Home Experience Services Contact Us Seminars Attorney's Guide to Perception Resources
33 Reasons For Not Seeing

Marc Green

Viewers often fail to see objects, such as pedestrians and other vehicles. Such failures-to-see have great importance in safety as well as in many other domains such marketing, medicine, and sports. Elsewhere, I have discussed seeing in general and why failure-to-see is so common. Here, I provide a broad overview of the specific reasons for failures-to-see. I present these in tables that are adapted from the book Roadway Human Factors: From Science To Application (Green, 2017), which discusses them in detail. However, such categorizations inevitably begin to fray at the edges. They are didactic and an attempt at cognitively simplifying the large number of visual factors affecting seeing.

The reasons can very broadly be divided into visibility and cognitive. The first table shows 5 visibility factors. External physical factors are partial or complete sightline obstructions. Physiological factors can be optical, neural, or motor. Psychophysical factors usually boil down to those affecting contrast detection. Lastly, these five entries can readily be decomposed further. There are more than 16 subfactors in contrast alone (Green, 2017).


Explanatory Factors



External Obstruction

Physical obstructions completely or partially block the sightline and prevent retinal image formation: e.g. buildings A-pillars, windshields.




Optical imperfections. Blur caused by mis-accommodation and spherical aberration. Cataracts and opacity.


Retinal limitations, e.g., field size, photoreceptor spacing, and scatomata.


Eye muscle movements also reduce vision, e.g., saccadic suppression and blinking.



Factors that determine whether an object differs sufficiently from the background to be visible: e.g., size, adaptation level, retinal eccentricity, etc.

In many cases, however, the cause of failure-to-see is cognitive - the viewer does not consciously see an object which is theoretically highly visible. The usual culprit is attention. The failure to see is especially likely in given circumstances. There is vast a literature on attention and cognition that provides many potential explanations for failure to see in a given set of circumstances. The table below boils this literature down to a single list of 28 cognitive failure-to-see "constructs" that are again grouped into rough categories purely for didactic purposes. Further, the chart constructs require further elaboration.


Explanatory Constructs





Salience is the convergence of sensory conspicuity factors. Objects that are visible but have low sensory impact are less likely to be noticed.


Objects in the foreground of the figure-ground segregation (e.g., bright) are more noticeable.





Attention tuned to the location and objects of highest anticipated meaningfulness, so others not noticed.


Tasks are performed with minimal attentional control, so little attentional is paid to "irrelevant" information.

Familiarity Blindness

The failure to notice scene information that has been irrelevant in the past.



Tunnel vision

Attention is concentrated in the center of the visual field, so information in peripheral vision is unnoticed.


Novices have not yet learned to reduce attention by developing schemata or by using automatic control.

Stress/Narrowing/ Hypervigilance

Stress focuses attention on a narrow set of information, so other information not noticed.

Mental Workload

Attention consumed by one task leaves less capacity to perform others.



Inattentional blindness

Failure to notice even highly visible objects located in the center of the visual field.

Change blindness

Failure to notice a scene change over time. The change may be very fast or occur gradually.

Mind wandering (Internal distraction)

Attention is focused internally rather than externally.



The second in a sequence of two identical objects is not noticed.



Clutter masking

Background contours interfere with attention to foreground objects. (It has some sensory component.)


Attention attracted to the most salient scene object and away from other information.

Cue generalization

Viewers direct attention to the most easily discriminated cue, such as color.


Visible objects in peripheral vision merge and cannot be identified. (It may be a sensory, not a cognitive effect.)



Visual space asymmetry

Different parts of the 3-D visual space are specialized for different attentional tasks, e.g., peripersonal vs. extrapersonal space.

Switching Costs/ Attentional blink

Changing the focus of attention is slow and effortful.

Satisfaction of search

Termination of serial search before reaching the critical information.

Inhibition of return

Once a scene area is searched, the probability of making a return saccade to re-search is reduced.


(Low Capacity and low arousal)

Fatigue/ Lack Of sleep

Fatigue is difficult to define, but usually explained as lowered performance due lack of sleep.

Vigilance decrement

The ability to notice information falls within the first half hour in routine tasks.

Low Workload/ Boredom/Monotony

Low arousal level with long time in dull and unchanging environments.

Circadian Rhythm

Arousal is lower in the troughs of the day 24-hour arousal cycle.


Older viewers are both slower and have lower attentional capacity.



Biases & Heuristics

Attention and decision are guided by mental short cuts designed to increase efficiency.


Attention becomes unnecessary once a "good-enough" solution achieved, then search ceases.

The cognitive construct list is long, but the phenomena overlap considerably because they are different manifestations of our fundamental cognitive architecture and its consequences: 1) conscious awareness is limited by capacity, 2) attention is selective, 3) attention is efficient, trying to use the easiest and simplest selection criterion, and 4) attention generally selects information that is most meaningful based on past experience and on expectation of the future. This overlap is why I call them "constructs" rather than variables or causes1. The criterion for inclusion in the list is only that the authors frequently use the "construct" to explain a cognitive failure-to- see. In many cases, the construct name specifies little more than an experimental paradigm, e.g. "change blindness".

Lastly, the constructs also represent different granularities of explanation. "Inattentional blindness" (failure-to-see central objects) and "tunnel vision" (failure to see peripheral objects) are really just descriptive terms for phenomena that are ultimately caused by more specific constructs (expectation, hypervigilance, mental workload, etc.). They can be useful as umbrella terms when merely describing the functional loss, but they are not very specific. The appropriate level of analysis depends on the discussion context.


1 The definition of "causation" is a knotty philosophical problem. In practice, however, it is often merely a "symptom" that doesn't require an explanation. Causal reasoning normally works through a chain where symptoms have causes which then become symptoms for other causes. Here's an example. A driver is blamed because he responded too slowly in seeing a pedestrian at night. Slow perception-response time is a symptom and low visibility is the cause. Low visibility is then the symptom, and a burned out streetlamp is the cause. The burned out streetlamp is now the symptom, and poor maintenance is the cause. Poor maintenance is the symptom... and so on. The chain stops when the symptom need not be explained. In this example, a human factors analysis doesn't need to know what caused the streetlamp to be burned out, so the cause need not be determined. For an investigating utility company, the causal chain would continue until the poor maintenance were explained.

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
  • Human Error And Fault Tolerance
  • Why Pedestrians Die
  • Bicyclists! Read This To Save Your Life
  • 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
  • Personal Injury: Other
  • Diving Accidents in Pools
  • Falls Down Steps
  • Medical Error
  • Computer & Medical 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
  • Visual Human Factors
  • 33 Reasons For Not Seeing
  • Determining Visibility
  • "Inattentional Blindness" & Conspicuity
  • Computer animation has perceptual limitations
  • Photographs vs. Reality
  • The Six Laws Of Attention
  • What is "inattention?"

  • | Home | Experience | Services | Contact Us  | Seminars/CLE | Attorney's Guide  | Resources |

    Send this link to someone

    Copyright © 2013 Marc Green, Phd
    Home Page:
    Contact Us