visual target detection models for civil twilight and night driving conditions

7/21/2019 Visual Target Detection Models for Civil Twilight and Night Driving Conditions

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Visual Target Detection Models for Civil Twilight and Night Driving Conditions

Helmut T. Zwahlen, Ph.D.

Russ Professor

Human Factors and Ergonomics Laboratory

Department of Industrial and Manufacturing Systems Engineering

Ohio University, Athens Ohio 45701-2979

(740) 593-1550

Thomas Schnell, Ph.D.

Assistant Professor

Cognitive Human Factors Laboratory

Department of Industrial Engineering

The University of Iowa

Iowa City, Iowa 52242-1527


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ABSTRACT

A luminance contrast based computer visibility model is discussed in this paper and compared with the

civil twilight method which has recently been introduced. The civil twilight method attempts to predict the

visibility of ordinary objects (reflectance 3% to 79%, average size) using only the headlamp illuminance

at the target. It is suggested by the authors that the one-factor approach used by the civil twilight method

is insufficient to satisfactorily address target visibility in the field. Developers of more advanced visibility

models generally attempt to design their models based on the current state of the visibility research and

with enough capability to obtain a reasonable degree of realism. The authors consider the level of the

benchmark illuminance (3.2 lx) used in the civil twilight method to be too high, leading to very short

detection distances for pedestrians (refer to literature review) under automobile headlamp illumination at

night. The developers of the civil twilight method claim that the 3.2 lx visibility benchmark is based on

systematic visual observations made by astronomers over a century ago. The use of the civil twilight

method for pedestrian detection under automobile headlamp illumination at night is strongly discouraged

by the authors of this paper, because the method may be misused by forensic experts if there is a

need to produce arbitrarily short pedestrian detection distances, irrespective of factors including the

clothing reflectance, contrast, pedestrian size, windshield transmittance, atmospheric transmissivity, etc.


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INTRODUCTION

The detection of targets under low light conditions has been of great interest to the scientific visibility

community in the recent past. A rather large body of research is available, providing the scientific

framework for target detection and target visibility, both from a theoretical and empirical point of view.

The list of visibility research that investigated human visual performance is too long to be completely

covered within the scope of this paper. It is, however, generally accepted that Blackwell [1][2][3] was a

major contributor of visual performance data forming the basis of our understanding of the capabilities

and limitations of the human visual system. More or less elaborate visibility models have been

developed in recent years based on such empirical visual performance data. Visibility models have

become an invaluable tool for predicting the visibility of targets under a wide range of viewing conditions.

Recent visibility models include the TARDEC model (army model) [4][5][6][7], the CIE (Committee

Internationale de LEclairage) model [8], the PCDETECT model [9], Adrians model [10], and computer

based visibility models developed by the authors [11][12][13][14]. These models are fairly elaborate and

were generally designed based on recent visibility research. The aim generally is to provide a visibility

model that is sufficiently precise (near true target value) and sufficiently accurate (small dispersion) in its

predictions under selected (possibly wide) range of input parameters. Another approach to target

visibility predictions was taken by Owens et al [15] and Andre and Owens [16] with their civil twilight

method. On the surface, their approach appears to provide a simple, holistic benchmark method to

address the issue of target visibility under automobile low-beam illumination at night. However, their

method has only 1 input parameter, namely the illumination at a point ahead of an automobile at a

selected height, on a plane normal to the illumination axis. It should be evident to readers who are

familiar with the basics of human visual performance at low light levels, that a one factor approach is

insufficient address target visibility in the field. It seems that the use of illumination at the target as a

visibility benchmark is a poor choice anyway. A better choice would be to use the target luminance,

which at least would implicitly account for target reflectance. That would, of course require exact

knowledge of the target reflectance properties. The civil twilight method was deliberately kept simple by


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Owens et al. [15] and Andre and Owens [16], so that the only instrument needed to conduct

measurements in the field would be a illuminance meter.

The members of the visibility research community should rightfully be concerned about the

proliferation of general, holistic approaches such as the civil twilight method. Such a holistic approach to

a specialized field of research seems to be an attempt to undermine and reverse recent advances in

visibility modeling. Human visual performance is not a field that should be trivialized by one-size-fits all

benchmarks such as the civil twilight method. In light of the vast scientific knowledge that was gathered

to date by many visibility researchers, it is hard to understand why Andre and Owens [16] pose the

general question of how much light do we need to see ? The answer is, it depends. It depends on the

visual target characteristics, on the background characteristics, on characteristics of the light source, on

environmental characteristics such as atmospheric transmissivity, ambient illumination, and on a

multitude of observer characteristics including (but not limited to) age, luminance and color contrast

sensitivity, adaptation level, probability of detection, arousal, expectancy and retinal eccentricity [17]. In

dynamic settings, the list of independent variables may be significantly expanded. It should be noted that

even the most advanced visibility model couldn't guarantee that a given observer would really be able to

detect a target exactly as predicted by the model all the time. In real world settings there may be

unknown physical factors such as dirt on the headlamps or the windshield, transient glare, or unknown

cognitive factors such as focus of attention, distraction, workload, etc. that could potentially influence the

visibility of a target. Although the influence of many of these factors can be modeled, it is not always

sure that their presence is known in a specific case. While even the most advanced current visibility

models are not able to explain all of the variability, it is fairly self evident that they provide far more

precise (near true target value) and accurate (small dispersion) predictions for a specific visibility

situation than a simple model such as the civil twilight method.


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Statement of the Problem

Luminance contrast based driver visibility models represent an important cornerstone in the design of

automobile lighting systems, the design of retro-reflective sheeting materials and pavement markings,

the design of traffic signals, etc. Visibility models are also often used by forensics experts in litigation

cases involving automobile accidents at night. There are a number of relatively advanced and higher

fidelity visibility models [4][5][6][7][8][9][10][11][12][13][14] that are based on the recent and/or present

state of the art in visibility research. These models consider many of the relevant factors of influence.

Thanks to being packaged in computer software, most of these models can be used by specialists with

relative ease.

Owens at al. [15] and Andre and Owens [16] suggest a method by which almost anyone, non-

experts and experts, could categorize a visual target seen under automobile headlamp illumination into

the visible or not visible category, simply by using an illuminance meter. It is our opinion that visibility

modeling should remain the domain of visibility researchers only. Non-experts should consult experts if

they seek a simple answer to the difficult question of how much light do we need to see. Serious

visibility experts know that the answer to that question depends on many issues, non-experts on the

other hand may not be aware of this. The civil twilight method attempts to determine the detection

distance of a target seen at a selected height under automobile headlamp illumination by using the

amount of headlamp illuminance at the target, on a plane normal to the illumination axis as the only

driving factor.

