Traffic Accident Analysis of Sun Glare and Twilight Shortly Before and after in Chiba Prefecture, Japan Kenji Hagita a, Kenji Mori b a,b Traffic Science Division, National Research Institute of Police Science, Chiba, 277-0882, Japan a E-mail: hagita@nrips.go.jp b E-mail: mori@nrips.go.jp Abstract: Traffic accidents shortly before and after sunset in Chiba were analyzed. The rate of traffic accidents shortly after sunset was found to be higher than at any other time, whereas the rate shortly before sunset was found to be lower. Traffic accidents caused by sun glare tended not to occur shortly before sunset, because the presence of topographic features meant that the sun had already effectively set. The rate of pedestrian accidents shortly after sunset was higher than that of any other time; the rate shortly before sunset was slightly lower than in day and shortly after sunset, because it was not dark and drivers were not dazzled by sun glare before sunset. However, it is already the case that many vehicle headlights are on shortly after sunset, so it is difficult to decrease pedestrian accidents only by advising drivers to use their lights in the hour before sunset. Keywords: Safety,, Sun Glare, Traffic Accident 1. INTRODUCTION The likelihood of traffic accidents is widely known to depend on weather conditions. Many studies have shown that adverse weather, such as rain or snow, increases the likelihood of traffic accidents. The position of the sun relative to the travel direction of vehicles involved in traffic accidents was analyzed, it was found that traffic accidents tended to be more frequent when the sun was in front of the first vehicle concerned. Traffic accidents were also found to occur more in twilight. As a measure against such accidents, the National Police Agency (NPA) of Japan recommends that drivers use their headlights from 1 hour before sunset. When drivers are dazzled by sun glare, the sun tends to be near the horizon. So the period between the time when the sun is dazzling and sunset is not so long. However, the traffic visibility conditions of these two periods are very different, so traffic accidents for these two periods were analyzed for comparison, and traffic safety measures related to visibility should be examined. 2. REVIEW In studying how sun glare contributes to traffic accident occurrence in Japan, (Hagita and Mori, 2011a, 2011b) found that the traffic accident rate tended to be higher when the sun was in front of the vehicle. Pedestrian accidents also were found to occur with greater frequency than other types of accidents when the sun was in front of the vehicle (Hagita and Mori; 2011c). That paper did not find pedestrian accident black spots that were associated with sun glare. Regarding accidents broken down by vehicle type (Hagita and Mori; 2011d), it was 1999
inferred that sun glare in front of heavy vehicles did not increase the accident rate of such vehicles. Sun glare was found to contribute to accidents by two-wheeled motorized vehicles when the sun was immediately in front. This is attributed to the fact that the viewing angle of a two-wheeled motorized vehicle driver wearing a full-face helmet is narrower than those of other vehicle drivers. Mitra (2008) showed that traffic accident rates are higher at dusk and dawn than at other times, in an analysis of traffic accident data, sunset and sunrise times, and road travel directions in Arizona, USA. This analysis was not precise, for three reasons. First, the road travel directions of the subjects were classified into only four categories: north, south, east and west. Second, the time periods in which sun glare was considered to contribute to traffic accidents were defined roughly as one hour after sunrise and one hour before sunset. Third, the influence of weather was not addressed. (Jurado-Pina and Pardillo-Mayora, 2009) estimated the solar position at the time of traffic accidents, and they suggested a way of analyzing sun-glare-influenced traffic accidents. Jurado-Pina et al.(2010) analyzed the adverse influence of sun glare at tunnel exits and suggested road designs to reduce sun glare. According to an analysis of traffic volume and travel speed during adverse weather, Daniel et al. (2009) showed that vehicle speeds are lower at dusk and dawn than at other times. Analyses of traffic accidents at twilight (Nishida, 2003; Morita, 2000) found that traffic accidents tended to occur more at twilight. However, the definition of twilight is ambiguous in these accident studies, and the period between the time when the sun is dazzling and sunset is not so long. So, there are no studies in which the traffic accident rate for a time series during twilight was analyzed and traffic safety countermeasures were proposed. Therefore, sunset is defined as a standard time at Chiba Prefectural Headquarters in this study. Traffic accidents in Chiba Prefecture shortly before and after sunset were analyzed, to calculate the traffic accident rate for a time series and to reveal problems of traffic visibility conditions shortly before and after sunset. 3. METHODS 3.1 Traffic Accident Database of Chiba Prefecture To calculate the position of the sun relative to the first vehicle concerned at the time of an accident, it is necessary to acquire two items that are not included in the national traffic accident database in Japan: the latitude and longitude of the accident spot, and the vehicle travel direction. Because these two items are not collected in the national traffic accident database in Japan, we analyzed the traffic accident database of Chiba Prefecture, which does include these two items. In Japan, traffic accident data to be collected are specified by the National Police Agency (NPA). All traffic accidents resulting in injury are included in this database. Local police departments are required to keep records of traffic accidents and to share these records with the NPA. NPA regulations state that the following be recorded: occurrence time and address, road traffic environment (weather, road condition, etc.), accident type, driver attributes (age and gender), vehicle attributes (vehicle type), safety facilities, etc. However, these items do not permit the solar position and the vehicle travel direction to be determined. The Chiba Prefectural Police Headquarters specifies the additional items of accident latitude and longitude, and vehicle travel direction. These data can be represented on a GIS, as shown in Figure 1. Vehicle travel directions of the first vehicle concerned are shown by black arrows, and those of the second vehicle concerned are shown by white arrows. Travel 2000
