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Seeing Pedestrians At Night
Marc Green
The visibility of pedestrians at night is a complex phenomenon. Below, I outline some of the major issues. See Green (2024) and the page Why Pedestrians Die for more additional discussion.
Pedestrians Usually Appear In Peripheral Vision
A Driver can only avoid an accident if he sees the pedestrian
relatively far ahead. The driver must detect the pedestrian, recognize the
impending collision, and then react by pressing the brakes. Suppose a driver is
traveling at 60 mph (88 feet per second) and suddenly sees a pedestrian. "Normal" perception-reaction time1 for a lane incursion by a
pedestrian (Green, 2000) is about
1.5 seconds. During this time, the car will have moved forward 132 feet (1.5 x
88). Cars don"t stop instantaneously, however, so the vehicle continues forward
after brake depression until friction halts all motion. This would require
another 150 feet and take about 3.4 seconds. As a result, the driver must see
the pedestrian 280 feet and 4.9 seconds in advance. At city driving speeds of
35 mph, the stopping distance is still 138 feet and the stopping time is 3.6
seconds. These calculations also assume good conditions and ignore brake lag, the time from touching the pedal to full depression and lock up, which is likely .25-.50 sec. The requirements
increase when the road is wet, the vehicle is heavier (a fully loaded tractor
trailer can take 400 feet or more), the terrain slopes downhill or the driver"s
reaction time is slowed by age, alcohol, fatigue or distraction. (Think cell
phone.)
The figure below shows what occurs when a driver approaches
a pedestrian. First, suppose the pedestrian is walking at a steady pace toward
the roadway and across the car"s path.
The driver is usually looking directly ahead. The pedestrian
must be seen in peripheral vision because there is an angle between the
driver"s direction of gaze and the pedestrian"s location. The pedestrian is
hard to detect because people are much poorer at seeing objects off the direct
line-of-sight. The eye has lower peripheral acuity and visual attention is
usually concentrated where the viewer fixates directly ahead. Attention
becomes more focused ahead when visibility conditions are poor or traffic is
heavy.
Next, the driver travels forward and the pedestrian walks
leftward. If the two are on a collision course (figure middle), the angle
remains constant. The pedestrian never leaves peripheral vision and may go
unseen. If the pedestrian speeds up (say, to beat a crossing signal) or the
driver slows, the angle shrinks and then s/he will move toward the center of
vision. The driver will then see the pedestrian suddenly appear out of nowhere.
In some cases, this may be literally true if the pedestrian had been hidden
behind a view-blocking windshield post.
The driver will most likely detect the pedestrian only if
there is an eye movement to the right. This may occur by chance or if the
pedestrian is highly conspicuous for some reason, but drivers are usually
looking for pedestrians only at likely locations (Green, 2002). Drivers won"t
expect to see pedestrians in rural roads or other areas where they have learned
through experience that pedestrians are unlikely to appear. For example, three
quarters of the pedestrians killed in road accidents are hit outside of
crosswalks, probably because drivers do not expect them there.
In another common scenario, the driver approaches with the
pedestrian remaining stationary on the sidewalk (right figure). As the distance
between the two shrinks, the angle grows. The pedestrian moves farther into
peripheral vision and becomes increasingly difficult to detect. Once the
pedestrian starts walking or running, the situation turns into the middle case.
This scenario reveals an interesting anomaly; the best
opportunity for the driver to see the pedestrian often occurs when the car is
still far away. As the car approaches, the pedestrian recedes farther into the
driver"s peripheral vision. At larger distances, the pedestrian is closer to
the driver"s line of sight and is more detectable.
Unfortunately, there is a catch-22 because at greater
distances the pedestrian"s image is smaller, which decreases visibility. If the
driver is 4 seconds away from the pedestrian, the sight distance is about 350
feet at 60 mph and about 200 feet at 35 mph. These are great distances to see a
pedestrian, possibly against a cluttered background.
In sum, the geometry of the situation, and the innate human
visual loss for objects located in the periphery and for small objects,
conspire to make pedestrian detection a difficult task. The situation is even
worse at night because headlamps do not allow drivers to see very far ahead.
Pedestrian Visibility Dims At Night
When scaled by the number of miles driven, pedestrian
fatality rate is three times higher at night. Part of the reason is greater
chance of driver drinking and fatigue, but the critical factor is lower
visibility due to reduced ambient illumination. People have contrast sensitivity in dim
light. Moreover, the eye exhibits "night myopia" focusing too near and causing
distant objects to blur.