In addition, the level of the benchmark illuminance (3.2 lx) chosen by Owens et al [15] is

considered to be too high, leading to very short detection distances (refer to literature review) under

automobile headlamp illumination at night. Also, the civil twilight method proposed by Owens et al. [15]

does not consider two of the major independent variables, namely target size and reflectance. There is a

need to discuss the advantages, disadvantages, and the structure of visual target detection models as

they relate to dawn, dusk, and night driving conditions.


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REVIEW OF THE TECHNICAL LITERATURE

Owens, Francis, and Leibowitz [15] proposed a simple, functional approach, the civil twilight method,

that according to their claims, allows for quantification of the nighttime motorist visibility under

automobile illumination. The reader should refer to [18] for an additional discussion of the civil twilight

method [15].

The civil twilight method proposed by Owens et al. [15] determines the visibility distance of a

target seen (at a selected height) under automobile headlamp illumination by locating the distance

ahead of the vehicle at which the illuminance provided by the headlamps equals the twilight illuminance

ETD[lx] as indicated by Equation (1)

TD = CD0.3

(1)

where TD is the twilight distance in feet, CD is the luminous intensity in candelas of the (Cyclops)

headlamp in the direction of the target, and 0.3 is the twilight illumination in footcandles (3.2 lx). It should

be noted, that Owens et al. [15] do not consider the two headlamps to be laterally separated but rather

to be both located in the center of the vehicle (cyclops geometry). Owens et al. [15] only refer to the

targets as being ordinary objects, implying that the target reflectance may vary from R=3% to R=79%.

Simply claiming that a visibility benchmark method works for all ordinary objects is vague and

misleading, and it appears that such a method makes very little use of the current state of the art in

visibility research.

Andre and Owens [16] state that they have tested the predictions of their civil twilight method in

the field. They used an automobile (1990 Buick Skylark) with aligned headlamps and simply measured

the illumination from the headlamps at specific grid-points ahead of the automobile at a height of 12,

35, and 59. Andre and Owens [16] found that the distances at which an illuminance of 3.2 lx was

measured agreed rather well with the twilight plateaus provided by their civil twilight method. It should be


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noted that calling such measurements a test of predictions provided by a visibility model is highly

misleading. Andre and Owens [16] simply established the iso-illuminance distances at selected grid

points and vertical heights, nothing more. Their measurements prove nothing, whatsoever, about the

visual performance of human observers under specific conditions. One may agree that such

measurements are useful for checking the accuracy of physical, photometrical quantities. A meaningful

validation, however, would have to include a wide range of observers whose task is to detect a wide

range of targets under a wide range of conditions. Such an experiment would be bound to disclose the

shortcomings of the civil twilight method because this method is unable to explain the large variation that

would be found in such a field experiment.

Other target detection refernces that are discussed in detail in [18] are Hazlett, and Allen [19],

Shinar [20], Chrysler et al. [21], Blomberg et al. [22], Olson and Sivak [23], Strickland, Ward, and Allen

[24], Austin, Klassen, and Vanstrum [25], Zwahlen and Schnell [26], Blackwell [1], Blackwell and

Blackwell [2]. Target detection distances from some of the above references are tabulated in Table 1.


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Basic Issues Regarding Visibility Modeling

In general, models of processes are often developed to demonstrate a concept, to facilitate the

understanding of a concept or process and to describe, in an abstract form, a process the way we think

it works. Sometimes, models are used to predict the behavior of a system without disturbing the system

itself. While approximations are sometimes used in engineering, one should be aware that for such

approximations to be useful, they must provide fairly accurate (small dispersion) and precise (near true

target value) results, they must be validated, and they should be accepted by the majority of the

scientists and engineers. The reader should note that the human performance part of the civil twilight

method has not been validated in the field. Certainly, it would be relatively simple to come up with test

cases involving the factors that are neglected by the civil twilight method to demonstrate that the civil

twilight method may provide completely inaccurate detection distance predictions. Figure 1a illustrates a

basic fact of model building. In order to increase the degree of realism obtained with a model, one has to

increase the complexity and the completeness. It should be noted that Owens et al. [15] claim that the

civil twilight method implicitly accounts for a number of factors by virtue of the generality of the civil

twilight method. The civil twilight method is supposed to encompass the visibility of all targets with an

imaginary average reflectance, an average size, an average background luminance, etc. It is, however,

quite evident that the civil twilight method with its rigid 3.2 lx benchmark value is unable to explain the

large variation of detection distances that are observed for different targets illuminated under different

conditions, and observed by different observers. In model building, one aims at determining the main

effects and as many interaction effects as possible, in order to obtain a high correlation between the

predicted and observed values. Usually, the model with the highest correlation (R2) should be selected

for use until a better model is found.

The visibility research community is called upon to conduct the research needed to build even

more complete models. There should be no doubt that todays state of the art visibility models far

exceed the capability of the civil twilight method in explaining the variability found in the visual detection


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of targets. By no means, however, should one expect a visibility model ever to be free of variation and to

be able to provide precise pinpoint predictions of target visibility. It is well known that human visual

performance follows psychometric functions such as the ones established by Blackwell [1][2]. There will

always be a less than perfect fit (R2


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computers it is fairly easy to obtain the results of target detection models almost instantaneously, thus

further reducing the need for a simple approach such as the civil twilight method.

The Civil Twilight Method, Old Science Taken Out of Context

Owens et al. [15] and Andre and Owens [16] state that their 3.2 lx visibility benchmark is based on

systematic visual observations made by astronomers over a century ago. The simple fact that some

research is over 100 years old does not necessarily lend more credence to its adequacy, especially not

if the research is taken out of context 100 years later. Astronomers back then and today are generally

more concerned about the visibility of astronomical objects rather than target visibility under automobile

illumination. During WWII, the office of scientific research and development established the Tiffany

foundation to further the state of the knowledge in visibility research. A large scale study funded by the

Tiffany foundation was conducted by Blackwell [1], based on the need of the US Navy to learn more

about the threshold contrast of the human eye. This large and relatively expensive study would not have

been commissioned, if the designers and visibility scientists at the time had felt that they already had a

handle on target visibility with the civil twilight observation data provided by astronomers. If these

researchers already felt they needed more adequate data to better understand target visibility it would

appear that more recent data provided by Blackwell [1][2] and other visibility researchers supersedes

observations made by astronomers over 100 years ago. This is especially true since modern

psychophysical experiments were conducted under controlled conditions, allowing for the isolation of

main effects and interaction effects. The human factors and Ergonomics Laboratory at Ohio University,

Athens, Ohio, used extensive visibility field research data to allow the use of laboratory visibility

research data [1][2] in models that predict target visibility under specific real-world conditions (calibration

of model). No psychophysical target detection field research was ever published to determine the validity

of the civil twilight method.


