1. Introduction
Glare is the condition of vision in which there is discomfort or a reduction in the ability to see details or objects, caused by an unsuitable distribution or range of luminance, or by extreme luminance contrasts [
1]. Glare from high-beam headlights is an adverse factor that affects the acquisition of visual information for oncoming drivers at night. Some studies [
2,
3,
4,
5,
6] have shown that glare caused by oncoming vehicles can reduce the visual ability, judgment, and response ability of pedestrians and drivers. According to statistics, half of the fatal accidents on U.S. roads occur at night, and the number of traffic accidents caused by glare from the high-beam headlights of oncoming traffic at night accounts for 12% to 15% of all traffic accidents [
7]. The installation of anti-glare facilities in the highway median strip is one of the most effective ways to block the glare from oncoming vehicles. Fernandes [
8] and Hammond et al. [
9] considered installing anti-glare facilities, which is one of the essential methods to solve the problem of high-beam headlight glare on highways, and can effectively improve the highway driving environment at night.
Many countries have introduced relevant standards for the height of anti-glare facilities. The existing standard in China stipulates that the height of the anti-glare facility should not exceed 2 m [
10]. According to research conducted by the Transportation Research Board, on a flat and level divided highway without cross slope, glare screens would have to be the same height as the average driver’s eye, or 1.14 m, in accordance with AASHTO standards [
11]. BSEN 12676-1:2000 gives the calculation formula for anti-glare facilities in the case of pavements with a constant longitudinal gradient [
12]. The Guidelines for Expressways published by the Indian Roads Congress state that the height of a glare-reduction device should be set at 1.4–1.5 m on the assumption of combinations of opposing passenger vehicles and of a passenger car and a large vehicle moving in opposite directions [
13]. Moreover, many scholars have also studied the height of anti-glare facilities on different freeway alignments. Liang used UC-win/Road simulation software to determine the acceptable height difference for drivers in the transition section; the results showed that the height difference should not exceed 6 cm when the radius of the concave vertical curve does not exceed 30,000 m [
14]. Bagui recommended that the height of an anti-glare screen barrier should be 1.85 m in the Indian context [
15]. Wu Yan put forward a calculation method for anti-glare plate heights on concave vertical curve sections. Research shows that when the lamp distance of car headlamps is 120 m and the radius of a concave vertical curve section is 12,000–32,000 m, the minimum design height of the anti-glare plate should be 1.72 m, and the maximum design height of the anti-glare plate should be 1.80 m [
16]. Although different countries and scholars recommend different heights for anti-glare facilities, most of the existing studies calculate the height of anti-glare facilities by considering the height of the high-beam headlights of opposing vehicles, the height of the driver’s line of sight, the lateral distance from vehicles to anti-glare facilities, and the minimum effective height of anti-glare facilities from the road surface. Ma Yang [
17] used a prismatic cone to simulate the spatial range variation of headlights in the actual driving process, and calculated the height of anti-glare facilities for any position. At present, the setting of anti-glare facilities mainly considers blocking disability glare; however, headlight leakage from the anti-glare facilities can still make drivers uncomfortable. This is not conducive to the driver’s identification of road obstacles, road conditions, and oncoming traffic conditions, and may cause traffic accidents. Therefore, it is crucial to study the illuminance thresholds and spatial distribution of glare perception caused by vehicle headlights on drivers.
The illuminance thresholds and spatial distribution of the glare from high-beam headlights on drivers fall under glare evaluation. Current research on glare focuses on both disability glare and discomfort glare. Disability glare is mainly evaluated by the threshold increment method [
18,
19]. Threshold increment (TI) is an evaluation index that expresses, as a percentage, the increase in the luminance contrast threshold required between an object and its background for it to be seen equally well with a source of glare present. The CIE 31:1976 report provides the formula [
20], as shown in Equation (1), with
and
in cd/m
2 and for a range 0.05 <
< 5.
where
is the average road luminance and
is the equivalent veiling luminance (cd/m
2);
is given by Equation (2):
where
, for practical purposes, is taken as 10 when
is expressed in degrees, or as
when
is expressed in radians. For a total installation, the individual equivalent veiling luminances
of each luminaire have to be added together, as follows:
.
is the illuminance on the tested driver’s eye produced by the glare source in the plane perpendicular to the line of sight, in lux, while
is the angle between the center of the glare source and the line of sight. The exponent of
is valid from ~1.5° to 60° (0.025 to 1.00 radian).
