The only choices are braking and steering. When very close, there is not enough braking time, so steering is the only alternative. However, the trailer was blocking the entire road. Steering leftward takes him across the road toward the tractor and its glaring headlamps. Steering rightward is possible, but the trailer extends off the roadway blocking the path. There is no escape. The problem is compounded by stress-produced, "perceptional narrowing," which many authors incorrectly equate with tunnel vision. In fact it is much more. Easterbrook7 , who popularized the term, meant that under stress viewers reduce the amount of information, both in the world and in memory, used in decision making and further minimize the set of alternative responses considered.

In sum, reaction time may be very long for the obvious reason that the viewer really doesn't want to choose an available response because none avoids the problem. All of his escape routes are blocked, so he has no favorable alternative to choose. It is no wonder that drivers in underride collisions frequently make no attempt at avoidance.

Driver Cognition

There is much more to seeing than mere visibility. In order to be consciously perceived, an object must engage attention and then be identified. Highly visible objects are more conspicuous, so the factors that reduce object visibility also reduce likelihood of the object drawing attention. However, even visible objects may not engage attention for many reasons. One factor that determines attention is expectation. Objects blocking the roadway are relatively rare effects, so drivers are less prone to notice them. Moreover, driving is not a series of discrete encounters with various road objects. Rather, it is a continuous task that smoothly proceeds through time. Drivers generally expect the future to unfold in the same way as the recent past. Imagine a driver who has been traveling on a relatively deserted dark road at night. He has encountered few if any conditions that require much attention or change in his steering or braking. He has little reason to expect a sudden road blockage. He would not be looking for it or expect it when it occurs.

Further, the tractor headlamps may mislead the driver. The driver must interpret the meaning of the oncoming tractor headlamps as he approaches. How far is it? Is it just the horizon? What does it mean? The visual ability to perceive "looming," the movement of objects directly toward the eye is limited until the object is very close. In the absence of looming cues, the oncoming driver will be unlikely to interpret the headlamps as belonging to a vehicle that is moving toward him at a normal rate of speed rather than to a stopped vehicle that is backing across or turning out of a side road or driveway.


Many accidents occur when drivers are in a low state of arousal, which lowers performance. The low arousal stems from two main sources. One is circadian rhythms, the normal 24 hour arousal cycle that all people experience. For people on a normal sleep-wake cycle, they will have a major arousal starting around midnight until 4 or 6 in the morning. Fatigue may also lower arousal during this period, but even a well-rested person should be expected to suffer declines in attention, object identification, situational awareness and perception-reaction time. The second source of lowered arousal is "vigilance decrement." People who have been performing a monitoring task, such as drivers on the roadway, typically exhibit a loss in vigilance in as little as 30 minutes, especially if the task is monotonous. A person driving a dark road with little to see is a prime candidate for such a decrement.

Documentation By Nighttime Photographs

Scene investigators often make extensive photographic documentation of the collision. These should not be used in any attempt to portray visibility conditions at the time of the accident. They are not only useless, but they are highly misleading. The trailer and especially the retroreflective tape will generally appear far more visible and conspicuous in the photograph than they would have been to the driver. As a result, they should not be shown to jurors.

The reasons that nighttime photographs are misleading lies in both the photographs themselves and in the responses that they elicit from viewers. I have discussed this issue in detail elsewhere8, so the following is just a brief overview.

First, some or all of the pictures may be taken with added light sources, such as a flash or the headlamps or flashers of other vehicles which arrived after the collision.

Second, cameras and photographs create distorted images, especially of night scenes. Camera light meters set exposure on the assumption scenes are evenly illuminated and that average scene reflectance is 18%. The camera sees a mostly dark field and sets exposure to accommodate this low light level. Bright areas, such as side marker lamps and reflective tape, are then drastically overexposed and have unrealistically high contrast. As a result, lights appear far more visible than they would have been to an actual viewer.

Lastly, perception is a function of the viewer as much as the image. I have already described how the expert had many advantages in seeing the trailer. The jurors have the same advantages and perhaps more. The jurors' sensory systems are different than those of the driver. I have already explained how factors such as mis-adaptation and veiling glare would have lowered driver contrast sensitivity. Jurors also get to inspect the photographs at their leisure with no time constraints. Moreover, the jurors knew what was there, what was going to happen, and what the outcome would be.