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LUMINANCE AND ILLUMINANCE MESUREMENTS DURING CIVIL TWILIGHT ON AN OVERCAST

EVENING AND ON A CLEAR EVENING

It was hypothesized by the authors of this paper that the ambient illuminance and target luminances

during civil twilight are highly dependent on weather conditions and on the direction in which the

measurements are performed. The measurements reported in this section should provide the reader

with an idea of the range of available ambient natural illuminance and its directionality during the civil

twilight period on an overcast evening and on a clear evening. In addition, luminances were measured

on a gray target and on a white target to obtain the range of luminances as a function of time and

direction during the civil twilight period. It should be noted, that Owens et al. [15] chose an illuminance of

3.2 lx, obtained at the end of the civil twilight on a clear day, as their benchmark illuminance ETDin the

civil twilight method. An overcast evening was chosen for part of the measurements reported herein to

demonstrate that the twilight illuminances can be considerably lower than the value of 3.2 lx reported by

Owens et al. [15] under such conditions. The clear evening measurements were conducted to confirm

that one would obtain twilight illumination values that are considerably higher than the ones measured

during an overcast evening.

Twilight is defined as the transition period from day to night (or night to day) when the sky is not

completely dark. It is technically defined as the period of time beginning (or ending) when the center of

the (refracted) sun is lower than a given elevation. Civil twilight corresponds to the sun being between 0

and 6 below the horizon, nautical twilight to 6 and 12, and astronomical twilight to 12 and 18.

Measurement Site, Setup, and Method

The measurements were conducted on the beach parking lot of the Strouds Run State Park outside

Athens, Ohio (Longitude:82o

, 06', 20" W, Latitude:39o

, 20', 14"N) during the civil twilight on February 3,

1998 (overcast evening) and on March 26, 1998 (clear evening). The measurement cycle was repeated

from 30 minutes prior to the beginning of the evening civil twilight to 30 minutes after the end of the


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evening civil twilight. Care was taken to ensure that virtually no alternative (other than natural) sources

of illumination were present. The gray target had a reflectance of R=0.108 and on the white target had a

reflectance of R=0.92. Both targets were made of posterboard 0.6m x 0.6m. The gray target was spray

painted with a blend of Krylon Satin Black spray paint and Krylon Satin White spray paint. The white

target was left untreated. For the luminance measurements, the targets (see Figure 2) were positioned

in 8 directions: North, Northeast, East, Southeast, South, Southwest, West, and Northwest. The

measurements consisted of repeated cycles of one full revolution of luminance measurements (white

and gray target in each position as indicated in Figure 2) followed by one full revolution of illuminance

measurements including one measurement taken skywards (straight up). The targets were removed for

illuminance measurements. At the end of the illuminance measurement cycle, the Pritchard 1980A was

tilted 90oup to obtain the sky illuminance.

Results of the Luminance and Illuminance Measurements

Figure 3, Figure 4, and Figure 5 summarize the luminance and illuminance values measured on the

overcast evening (February 3, 1998) and on a clear evening (March 26, 1998) at the Strouds run parking

lot site. The measured luminance values (overcast condition) for the white target with a reflectance of

R=0.92 are shown in Figure 3a. In spite of the diffuse overcast sky conditions, a small directionality

effect was found as evidenced by the spread of the luminance curves in Figure 3a. At 6:19pm (end of

the overcast evening civil twilight on February 3, 1998), the white target provided an average luminance

of 0.012 cd/m2. Figure 3b shows the measured luminance values (overcast evening) for the gray target

with a reflectance of R=0.108. At 6:19pm (end of the evening civil twilight), the gray target provided an

average luminance of 0.0012 cd/m2.

Figure 4a shows the luminance values that were measured during the evening civil twilight of

March 26, 1998 (clear evening) on the white target. Again, a substantial directionality effect is observed

and the average luminance of the white target at the end of the civil twilight is 0.52 cd/m2. The clear


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evening civil twilight luminance values of the gray target are shown in Figure 4b. The average luminance

of the gray target at the end of the civil twilight is 0.042 cd/m2.

The ambient illuminances that were measured on February 3, 1998 (overcast evening) at the

beach parking lot of the Strouds Run State park are shown in Figure 5a. Again, a directionality effect

was found. At 6:19pm (end of the evening civil twilight) the average illuminance measured was 0.08 lx.

Figure 5b shows the ambient illuminance [lx] as a function of time with direction as parameter, during the

evening civil twilight of March 26, 1998 (clear sky). As in the overcast evening data (Figure 5a), there

was a fairly large directionality effect. The average illuminance over all measured directions at the end of

the civil twilight was 1.7 lx, which is still 46.9% lower than the 3.2 lx stated by Owens et al. [15].

It appears that the series of measurements conducted under clear sky conditions come much

closer to the 3.2 lx used by the civil twilight method. Benchmarks should not be based on highly variable

phenomena but rather on highly reproducible phenomena. For the specific case of target visibility

modeling the authors recommend against the use of benchmarks, whatsoever. However, the authors

recommend the use of more precise, calibrated visibility models that produce unbiased estimates and

account for as much of the variation as the current state of research permits. Looking at Figure 5 one

can see that each illuminance line is a well behaved, monotonously decreasing function over time.

Therefore, selecting an end of civil twilight illuminance of 3.2 lx is definitely arbitrary since the measured

illuminances at the end of the civil twilight ranged from 0.06 lx (overcast sky) to 3.2 lx (clear sky). The

luminances measured on the targets at the end of the civil twilight period ranged from 0.0009 cd/m2

(gray target, overcast sky) to 1.2 cd/m2 (white target, clear sky). The measured luminance range

practically covers the entire mesopic range. The practice of using the end of civil twilight illuminance as

benchmark is highly questionable, since the photometric conditions associated with the end of civil

twilight appear to span the entire mesopic range.