The obvious solution, increased highway lighting, is
impractical due to high energy costs. Moreover, there would also be a
great outcry against the resulting "light pollution." Since road lighting cannot be made sufficiently high for daytime safety levels, drivers typically
rely on headlamp illumination to detect pedestrians. However, normal low-beams
make pedestrians visible only at relatively short distances. The key concept is
"assured-clear-distance," which refers to the distance ahead that a driver can
see a pedestrian on the road. Most drivers are taught to drive slowly enough
that they could stop for a pedestrian who just falls within their
assured-clear-distance, otherwise they would be "overdriving" their headlamps.
Some US states have even made this a matter of law; anyone who overdrives
his/her headlamps and has an accident is automatically guilty. However, automobile
headlamps provide such a short assured-clear-distance that even drivers who
obey the speed limit are often overdriving their headlamps.
People fail to slow sufficiently at night because they are
unaware how poor their vision has become. Humans have two distinctly different
types of vision, focal and ambient. They differ in the visual tasks that they
perform, the parts of the visual field they examine and their pathways through
the brain. Roughly speaking, focal vision tells us "what is there" while ambient
vision tells us "where we are"
Focal vision is used for detecting and recognizing objects,
such as pedestrians. It is centered along the line of sight, so when we want to
recognize an object, we turn our eyes to look directly at it. Focal vision declines
rapidly in dim light. In fact, many people with impaired vision who are unable
to receive driver"s license still have vision superior to that of a normal
person at night.
Ambient vision is used for determining location in space and
orientation in the environment and to perform tasks such as steering a car. It
operates out in the visual periphery and needs only detect faint large shapes.
Most significantly, ambient vision is not greatly impaired when light level
declines. Drivers can steer the car just as well at night as during the day and
feel little need to slow. They do not realize that their ability to see
pedestrians has been greatly reduced.
Many studies have investigated the exact distance at which
normal headlamps permit pedestrian detection. There is no single estimate
because the distance is a function of many factors, including pedestrian
clothing and location, and driver age and expectation.
In one study, the average driver saw dark-clothed
pedestrians standing "a foot or two" to the right of the car at a range of 150 feet
while 90% of the drivers fell in a range from 50-250 feet. When pedestrians
stood to the left, the visibility distances halved. Using some reasonable
assumptions, the authors concluded that a driver traveling 55 mph would fail to
see a pedestrian on the right in time to avoid collision 45% of the time. If
the pedestrian is standing to the left, the number grows to 95% - the driver
will almost always hit the pedestrian.
Switching from dark clothing to a white vest markedly
improved visibility. Drivers then detected pedestrians on the right at about
300 feet and pedestrians on the left at about 200 feet. Theoretically, the vest
reduces the number of accidents to 3% right and 9% left, or an improvement by a
factor of 10.
The benefit of lighter clothing is to be expected since detection depends
on the amount of light reflected from the pedestrian back to the driver"s eye. This, in turn, is the amount of light falling on the
pedestrian multiplied by reflectivity of the clothing. Pedestrians could be
made more visible either by being more brightly illuminated by a closer
headlamp or by wearing lighter, more reflective clothing.
Driver age is another
important variable. A group of older drivers detected the pedestrians at only
60% of the distance of a younger group. Older drivers traveling 55 mph would
then hit almost all dark-clad pedestrians. Even at 35 mph, there would be
insufficient time to stop for half of the pedestrians standing on the right.
Although these predicted accident rates may seem alarming,
the reality is even worse. Most studies use drivers who were told to look for a
pedestrian ahead. They expected to see the pedestrian and knew the approximate
location. Real drivers have no such knowledge and certainty. When drivers are
tested without being told that a pedestrian would be ahead, visibility
distances decreased by up to 50%. As a
rule of thumb, unalerted drivers will seldom see a pedestrian at distances much
greater than 100 feet. Recall from above that drivers traveling at city speeds
of 35 mph (about 60 kph) take about 138 feet to stop. They are overdriving
their headlamps by a large margin.