However, it is impossible to guarantee driving safety if only disability glare is considered as the basis for the height of anti-glare facilities. Headlights can still interfere with drivers’ ability to recognize objects ahead and, thus, can still cause traffic accidents.
In the actual driving process, discomfort glare may divert the attention of drivers away from the scene to be surveyed towards the bright glare source [
21]. However, not all discomfort glare will adversely affect driver safety, so it is crucial to classify discomfort glare, and to define the level of discomfort glare that is acceptable for drivers. The psychophysical method is a method that mainly evaluates the degree of discomfort glare, and the evaluation process is divided into three parts: (1) selection of representative subjects for subjective evaluation of uncomfortable feelings, (2) measurement of photometric parameters within the field of view, and (3) establishment of a correlation model between the subjective evaluation of feeling levels and photometric parameters [
22]. Scholars have conducted many studies on the evaluation models of discomfort glare, and obtained evaluation models of the degree of discomfort glare for different scenes [
23,
24,
25,
26,
27]. To describe perceptions of discomfort glare, various scholars have developed scales consisting of several words [
25,
26,
28,
29], among which the nine-point scale designed by de Boer et al. has been widely used [
30]. However, some scholars have also raised different opinions to de Boer’s nine-point scale. Theeuwes et al. found that the commonly used de Boer glare scale is not suitable for testing driver performance [
31]. There is no validated Chinese description of the de Boer scale; this causes differences in individual understanding of the Chinese meaning of the scale, leading to differences in test results [
30].
In view of the above problems, this article proposes a subjective glare scale based on the analysis of the mechanism of glare from high-beam headlights. We study the threshold values and spatial distribution of glare perception at different longitudinal distances. The results can provide scientific evidence for calculating the reasonable heights of anti-glare facilities for expressways with various alignments.
2. Mechanism and Evaluation Method of Vehicle High-Beam Headlights Glare
Light with information about the illuminated object enters the subject’s eye through the pupil and is refracted by the crystalline lens, passing though the vitreous humor, and then falling on the retina. Light with information about the illuminated object is transmitted to the brain in the form of electronic impulse signals through the optic nerve, finally forming vision through a series of chemical reactions and transformations [
32]. Moderate light provides environmental brightness and visual guidance for driving, but unreasonable light will have a negative impact on driving safety. Drivers need to complete the task of transportation and ensure the safety of driving during dynamic processes. For the particular characteristics of the driver, we propose the concept of traffic glare, which refers to the glare that results in the driver being unable to recognize the road information ahead and the surrounding environment during a given driving task because of visual discomfort or reduced visual ability.
High-beam headlight glare is one form of traffic glare. High-beam headlight glare affects safe driving in two primary ways: (1) First, the intense light from high-beam headlights scatters in the opposite driver’s eye to form a bright veil, reducing retinal image contrast and, thus, reducing the overall visibility of objects laying ahead. Light scatter in the eye due to high-beam headlight glare is shown in
Figure 1 [
31]. Reduced visibility may affect the performance of visual tasks related to safe driving. (2) Second, the visual disturbance caused by the glare from high-beam headlights results in discomfort, and Berman et al. [
33,
34] concluded that discomfort glare effects coincide with uncomfortable contractions of the iris and the muscles surrounding the eyes.
To characterize the impact of high-beam headlight glare on driver safety, a combination of quantitative and qualitative methods was used to study the influence of opposing glare from vehicles’ high-beam headlights on drivers’ visual ability. The primary function of the vehicle headlamp is to provide a light environment for the driver to identify road traffic conditions within a certain distance, including information on the road ahead and the surrounding environment. According to the visual recognition requirements of the most unfavorable conditions, the criterion of traffic glare is whether the driver can find the gray target within a certain distance ahead in time under the interference of vehicle headlights. The 20 cm × 20 cm × 20 cm gray cube recommended by the Commission International de l’Eclairage is used as the visual target, and its surface reflection coefficient is 0.2.