Figure 6a shows twilight distance plateaus for an automobile using Ford Taurus low-beam

headlamps and a target (size and reflectance are not considered by twilight distance method) with an


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assumed vertical target center at 0.53m above ground. The plateaus were constructed for various

threshold twilight illuminances. Using a twilight illuminance of 3.2 lx, it can be seen that the twilight

distance is about 43m, if the target is placed 1.5m to the right of the longitudinal vehicle axis. A twilight

illuminance of 1.6 lx allows the twilight distance to increase to 60m, if the target is located about 2.2m to

the right of the longitudinal vehicle axis. Decreasing the twilight illuminance to 0.8 lx allows the twilight

distance to increase to 87m, if the target is located about 3.5m to the right of the longitudinal vehicle

axis. A twilight distance of over 100m can be obtained if the twilight illuminance is assumed to be only

0.4 lx and the target is located 2.5m to the right of the longitudinal vehicle axis. Figure 6a basically

illustrates that an arbitrary selection of the twilight illuminance will result in an equally arbitrary twilight

distance. The illuminance measurements have clearly shown, that an illuminance of 3.2 lx seems quite

high. Choosing a twilight illumination of 3.2 lx will consequently lead to relatively short twilight distances

as indicated by Figure 6a. Again, it seems that the twilight distance method is a completely inadequate

approach to solving a complex visual detection modeling problem.

Comparison of Detection Distances Obtained Using the Civil Twilight Method With Detection

Distances Obtained Using a State of the Art Computer Model

It was previously mentioned that the civil twilight method neglects a number of factors that are known to

significantly affect the visibility of a target. This section demonstrates the effect of the angular target size,

the target reflectance, and the driver age upon the detection distance of selected targets. The obtained

detection distances are compared to the corresponding twilight distances.

A proprietary computer based visibility model was used by the authors to calculate the detection

distances for the selected targets. This computer visibility model is based on the Blackwell human

threshold contrast database [1]. The age function [11] used in the computer visibility model is based on


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data provided by Blackwell for 156 normal observers of various ages [2]. The headlamps, the observer,

and the targets are accurately accounted for in 3D space.

Figure 6b illustrates the setup used for the calculations of the detection distance of diffuse targets

located on the right road shoulder. The center of the targets are located 0.53 m above the road surface.

To demonstrate the effect of the target size it is assumed that the target size is increased along the

vertical dimension from 0.13m up to 1.06m. Two target reflectances R1=0.3 and R2=0.1 are used to

demonstrate the effect of target reflectance. The driver eye location and the headlamp location are valid

for a 50 percentile adult in an average car [36]. In computing the visibility of a given target, the

proprietary computer based visibility model first determines the luminance of the background along the

longitudinal target axis by using a road surface reflectance matrix (used asphalt in this case). It should

be noted, that only the luminance of the pavement was used as a target background. It was assumed,

that the target did not extend into the horizon sky. Candlepower matrices are used for each headlamp

separately to determine the illuminance at the target. Then the computer model determines the

luminance of the target along the longitudinal target axis, using the reflectance of the diffuse target. With

the target luminance LT and background luminance LB, the computer model determines the actual

contrast along the longitudinal target axis using Equation (2).

CACT =LT L B

L B(2)

Then, the computer visibility model compares the actual contrast with the threshold contrast CTH

determined from Blackwells human threshold contrast database [1]. The reader is referred to [14] for a

detailed description of the interpolation method used to determine CTH as a function of the visual angle

subtended by the target and the available background Luminance LB. In a final step, the computer

visibility model applies a number of contrast multipliers to account for the observer age [2][11] (25 years


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and 65 years used in present paper), probability of detection (P=99.93%1used in this paper), exposure

time [37] (0.65 seconds used in this paper), and a field factor of 10 to account for the difference between

Blackwells trained observers in the laboratory [1] and normal observers driving in an automobile. The

detection distance is given by the longitudinal location at which the actual contrast CACT

equals the

adjusted threshold contrast CTH.

Figure 7 clearly illustrates the effect of Target Size and Target Reflectance on the detection

distance using the proprietary computer based visibility model. Figure 7a shows that young observers

(25 years) can detect the dark achromatic targets with a reflectance of R=0.1 at a detection distance of

30.0m, 36.0m, 39.3m, and 41.5m for a target height of 0.13m, 0.26m, 0.53m, and 1.06m, respectively.

All of these distances are shorter than the twilight distance (headlamp illumination equals 3.2 lx, 0.53m

above ground) of 53m. Figure 7b shows that young observers (25 years) can detect the light achromatic

targets with a reflectance of R=0.3 at a distance of 62.5m, 77.5m, 87.0m, and 93.0m for a target height

of 0.13m, 0.26m, 0.53m, and 1.06m, respectively. All of these distances are longer than the twilight

distance (headlamp illumination equals 3.2 lx, 0.53m above ground) of 53m. Old observers (Figure 7b)

detect the 1.06m high target with a reflectance of R=0.3 at a distance of only 77.5m which equates to a

loss in the visibility distance of 16.6% over the young observers. Figure 7 provides evidence that the civil

twilight distance method is not only unable to account for target size, but also fails to predict the severe

reduction in detection distance when using dark, diffuse target materials. Also, the civil twilight distance

method does not consider the factors of contrast and observer age, to name a few.

1Probability of Detection of 99.93% (3.2) means that in 9,993 out of 10,000 observations the target is

detected


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Summary, Discussion, and Conclusion

This paper investigated and identified the shortcomings of the civil twilight method proposed by Owens

et al. [15] and Andre and Owens [16] as a simple functional approach to determine the visibility distance

of an ordinary object under automobile illumination at night. The civil twilight method attempts to

determine the detection distance of a target seen under automobile headlamp illumination at night by

using the amount of illuminance provided by the headlamps as the only benchmark factor. The civil

twilight method completely neglects target size and reflectance. In other words, for the civil twilight

method it does not matter whether the target is a dark clad pedestrian or a large light colored farm

animal. Other factors that are neglected by the civil twilight method, in spite of scientific evidence

indicating them to be important for visbility modeling are driver age, windshield transmittance, glare,

atmospheric transmissivity, and exposure time, just to name a few.

A series of luminance and illuminance measurements were conducted during the evening civil

twilight of February 3, 1998 (overcast evening) and March 26, 1998 (clear evening). A strong

dependency of the measured illuminance on the prevailing weather condition and a directionality effect

were found. The measured illuminance of 0.08 lx at the end of the civil twilight during the overcast

evening is 40 times lower than the 3.2 lx cited by Owens et al. [15]. The measured illuminance of 1.7 lx

at the end of the civil twilight during the clear evening is still 1.9 times lower than the 3.2 lx cited by

Owens et al. [15].