Headlamps Aim In The Wrong Direction
Given the dangerously short assured-clear-distance that headlamps provide,
it might be expected that manufacturers would increase headlamp power. This is
not possible, however, because headlamps must operate under an important
constraint while making pedestrians more visible, they must not put blinding
glare in the eyes of oncoming motorists. Headlamp brightness must be limited,
lowering the distance at which they could potentially illuminate pedestrians.
Further, the headlamps must aim away from the eyes of the
oncoming drivers. The figure shows a typical beam pattern, the spread of light
leaving the car from a properly aligned headlamp. The horizontal and vertical
lines intersect at the center of the view ahead and the red x marks a circular
area that is the highest intensity in the beam"s center. The roughly circular
areas expanding outward indicate areas of equal illumination. The farther from
the beam center, the lower the light level.
The diagram shows that the beam"s center aims down and to the right hand
quadrant. An oncoming driver"s eyes would be located in the upper left-hand
quadrant, so the beam will not hit the oncoming driver square in the eyes and
cause blinding glare.
The down-right aim, however, greatly decreases pedestrian visibility. The
beam hits the road close to the vehicle, rather than illuminating objects at a greater
distance. Moreover, pedestrians approaching from the left will be far less
visible than those approaching from the right. I have already noted that
studies find visibility distance is much shorter for pedestrians located to the
vehicle"s left.
The downward aim also causes headlamps to illuminate pedestrians in the
worst possible location, near the ground. To see why, consider where the light
from car headlamps hits the pedestrian and background (see Figure). Drivers
detect pedestrians by their contrast, the difference in brightness between
pedestrian and background. In an ideal world, headlamps would maximize contrast
by illuminating the pedestrian but not the background. Headlamp illumination
falls with distance, so the farther the background from the pedestrian, the
less light it receives and reflects back and the more the ideal situation is
approximated.
The figure shows that the beam aim creates poor lighting
conditions. Point A, the pedestrian"s leg, is only slightly in front of point
B, the road just behind the leg. The down-aiming car headlamp then illuminates
the background almost as well as the pedestrian"s lower body. Visibility
depends on the difference in reflectance between the pedestrian"s pants and the
roadway, so they receive similar intensity light.
Now look at point C high up on the
pedestrian"s body. The background is far away, so it does not reflect
significant headlamp illumination. The contrast between object and background
will then approach the ideal situation. Unfortunately, the downward pointing
headlamps throw the most light where it does the least good - where the
distance between pedestrian and background is small - and illuminates the least
where it does the most good - where the distance between pedestrian and background
is large.
This partly explains why children are difficult to see on
the road. They are closer to the ground, so they must be detected in a
situation where the headlamps illuminate pedestrian and roadway similarly. Of
course, the problem is compounded by their small size and by their walking gait,
which drivers do not readily recognize as belonging to a person.
Lastly, many factors can reduce or alter headlamp
illumination. Headlamps are often misaligned. Even the weight distribution in
the car can change headlamp aim. For example, heavy objects in the trunk lower
the car"s rear and cause the headlamps to aim upward. This may help the driver see farther, but the resulting glare is
sure to reduce pedestrian visibility for the oncoming motorists who are blinded.
Conversely, heavy objects in the front seat cause headlamps to aim farther
downward. Research also shows that most people are driving cars where dirt is
significantly reducing their headlamps' effective output. The problem is
especially great in rain, when dirty water splashes up from the road coats the
headlamps.
So far, it sounds as if the more light that
headlamps throw on the pedestrian, the greater the visibility. Further, light
colored clothing should increase visibility since more light is reflected back
to the driver"s eye. This view, however, is an oversimplification because
increased light levels can actually decrease pedestrian visibility.
The reason is that people don"t see light; they see
contrast. A driver may see a pedestrian in either positive or negative
contrast. Negative contrast occurs when a dark object lies on a bright
background (left) while Positive contrast occurs when a bright object lies on a dark
background (right). In the driving literature, negative contrast is often called "silhouette" and positive
contrast is termed "reverse silhouette."
In real road situations, positive and negative contrast can
compete, rendering the pedestrian invisible. Imagine a car on a dark rural road
at night. The driver will likely see a pedestrian in positive contrast because
the light from the headlamps reflects off clothing back to the eye and
because the background is dark - there is little or no background lighting. In
this case, pedestrian clothing is an important factor in visibility. If wearing
light clothes, then most of the headlamp illumination reflects back to the
driver"s eye. Visibility will be far lower for pedestrians wearing dark clothes
that reflect less light. The studies discussed above all used dark roads where
positive contrast would be maximized.