The illuminance at the eye of the driver is the main factor affecting the glare experienced by the driver. To quantify the impact of high-beam headlights on the glare experienced by the driver, the vertical illuminance at the eye of the tested driver is measured. The existing glare research shows that both disability glare and discomfort glare are related to the relative position between the light source and the subject [
22,
35]. When the relative position between the driver and the high-beam headlights is different, the glare experienced by the driver is also different. The glare caused by the high-beam headlights will not always impact the driver’s ability to drive safely. When the included angle between the driver and the high-beam headlights is small or close, it is easy to produce disability glare, resulting in the driver’s inability to see the target in front within a short time, reducing driving safety. As the included angle between the driver and the high-beam headlights is enlarged or the distance increased, although the glare generated by the high-beam headlights is weakened, it still interferes with the driver’s recognition of the target ahead. Drivers want to avoid the glare by turning their heads, and traffic accidents can easily occur when the vehicle is moving at high speeds. When the angle between the driver and the high-beam headlights is further enlarged or the distance increased, although the driver will still feel the glare generated by the high-beam headlights, the glare experienced is weaker; the driver can recognize the target ahead, and can drive safely under these conditions. According to the level of glare experienced by the driver, the discomfort glare is classified into interference glare and acceptability glare. A subjective headlight glare scale and relevant description is shown in
Table 1; according to the influence of glare on the driver’s vision, from strong to weak, it is divided into disability glare, interference glare, and acceptability glare. The degree of influence of different glare levels is described in combination with the test scene to facilitate the test driver’s distinguishing of different levels of glare.
The combination of quantitative measurements of vertical illuminance at the driver’s eye and subjective visual recognition evaluation of the tested driver was used to study the illuminance thresholds and the spatial distributions of disability glare–interference glare (DGIG) and interference glare–acceptability glare (IGAG) from high-beam headlights under different longitudinal distances. The results can provide a scientific basis for calculating the reasonable height of anti-glare facilities under different alignments.
5. Discussion
The cutoff curves of DGIG and IGAG (as shown in
Figure 11) were obtained through the glare effect test. The assumption that the distribution diagrams of different glare levels of the tested drivers are similar to the isoilluminance diagram of high-beam headlights is correct.
Figure 11 shows that the glare level of the tested drivers was reduced from disability glare to interference glare, and then to acceptability glare, with the increase in lateral distance or vertical distance at the same longitudinal distance. The main reason for this is that the angle between the main optical axis of the headlights and the tested driver’s line of sight increases with the change in the vertical or lateral distance between the tested driver and the headlights; the light entering the tested driver’s eyes is thus reduced, which reduces the glare experienced by the tested driver.
It can be seen from
Figure 12 that the vertical illuminance thresholds of DGIG and IGAG are almost equal under the same longitudinal distance.
Table 5 shows that the vertical illuminance thresholds decrease with the increase in longitudinal distance. This may be because the light source not only provides the required light environment for the test, but also provides the background light environment. However, with the increasing longitudinal distance between the tested drivers and the light source, the gradient of the background luminance generated by the light source decreases more quickly. When the background luminance decreases, it is easy to form luminance contrast in the eyes of the tested driver, making it more likely to produce disability glare or interference glare.
In existing studies, the height design of anti-glare facilities primarily takes into account the height of the driver’s line of sight and that of the high-beam headlights, as well as lane width and road alignment [
10,
12,
43,
44]. Ma Yang [
17] used a prismatic cone with the center of the headlight line as the vertex of the vehicle light irradiation range as the basis for calculating the height of anti-glare facilities. Both of these methods have shortcomings. On the one hand, the influence of headlights is calculated by a ray of light only, which may lead to light leakage in the special linear road section. On the other hand, the entire influence range of headlights is included in the calculation of the height of anti-glare facilities, which may lead to the excessive height of said facilities, as the highway anti-glare facilities should be adjusted in light of not only their anti-glare effect, but also the effect of wind load on them [
43]. If the anti-glare facilities are too high, they may be unsafe due to the wind load. It was found through the glare effect tests that the entire irradiation range of headlights does not have an impact on driver safety. The spatial distribution of the high-beam headlight on the driver is a pyramid-like shape. Parameters such as the relative distance between high-beam headlights and the driver, the width of the driving lane, driver-sight height, and the height of the headlamp were considered, and the models of the height of the anti-glare facilities as a function of the most unfavorable angle for expressways with different alignments were put forward. The height of the anti-glare facilities determined by the calculation method can meet the visual recognition requirements of driving safety, and the safety of highway driving at night can be improved.