It is the opinion of the authors of this paper that the civil twilight method as a whole appears to be

inadequate and insensitive, and should not be used at all. Owens et al. [15] feel that the value of their

method lies in its simplicity rather than in its accuracy. It is the opinion of the authors of this paper that

proposing a simple, non-validated approach such as the civil twilight method to solve target detection

problems under automobile headlamp illumination at night, is highly questionable and the results of the

civil twilight method may be misleading at best. There really is no need for a simple target detection


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algorithm like the civil twilight method, since advanced target detection/visibility/legibility models are

available as computer programs that can be used by specialists. At this point in time, no claim should be

made that the civil twilight method is an accepted, validated, adequate, scientific method to determine

the visibility of targets.

The use of the civil twilight method for pedestrian detection under automobile headlamp

illumination at night is strongly discouraged by the authors of this paper, because the method may be

misused by forensics experts if there is a need to produce arbitrarily short pedestrian detection

distances, irrespective of factors including the clothing reflectance, contrast, pedestrian size, windshield

transmittance, atmospheric transmissivity, etc. Advanced visibility models that are calibrated with field

data and which consider a multitude of factors provide predictions that are more precise (near true target

value) and more accurate (small dispersion) than predictions made by the civil twilight method.


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0.001

0.01

0.1

1

10

100

5:30 5:35 5:40 5:45 5:50 5:55 6:00 6:05 6:10 6:15 6:20 6:25 6:30 6:35 6:40 6:45

Time Begin

Luminance[cd/m^2]

SE

S

SW

W

NW

N

NE

E

Sunset: 5:52pm

End of Civil Twilight : 6:19pm

Luminance of a white target taken in

different directions, 100% overcast.

Strouds Run State Park, Athens, Ohio.

Lat: 39:18:58N, Long: 82:05:42W

Civil Twilight begins: 5:52pm, ends: 6:19pm.

a. White Target, R=0.92

0.0001

0.001

0.01

0.1

1

10

5:30 5:35 5:40 5:45 5:50 5:55 6:00 6:05 6:10 6:15 6:20 6:25 6:30 6:35 6:40 6:45

Time Begin

Luminance[cd/m

^2]

SE

S

SW

W

NW

N

NE

ESunset: 5:52pm

End of Civil Twilight: 6:19pm

Luminance of a gray target taken in

different directions, 100% overcast.


Lat: 39:18:58N, Long: 82:05:42W


b. Gray Target, R=0.108

Luminance Measurements Conducted on the Beach Parking Lot of the Strouds Run State Park Outside Athens, Ohio(Longitude:82

o06 20" W, Latitude:39

o20 14"N) During the Evening Civil Twilight on February 3, 1998

Figure 3. Overcast Evening: Target Luminance [cd/m2] as a Function of Time with Direction as

Parameter, During Evening Civil Twilight


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0.0001

0.001

0.01

0.1

1

10

100

1000

6:15 6:20 6:25 6:30 6:35 6:40 6:45 6:50 6:55 7:00 7:05 7:10 7:15 7:20 7:25 7:30 7:35 7:40

Time at Beginning of Measurements

Luminance[cd/m^2]

SE White

S White

SW White

W White

NW White

N White

NE White

E WhiteBeginning of Civil Tw ilight, 6:46

End of Civil Tw ilight, 7:13

Luminance of w hite square target in

diff erent directions during Civil Tw ilight.

26 March 1998. Clear sky. Strouds Run, Athens, OH.

Latitude: 39:18:58N, Longitude: 82:05:42W

a. White Target, R=0.92

0.0001

0.001

0.01

0.1

1

10

100

1000

6:15 6:20 6:25 6:30 6:35 6:40 6:45 6:50 6:55 7:00 7:05 7:10 7:15 7:20 7:25 7:30 7:35 7:40


Luminance

[cd/m^2]

SE Gray

S Gray

SW Gray

W Gray

NW Gray

N Gray

NE Gray

E GrayBeginning of Civil Tw ilight, 6:46

End of Civil Tw ilight, 7:13

Luminance of gray square target in diff erent

directions dur ing Civil Twilight.


Latitude: 39:18:58N, Longitude: 82:05:42W

b. Gray Target, R=0.108Luminance Measurements Conducted on the Beach Parking Lot of the Strouds Run State Park Outside Athens, Ohio(Longitude:82

o06 20" W, Latitude:39

o20 14"N) During the Evening Civil Twilight on March 26, 1998

Figure 4. Clear Evening: Target Luminance [cd/m2] as a Function of Time with Direction as Parameter,

During Evening Civil Twilight


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0.001

0.01

0.1

1

10

100

1000

5:40 5:45 5:50 5:55 6:00 6:05 6:10 6:15 6:20 6:25 6:30 6:35 6:40 6:45

Time Begin

Illuminance[lux]

SE

S

SW

W

NWN

NE

E

Up

3.2lx

Illuminance taken in different directions, 100% overcast.


Lat: 39:18:58N, Long: 82:05:42W


3.2lx at 6:01pm

when

measuring NE

3.2lx at 6:04pm

when

measuring up

0.08lx at end

of

civil twilight

a. Overcast Evening, February 3, 1998

0.001

0.01

0.1

1

10

100

1000

6:35 6:40 6:45 6:50 6:55 7:00 7:05 7:10 7:15 7:20 7:25 7:30 7:35 7:40 7:45 7:50


Illuminanc

e[lux]

SE

S

SW

W

NW

N

NE

E

Up

3.2 lx

Illuminance in different directions during Civil Twilight.


Latitude: 39:18:58N Longitude: 82:05:42W

Civil twilight begins 6:46pm, ends 7:13pm

Stated value of illuminance

at the end of Civil Twilight, 3.2 lux.

3.2lx at 7:06pmwhen

measuring

East

3.2lx at 7:13pm

when

measuring

West

b. Clear Evening, March 26, 1998Illuminance Measurements Conducted on the Beach Parking Lot of the Strouds Run State Park Outside Athens, Ohio(Longitude:82o06 20" W, Latitude:39o20 14"N) During the Evening Civil Twilight on February 3, 1998

Figure 5. Illuminance [lx] as a Function of Time with Direction as Parameter, During Evening Civil

Twilight


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24

a. Twilight Distance Plateaus for Ford Taurus Low-Beam Headlamps, Assumed Vertical Target Centeris 0.53m above Ground

Area enclosed by each plateau receives an illuminance at or above the indicated value in lux

=Origin

Driver

LongitudinalOffset2.05m

Longitudinal Dista nce to Target

DriverLateral

Offset0.32m

Target Heightto Center 0.53m

HeadlampHeight0.607m

DriverEye Height

1.16m

Lane Width3.65m(12ft)

1/2 Lane Width

1.82m(6ft)

1/2 Lane Width

1.82m(6ft)

Headlamp

Separation1.11m(4ft)

Vertical TargetDimension

1.06m0.53m0.26m

0.13m

Diffuse TargetLocated on RightRoad Shoulder

Reflectance10% and 30%Width 0.13m

b. Setup used for the Calculation of the Detection Distance of Diffuse, Achromatic Targets

Figure 6. Iso Twilight Distance Plateaus for Various Twilight Illuminations and Setup Used in Detection

Distance Calculations


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0.001

0.01

0.1

1

10

0 5 10 15 20 25 30 35 40 45 50 55 60

Distance [m]

Contrast

Cth for 1.06m high targetCth for 0.53m high targetCth for 0.26m high targetCth for 0.13m high targetCact

Object with center located along right edge line (1.82m from car

center) and 0.53 m above ground.