Negative contrast occurs when the prevailing light comes
from behind. If there is backlighting from streetlamps, stores, sky glow,
headlamps behind, moon, etc, then a pedestrian may appear as a silhouette
against the lighted backdrop. In this case, the pedestrian"s outline is black
regardless of clothing. This might seem fortunate because viewers have slightly greater contrast sensitivity to negative contrast. However, it is probably more difficult to notice an object with negative contrast. When people first view a scene, they perform "figure-ground" segregation, the task of breaking the scene into foreground objects that deserve against a background which does not. Viewers tend to judge bright parts of the scene as foreground, which makes dark objects unnoticed in the background. A dark pedestrian is especially a hole in the scene. Moreover, viewers are better at noticing and identifying objects if their internal 3D structure is visible. The silhouette seen in negative contrast has no visible internal structure.
Complications arise because positive and negative contrast
can cancel one another. For example, light colored clothing decreases negative
contrast by better reflecting any ambient front light back to the driver"s eye.
With negative contrast, dark clothing creates better visibility. Finally, a pedestrian wearing different
color top and bottom clothing can appear as two smaller objects, one of
positive contrast and the other of negative. This further impairs pedestrian
visibility because 1) each piece is small and therefore less detectable and 2)
the object is more difficult to identify as being a pedestrian.
The dominant contrast polarity often depends on viewing
distance. Since car low beam headlamps provide usable light for a distance of
only 100 feet, only shorter distances promote positive contrast. At longer
distances, negative contrast is favored if there is much backlighting. The
pedestrian then switches from negative to positive contrast as the driver
approaches. At some point during the transition, however, the pedestrian must
go through a period of zero contrast and be literally invisible.
The conflict between positive and negative polarity also
explains why twilight is an especially dangerous time for pedestrians. The
overall illumination decreases quickly, so human contrast perception degrades
rapidly. Yet, there is still some ambient illumination to provide both front
and back lighting and hence both positive and negative contrast. As a result,
the driver has reduced contrast vision while the positive and negative
contrasts are canceling one another.
Lastly, negative contrast can be good for providing visibility, but it is poor for conspicuity and for recognition. In positive contrast, the viewer can distinguish the pants, shirt, face, etc. and can more readily recognize the object as a person. Negative contrast provides just an outline that has few cues as to the object's meaning, which is important for capturing attention. This recognition problem will be especially bad for a pedestrian walking in the same or opposite direction to the car. In this case, the outline is simply an oblong with few form or motion cues that would mark it as a person.
Pedestrians Help Make Themselves Invisible
Pedestrians are themselves partly to blame for their
invisibility and high accident rate. I"ve already noted that pedestrians
frequently confound driver expectation by crossing roadways at arbitrary
locations. Further, people also generally prefer dark clothing. Safety
authorities often suggest that pedestrians would become more conspicuous if
they would wear reflective material that sends more light back to the driver"s
eye. Research typically confirms that pedestrians are visible at greater
distances when they wear a reflective tag or vest. However, there are some
drawbacks to reflective material. One is that reflective material sends light
primarily in one direction. If the headlamps hit the material at the wrong
angle, the reflected light goes in the wrong direction and does not hit the
driver"s eye, and the reflector will appear dark. Further, if the reflective
material covers a small part of the body, then the driver may detect its light
but not recognize it as being a person.
Reflective material may also cause pedestrians to be
overconfident. Pedestrians already greatly overestimate their visibility at
night. They see a highly conspicuous car coming toward them and imagine that
they must be just as visible to the driver as the car is to them. Studies
show that pedestrians typically overrate their visibility distance by a factor
of two.
The use of reflective material is likely to amplify
pedestrian overconfidence. Introduction of new safety devices often makes
people feel more secure, so they engage in riskier behavior (Green, 2001a,b). A
pedestrian wearing reflective material may be more likely to assume high
visibility and take more risks.
Lastly, many, if not most, pedestrians killed in automobile
accidents had been drinking or taking drugs. Even moderate doses of alcohol
impair perception, attention and motor skill (the ability to move
quickly). Accident statistics reflect
these effects by showing that alcohol ingestion greatly increases both the
likelihood that a pedestrian will be involved in an automobile accident and the
severity of the resulting injury. One study found that Ÿ of all pedestrians
killed by automobiles had alcohol in their blood streams while another
concluded that alcohol increases the odds of a pedestrian being involved in a
fatal accident by a factor of 5. In contrast, drinking and driving increases
accident rates by a factor of "only" 4. Drunken walking may be even more
dangerous than drunken driving!