The task faced by a driver approaching a trailer blocking a dark road is formidable. He must sense, and notice the trailer. He must correctly identify it and assess its distance. He may have to perform these tasks while facing the glare of tractor headlamps and be impaired by the effects of aging, bad weather or low arousal. He must then select a response by attempting to resolve an avoidance-avoidance conflict while under stress and perceptual narrowing.

It is relatively easy to point out the physical variables that cause these difficulties. Headlamps change the eye's adaptation state and create scattered glare in the driver's eye. Trailer angle reduces the effectiveness of the retroreflective tape, shrinks the size of the side maker lamps and reduces the amount of headlamp luminous intensity reflected back to the driver. Some viewer variables are also obvious. Aging increases glare susceptibility and lowers contrast sensitivity. Contrary to common lore, aging can also dramatically increase perception-reaction time in uncertain situations, such as identifying the trailer and resolving the avoidance-avoidance conflict.


1 See Green, M. et al. 2008. Forensic Vision: With Applications To Highway Safety. Lawyers & Judges Publishing: Tucson.

2 Headlamp aim is a key factor in creating glare. Headlamps misaimed upward or leftward cause more glare. However, it is usually impossible to go back and to determine the actual headlamp aim at the time of the collision.

3 3M Product Bulletin 4000, 2005.

4 Morgan, C. (2001) NHTSA Report Number DOT HS 809 222.

5 "How Long Does it Take to Stop? Methodological Analysis of Driver Perception Brake Times," Transportation Human Factors, Vol. 2, pp. 195-216, 2000.

6 E. g., Simon, J., and A. Pouraghabagher 1978. The Effect of Aging on the Stages of Processing in a Choice Reaction Time Task. Journal of Gerontology, 33. 553-561.

7 Easterbrook, J. A. 1959. The effect of emotion on cue utilization and the organization of behavior. Psychological Review 66, 183-201.

8 Supra note 1.

Other Topics
Personal Injury: Road Accidents
  • Is The Moth-Effect Real?
  • Human Error in Road Accidents
  • Reaction Time
  • Let's Get Real About Perception-Reaction Time
  • Why PRT Is Not Like Gravity
  • Vision in Older Drivers
  • Weather and Accidents: Rain & Fog
  • Accidents At Rail-Highway Crossings
  • Seeing Pedestrians At Night
  • Underride Accidents
  • Rear End Collision: Looming
  • Night Vision
  • Distracted Pedestrians
  • Failure To See
  • Perception-Reaction Time (PRT) Programs
  • Twilight (3.3 lux) As A visibility Criterion
  • Perception-Reaction Time (PRT) Programs
  • Human Error And Fault Tolerance
  • Junk Science Meets Impaired Drivers
  • Personal Injury: Warnings & Product Defects
  • Warnings and Warning Labels
  • Warning Effectiveness Checklist
  • The Psychology of Warnings
  • Drugs, Adverse Effects & Warnings
  • Are Warnings Effective?
  • Human Error Vs. Design Error
  • Product Misuse And "Affordances"
  • Safety Hierarchy: Design Vs. Warning
  • Thinking Like A Human Factors Expert
  • Personal Injury: Other
  • Diving Accidents in Pools
  • Falls Down Steps
  • Medical Error
  • Computer & Medical Error
  • Nursing Error
  • Criminal & Police
  • Errors in Eyewitness Identifications
  • Perceptual Error in Police Shootings
  • Eyewitness Memory Is Unreliable
  • Human Factors In Forensic Evidence
  • Intellectual Property
  • "Any Fool Can See The Trademarks Are Different"
  • Measuring Confusion For Intellectual Property
  • Color in Trademark and Tradedress Disputes
  • Color Functionality: A Case Example
  • Forensic Human Factors
  • Seeing Color
  • Determining Visibility
  • "Inattentional Blindness" & Conspicuity
  • Computer animation has perceptual limitations
  • Photographs vs. Reality
  • The Six Laws Of Attention
  • What is "inattention?"

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