Reflectance = 10%, H6054 low beams, Young Observer 25 years

Probability of Detection P=99.93%

Exposure Time 0.65seconds (85th percentile eye fixation duration)

Field Factor = 10

Twilight Distance

= 53m

(3.2 lux at target)

30.0m 36.0m

39.3m

41.5m

a. Reflectance R=0.1

0.01

0.1

1

10

100

0 10 20 30 40 50 60 70 80 90 100 110 120

Distance [m]

Contra

st

Cth for 1.06m high target, 65 yearsCth for 1.06m high target, 25 yearsCth for 0.53m high target, 25 yearsCth for 0.26m high target, 25 yearsCth for 0.13m high target, 25 yearsCact

Twilight Distance

= 53m

(3.2 lux at target)

Object with center located along right edge line (1.82m from car center)

and 0.53 m above ground.

Reflectance = 30%, H6054 low beams, Probability of Detection 99.93%

Exposure Time 0.65seconds (85th percentile eye fixation duration)

Field Factor = 10

62.5m 77.5m 87.0m

93.0m

b. Reflectance of R=0.3

Figure 7. Actual Contrast and Threshold Contrast as a Function of Longitudinal Distance for Diffuse,

Achromatic Targets Ranging in Height from 0.13m to 1.06m, Target Center 0.53m above

Ground, As Seen Against Used Asphalt, Reflectance R=0.1 and R=0.3


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Table 1. Comparison of Average Target Detection Distances Reported by Various Researchers

Note: Entries are sorted by increasing detection distance, entries shown in white on black indicate oncoming car glareconditions. NV=Not Visible

Researcher(s) Method Observer Age Beam Type Target TypeTarget

Reflectance/AppearanceTarget Size

Average

Detection

Distance [m]Owens et al. Twilight Distance Not considered Europe low beams Any target, 43.3" above ground Not considered Not considered NV

Olson and Sivak Field, Approaching Glare source young Low beams Human figure on Left Dark Adult human 18

Olson and Sivak Field, Approaching Glare source young Low beams Human figure on right Dark Adult human 24Owens et al. Twilight Distance Not considered US low beams Any target, 43.3" above ground Not considered Not considered 29

Leibowitz and Owens not stated Not considered Low beams Pedestrian in dark clothing Not specified Not specified 34

Leibowitz and Owens not stated Not considered High beams Pedestrian in white clothing Not specified Not specified 34Hazlett and Allen Field, Blood Alcohol Content = 0 Young Low beams Simulated pedestrian Black cloth, 9% .3m x .3m x 1.2m 34

Olson and Sivak Field, Approaching Glare source young Low beams Human figure on Left Light Adult human 37

Olson and Sivak Field, Approaching Glare source young High beams Human figure on right Dark Adult human 37Olson and Sivak Field, Approaching Glare source young High beams Human figure on Left Dark Adult human 37

Hazlett and Allen Field, Blood Alcohol Content = 0 Young Low beams Simulated pedestrian Gray cloth, 16% .3m x .3m x 1.2m 37

Owens et al. Twilight Distance Not considered Europe low beams Any target, 27" above ground Not considered Not considered 44

Leibowitz and Owens not stated Not considered High beams Pedestrian in dark clothing Not specified Not specified 49Olson and Sivak Field, Approaching Glare source young Low beams Human figure on right Light Adult human 49

Owens et al. Twilight Distance Not considered US low beams Any target, 27" above ground Not considered Not considered 55

Chrysler Field, rural test track 53-75 Low beams Small road hazard 31.5% Reflectance .17m x .33m 73Olson and Sivak Field, Approaching Glare source young High beams Human figure on right Light Adult human 73

Olson and Sivak Field, Approaching Glare source young High beams Human figure on Left Light Adult human 73

Owens et al. Twilight Distance Not considered US low beams Any target on ground level Not considered Not considered 86Chrysler Field, rural test track 53-75 Low beams Child mannequin 37.8% Reflectance 1.06m tall 88

Owens et al. Twilight Distance Not considered Europe low beams Any target on ground level Not considered Not considered 91

Bloomberg et al Field, dark test course Not specified Low beams Child, jacket Gray Garmet size 8 94

Chrysler Field, rural test track 19-25 Low beams Small road hazard 31.5% Reflectance .17m x .33m 96

Shinar Field, rural roads, target not expected 20-58 Low beams Pedestrian Khaki, 5% Reflectance Adult human 101Shinar Field, rural roads, target not expected 20-58 Low beams Pedestrian Khaki, 70% Reflectance Adult human 105

Bloomberg et al Field, dark test course Not specified Low beams Adult, Coverall Gray Garmet size , large 110Bloomberg et al Field, dark test course Not specified Low beams Child jacket White Garmet size 8 114

Owens et al. Twilight Distance Not considered US high beams Any target, 43.3" above ground Not considered Not considered 125

Chrysler Field, rural test track 19-25 Low beams Child mannequin 37.8% Reflectance 1.06m tall 125Owens et al. Twilight Distance Not considered US high beams Any target, 27" above ground Not considered Not considered 137

Bloomberg et al Field, dark test course Not specified Low beams Human adult figure target Gray Not specified 143

Owens et al. Twilight Distance Not considered US high beams Any target on ground level Not considered Not considered 143

Owens et al. Twilight Distance Not considered Europe High beams Any target on ground level Not considered Not considered 146Owens et al. Twilight Distance Not considered Europe High beams Any target, 27" above ground Not considered Not considered 146

Owens et al. Twilight Distance Not considered Europe High beams Any target, 43.3" above ground Not considered Not considered 146

Hazlett and Allen Field, Blood Alcohol Content = 0 Young Low beams Simulated pedestrian White cloth, 75% .3m x .3m x 1.2m 149