The reasons for pedestrian invisibility are complex, but
most lie in the normal operation of human vision and attention. For a driver to
avert a collision, the pedestrian must be seen far ahead, a time when he is likely
to appear in peripheral vision at night, the problems are exacerbated by the
lack of ambient illumination, the limited power and aim of headlights, the
conflict between positive and negative contrast and by night myopia and glare.
Lastly, pedestrians contribute significantly to their own invisibility by
choice of clothing, and risk-taking due to overconfidence in their conspicuity
and to drinking or taking drugs.
Pedestrian Visibility Distance
The question often arises: what is pedestrian visibility distance? Unfortunately, there is no simple answer. Visibility not even necessarily the appropriate criterion, since it is only the theoretical moment when contrast detection is possible. It would be more accurate to refer to pedestrian perception or recognition distance, which is usually much greater. As explained in Green (2024), moreover, determining precise absolute values for performance variables such as visibility/recognition distance, looming threshold, perception-responce time, is difficult to impossible for many reasons.
There are research studies that claim "visibility" distance of 300 feet or more, but these occurred under highly artificial conditions with expected pedestrians2. However, better controlled and more realistic studies put the distance for dark pedestrians to the vehicle left as much shorter. A commonly suggested number is about 100 feet, although I heard a state policeman put the distance at 40 feet.
However, absolute distances vary with the specific situation, making a general estimate impossible. Although exact distances are somewhat uncertain, there is no doubt that a large number of factors modulated pedestrian "visibility distance" at night. These have all been confirmed by road research, although they all are predicable from elementary physics and from the basic psychophysical and cognitive factors that determine contrast sensitivity. Some have already been discussed, but a more exhaustive list would include:
- The pedestrian is unexpected;
- The pedestrian is wearing dark clothing (when the background is dark);
- The pedestrian is wearing no retroreflective surfaces (on dark roads);
- The pedestrian is located to the weaker left (or right in Ireland, UK, etc.) of the headlamp beam;
- When crossing, the pedestrian is traveling faster (creating a larger bearing angle);
- The pedestrian is turned sideways to the driver showing a smaller surface area;
- The pedestrian is not moving his arms and/or standing still in the road;
- Opposing headlamp glare is present, especially for older drivers and for HID/LED headlamps;
- The vehicle lights are set to low-beam rather than high-beam;
- The background is cluttered;
- Atmospheric conditions are rainy, snowy or foggy;
- The roadway is wet and there are glare spots in the ground;
- The driver is older (or impaired in some other way); and
- Driver attention is divided by other vehicles, pedestrians or bicycles;
Endnotes
1At most, this is "normal" perception-reaction time that is expected only during good visibility in daylight condition, in good weather, with normal vision, etc., etc. At night, reaction time is likely longer, possible as long as infinity. Perception-reaction time is a very complex phenomenon, which is affected by many variables. In sum, there is no such thing as a global "normal" value. See Green (2009) for more details.
2Perception distance is difficult to measure since it involves looking into the driver's mind, which is obviously impossible. Instead, studies usually measure "response distance", the distance at which the driver responds. Some attempt to determine a true pedestrian perception distance by adding a hypothesized distance traveled during perception-response time to the measured response distance. The methodological issues in measuring visibility distance are discussed in detail in Green (2017).
References
Green, M. (2000). "How long does it take to stop?" Analysis
of brake reaction times, Transportation
Human Factors,, 2,195-216.
Green, M. (2001a). The psychology of warnings. Occupational Health and Safety Canada,
30-38, October/November, 30-38.
Green, M. (2001b). Do Mobile phones pose an unacceptable
risk? A Complete look at the adequacy of the evidence? Risk Management,
November, 40-48.
Green, M. (2002). Inattentional Blindness, Occupational Health & Safety Canada, Jan/Feb, 23-29.
Green, M. (2009). Perception-reaction time: Is Olson (& Sivak) all you need to know? Collision,
4, 88-95.
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A division of 2057949 Ontario, Inc.
Copyright © 2024 Marc Green, Ph. D.
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