Shinar

Field, rural roads, target expected within2km, laterally located either on center, left

or right 20-58 Low beams Pedestrian Khaki, 5% Reflectance Adult human 150

Shinar

Field, rural roads, target expected at exactlongitudinal distance, laterally located

either on center, left or right 20-58 Low beams Pedestrian Khaki, 5% Reflectance Adult human 152

Shinar

Field, rural roads, target walking away

from static car 20-58 Low beams Pedestrian Khaki, 70% Reflectance Adult human 160

Shinar

Field, rural roads, target walking away

from static car 20-58 Low beams Pedestrian Khaki, 5% Reflectance Adult human 165

Shinar

Field, rural roads, target expected within

2km, laterally located either on center, leftor right 20-58 Low beams Pedestrian Khaki, 70% Reflectance Adult human 175

Bloomberg et al Field, dark test course Not specified Low beams Adult, Jacket White Garmet size 44 178

Shinar

Field, rural roads, target expected at exact

longitudinal distance, laterally located

either on center, left or right 20-58 Low beams Pedestrian Khaki, 70% Reflectance Adult human 185Bloomberg et al Field, dark test course Not specified Low beams Adult, Trousers White Garmet size , large 219

Bloomberg et al Field, dark test course Not specified Low beams Human adult figure target White Not specified 257

Bloomberg et al Field, dark test course Not specified Low beams Adult, Coverall White Garmet size , large 275


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Table 2. Comparison of visibility Model Components

Component present in Visibility Method

Civil

TwilightMethod

Ohio

University

Visibility

Model

Component present in Visibility

Method

Civil

TwilightMethod

Ohio

University

Visibility

Model

Candlepower beam pattern yes yes Observer threshold contrast no yes

individually defined with 6 dof no yes Threshold illumination at the eye no

yes, based on

Blackwell

1946 data

Observer location with 6 dof no yes Adaptation of observer no

yes, based on

Blackwell

1946 data

Windshield transmission no yes Age-background luminance interaction no

yes, based on

data from

Blackwell

1946 and 1971

Atmosperic transmissivity no yes Target eye fixation duration no

yes, Ohio

University eye

scanning

research

Above horizon scene background

luminanceno yes Probability of detection no yes

Road surface luminance (for pavement

markings)no yes Disability glare no yes

4 dimensional coefficient of retro-

reflection matrices for micro-prismatic

retro-reflectors

no yes Contrast polarity no yes

2 dimensional coefficient of retro-

reflection matrices for beaded materialsno yes Effects of color contrast no no

2 dimensional coefficient of retro-reflectance matrices for pavement

markings and road surfaces

no yesEffects of non-uniform background

luminanceno no

Reflectance of diffuse reflectors no yes Observer attention/arousal no no

Target location and orientation in 6 dofno, 3 dof

onlyyes

Calibration with target and task

specific psychometric visibility field

data

no yes

Target size no yesValidation with target and task specific

psychometric visibility field datano yes

Model usable for legibility of text no yes

Peripheral detection no noObserver age no yes

dof = degrees of freedom


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REFERENCES

[1] Blackwell H.R., Contrast Thresholds of the Human Eye, Journal of the Optical Society of America,

36(11), 1946

[2] Blackwell O.M., Blackwell H.R., Visual Performance Data for 156 Normal Observers of Various Ages.

Journal of the Illuminating Engineering Society, 1(1):pp 3-13, 1971.

[3] Blackwell H.R., Specification of Interior Illumination Levels, Illuminating Engineering, 54, pp. 317-53,

1959

[4] Gerhart G., Meitzler T., Sohn E., Verification and Validation Status of the TARDEC Visual Model, In

Proceedings of the Ground Target Modeling and Validation Conference, Houghton, MI, 1995

[5] Lindquist G.H., Witus G., Cook T.H., Freeling J.R., Target Discrimination Using Computational Vision

Human Perception Models, In Proceedings of the 8thInternational Symposium of the SPIE, Orlando,

FL, 1994

[6] Meitzler T., Bryk D., Gerhart G., Karlsen R., Sohn E., Witus G., Anneberg L., Evaluation of the

Conspicuity of Civilian Vehicles Utilizing a Dual Purpose Visual Perception Laboratory [VPL] and

Visual Perception Model [VPL], In Proceedings of the 7th Annual Ground Target Modeling and

Validation Conference, Houghton, MI, 1996

[7] Meitzler T., Gerhart G., Sohn E., Witus G., Heckmann T., Cusmano E., Adapting an Army Vision

Model for Measuring Armored-Vehicle Camouflage to Evaluating the Conspicuity of Civilian

Vehicles, In Proceedings of the 5th Annual Ground Target Modeling and Validation Conference,

Houghton, MI, 1994

[8] Commision Internationale de L Eclairage, An Analythic Model For Describing The Influence Of

Lighting Parameters Upon Visual Performance, Publication CIE No. 19/2.1 (TC-3.1), Paris, France,

1989

[9] Farber E., C. Matle, PCDETECT: A Revisd Version of the DETECT Seeing Distance Model,

Transportation Research Record 1213, Transportation Research Board, National Research Council,

Washington, D.C. 1989, pp. 11-20


29/32

29

[10] Adrian W., Visibility of Targets, Transportation Research Record 1247, Transportation Research

Board, National Research Council, Washington, D.C. 1989, pp. 39-45

[11] Zwahlen, Helmut T. and Schnell, Thomas, "A Combined Age-Background Luminance Contrast

Multiplication Function to Adjust the Human Contrast Threshold More Accurately in Visibility and

Legibility Evaluations," paper presented at the PAL-Progress in Automobile Lighting-Symposium,

September 26-27, 1995, Technical University, Darmstadt, Germany. Published in PAL Symposium

Proceedings, Technische Hochschule Darmstadt, 1995, pp. 240-247.

[12] Schnell T., Zwahlen H.T., The Development of CARVE, Computer Aided Road-Marking Visibility

Evaluator, A PC-Based Pavement Marking Visibility Evaluation Software Package, Paper presented

at the TRB A3A04 Visibility Committee Midyear Symposium on Night Visibility and Driver Behavior,

Iowa City, IA, April 28-30, 1996

[13] Schnell, Thomas and Zwahlen, Helmut T., "Predicting the Visibility of Pavement Markings with

CARVE (Computer-Aided Road-Marking Visibility Evaluator)," paper presented at the Ohio

Transportation Engineering Conference, The Ohio State University, Columbus, Ohio, November 19-

20, 1996.

[14] Schnell T., Zwahlen H.T., Retroreflective Materials, Visibility Calculations for Prismatic or Micro-

Prismatic Materials, Paper presented at the Transportation Research Board 77th Annual Meeting,

January 11-15, 1998, Washington DC, 1998

[15] Owens D.A., Francis E.L., Leibowitz H.W., Visibility Distance with Headlights: A Functional

Approach, SAE Technical Paper Series, Paper Number 890684, Society of Automotive Engineers,

400 Commonwealth Drive, Warrendale, PA 15096, 1989

[16] Andre J.T., Owens, D.A. The Twilight Envelope: An Alternative Approach Toward Describing

Roadway Visibility at Night, In A. Gough (Ed.), Vision at Low Light Levels, Proceedings of the 4th

International Lighting Research Symposium, pp. 171-196, May 19-21, 1998, Orlando, Florida

[17] Adrian W., Fields of Visibility of the Nighttime Driver, Published in Proceedings of the PAL 97,

Progress in Automobile Lighting Conference, Darmstadt University of Technology, September 23-

24, 1997


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30

[18] Zwahlen H.T., Schnell T., Target Visibility During Civil Twilight, In A. Gough (Ed.), Vision at Low

Light Levels, Proceedings of the 4thInternational Lighting Research Symposium, pp. 171-196, May

19-21, 1998, Orlando, Florida

[19] Hazlett R.D., Allen M.J., The Ability to See a Pedestrian at Night : The Effects of Clothing,

Reflectorization and Driver Intoxication, American Journal of Optometry and Archives of American

Academy of Optometry, 45(4), 1968

[20] Shinar, David., The Effects of Expectancy, Clothing Reflectance, and Detection Criterion on

Nighttime Pedestrian Visibility. Human Factors. 27(3), 1985.

[21] Chrysler, Susan T., Danielson, Suzanne M., and Kirby, Virginia M. Age Differences in Visual

Abilities in Nighttime Driving Field Conditions. Proceedings of the Human Factors and Ergonomics

Society. Volume 2. Fortieth Annual Meeting, 1996

[22] Blomberg R.D., Leaf W.A., Jacobs H.H., Detection and Recognition of Pedestrians at Night,

Transportation Research Circular Number 229, Transportation Research Board, National Academy

of Sciences, Washington, DC, 1981

[23] Olson P.L., Sivak M., Visibility Problems in Nighttime Driving, Authors of Chapter 15 in Automotive

Engineering and Litigation, Garland Law Publishing, New York, 1984

[24] Strickland J., Ward B., Allen M.J., The Effect of Low vs. High Beam Headlights and Ametropia on

Highway Visibility at Night, American Journal of Optometry and Archives of American Academy of

Optometry, 45(2), 1968

[25] Austin R.L., Klassen D.J., Vanstrum R.C., Driver Perception of Pedestrian Conspicuousness, Under

Standard Headlight Illumination, 3m Company, St. Paul, MN

[26] Zwahlen, H.T. and Schnell, T., "Loss of Visibility Distance Due to Automobile Windshields at Night,"

paper presented at the Transportation Research Board's 12th Biennial Symposium on Improving

Visibility for the Night Traveler, May 23-24, 1994, National Academy of Sciences, Washington, D.C.

Published in Transportation Research Record No. 1495, 1995, pp. 128-139.

[27] Zwahlen Helmut T., Schnell Thomas, "The Visibility of Road Markings as a Function of Age and

Retro-Reflectivity under Low-Beam and High-Beam Illumination at Night", paper No. 980285,


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31

presented at the 77th Annual Meeting of the Transportation Research Board, Washington, DC,

January 11-15, 1998

[28] Zwahlen, Helmut T. and Schnell, Thomas, "Visibility of New Centerline and Edge Line Pavement

Markings," Paper No. 971166, presented at the 76th Annual Meeting of the Transportation Research

Board, Washington, DC, January 12-16, 1997.

[29] Zwahlen, Helmut T. and Schnell, Thomas, "Visibility of Yellow Center Line Pavement Markings as a

Function of Line Configuration and Line Width," paper presented at the 40th Annual Meeting of the

Human Factors and Ergonomics Society, September 2-6, 1996, Philadelphia, Pennsyvlania.

Published in Proceedings, 40th Annual Meeting of the Human Factors and Ergonomics Society,

1996, pp. 919-922.

[30] Zwahlen, Helmut T. and Schnell, Thomas, "Visibility of New Dashed Yellow and White Center

Stripes as a Function of Material Retro-Reflectivity," Paper No. 961268, presented at the 75th

Annual Meeting of the Transportation Research Board, January 7-11, 1996, Washington, DC.

Published in Transportation Research Record No. 1553, 1996, pp. 74-81.

[31] Zwahlen, Helmut T., Hagiwara, Toru, and Schnell, Thomas, "Visibility of New Yellow Center Stripes

as a Function of Obliteration," paper presented at the 74th Annual Meeting of the Transportation

Research Board, Paper No. 950993, January 22-26, 1995, Washington, DC. Published in

Transportation Research Record No. 1495, 1995, pp. 77-86.

[32] Zwahlen, Helmut T., Schnell, Thomas, and Hagiwara, Toru, "The Effects of Lateral Separation

Between Double Center Stripe Pavement Markings on Visibility Under Nighttime Driving Conditions,"

paper presented at the 74th Annual Meeting of the Transportation Research Board, Paper No.

950994, January 22-26, 1995, Washington, DC. Published in Transportation Research Record No.

1495, 1995, 87-98.

[33] Zwahlen H.T., Schnell T., Visibility of New Pavement Markings at Night Under Low Beam

Illumination, Transportation Research Record 1495, National Research Council, Washington,

D.C., 1995


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32

[34] Zwahlen, Helmut T. and Schnell, Thomas, "Visibility of New Centerline and Edge Line Pavement

Markings," Paper No. 971166, presented at the 76th Annual Meeting of the Transportation Research

Board, Washington, DC, January 12-16, 1997

[35] Zwahlen, H.T., Schnell T., Legibility of Traffic Sign Text and Symbols, Paper Presentated at the

Transportation Research Board 77th Annual Meeting, January 11-15, 1998, Washington, DC

[36] Schnell T., Zwahlen H.T., Driver-Headlamp Dimensions, Driver Characteristics, Vehicle And

Environmental Factors Used in Retro-Reflective Target Visibility Calculations, Paper presented at

the Transportation Research Board 77th Annual Meeting, January 11-15, 1998, Washington DC,

1998

[37] Pline J.L, Editor, ITE, Institute of Traffic Engineers, Traffic Engineering Handbook, 4th Edition,

Prentice Hall, Publishers, Englewood Cliffs, NJ, 1992

visual target detection models for civil twilight and night driving conditions

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