directions of single-vehicle accidents are shown by only a black arrow, and travel directions of multi-vehicle accidents are shown by black and white ones. The intersection of the two travel directions is the accident spot. From these traffic accident data, it is possible to calculate the position of the sun relative to the vehicle travel direction at the accident time and location. Travel Direction of First Vehicle Concerned Travel Direction of Second Vehicle Concerned Map source: Shobunsha Publications, Inc Figure 1. GIS representation of traffic accidents in Chiba, including the vehicle travel direction, used for calculating the solar position at the accident time and location As shown in Figure 2, the solar position at the accident time and location is represented by solar zenith angle (θ) and azimuthal angle (χ). The solar zenith angle is the angle between a line extending vertically from the accident spot and the line extending from the accident spot to the sun. The lower is the sun, the greater is the solar zenith angle. At solar zenith angles exceeding 90 degrees, it is night. The solar azimuthal angle is the angle between a line pointing to the North Pole and a line pointing to the solar position on the horizontal plane at the accident spot. So the solar azimuthal angle of the line to the North Pole is 0 degrees, with the angle increasing from 0 to 360 degrees clockwise. Vehicle travel direction is also defined as an angle from 0 to 360 degrees clockwise. Formulae for calculating solar zenith angle and azimuthal angle are explained here. This method was proposed by Murakami (2010). Solar declination (δ) and hour angle (t) are indices representing the solar position on the celestial sphere. The north celestial pole is expressed as +90 degrees and the south celestial pole is expressed as -90 degrees in solar declination (δ). Hour angle (t) is the angle between the celestial meridian and the great circle that includes solar position and both celestial poles. The position of the sun relative to the earth was estimated by calculating solar declination (δ) and hour angle (t). The solar zenith angle (θ) at the accident spot is determined by latitude and longitude, and the solar zenith angle (θ) was calculated in order to use the solar declination (δ), hour angle (t) and latitude and longitude at the accident spot, as shown in Formula (1). The solar azimuthal 2001
angle (θ) at the accident location is also determined by latitude and longitude, so as a first step in calculating the solar azimuthal angle (θ), the sine and cosine of χ are calculated using solar declination (δ), hour angle (t) and solar zenith angle (θ) from Formulae (8) and (9). The values of sine and cosine of χ are then Zenith Sun Vehicle Travel Direction North Solar Azimuthal Angle χ Solar Zenith Angle θ East West Accident Location (Latitude, Longitude) The Difference in Azimuthal Angle South Figure 2. Solar position at the accident location cos θ = sin δ sin φ + cos δ cos φ cos t (1) where θ: solar zenith angle (rad), δ: solar declination (rad), φ: latitude (deg., at accident location), λ: longitude (deg., at accident location), and t: hour angle (deg.). δ = 0.006918 0.399912co s A + 0.070257si n A 0.006758co s(2a) + 0.000907si n(2a) 0.002697co s(3a) + 0.00148si n(3a) (2) A = 2πJ (3) 365 where, J: day of the year (Julian day) (TST 12) t=15 π 180 TST=MST+ET (5) MST = GMT+ λ 15 (6) ET=(0.000075 + 0.001868 cos A 0.032077 sin A 0.014615 cos(2a) 0.040849 sin(2a)) 12 π (7) (4) 2002
where, TST: true solar time, MST: mean solar time (local time), GMT: Greenwich Mean Time, and ET: equation of time sin χ= cos δ sin t cos χ= cos θ cos φ sin δ+ sin φ cos δ cos t sin θ (8) (9) The calculated values of sinχ and cosχ are classified into cases, and χ1 and χ2 are calculated as follows. χ1=2π-χ if cosχ < 0 χ1=3π+χ if cosχ > 0 and sinχ < 0 If cosχ and sinχ do not fit the two above-mentioned conditions then χ1=π+χ. where, χ2=χ1-2π if χ1>2π If χ1 is less than 2π, then the calculated χ1 is the solar azimuthal angle. If the calculated χ1 is more than 2π, then the solar azimuthal angle is χ1-2π, which is equal to χ2. Therefore, either of the calculated χ1 or χ2 is the solar azimuthal angle. 3.2 Analysis Method The traffic accident database consisted of the 134,339 injury accidents that were reported in Chiba Prefecture from 2007 to 2011. At first, the solar position at the accident time and location was calculated for each accident by applying Formulae (1) to (9). Accidents that occurred in the afternoon were classified into the five categories of day (high sun), day (low sun), shortly before sunset (hereinafter: before sunset ), shortly after sunset (hereinafter: after sunset ) and night, by using solar zenith angle as a standard. is typically defined as when the sun has completely set; however, sunset is defined here as when the solar zenith angle reaches 90 degrees. As shown in TABLE 1, time definitions according to solar zenith angle are as follows: less than 50 degrees: day (high sun); 50 to 80 degrees: day (low sun); 80 to 90 degrees: before sunset; 90 to 110 degrees: after sunset; 110 degrees or more: night. Accidents were classified into the five categories by accident type (heavy vehicle, car, two-wheeled motorized vehicle, bicycle and pedestrian). The solar zenith angle at the Chiba Prefectural Headquarters was defined as the representative value for Chiba Prefecture. The frequencies of occurrence of solar zenith angles for vehicle drivers at Chiba Prefectural Headquarters for one year were calculated. The rate for each type of traffic accident (number/hour) and fatal accident (number/hour) was calculated for each time slot. 57,814 daytime accidents in fine weather were extracted to analyze effect of sun glare. Sun glare was regarded as a possible causal factor in these accidents. Nighttime accidents and accidents in weather that was not fine were used as control accident data. time accidents in fine weather were classified as being influenced by sun glare or not, using the position of the sun relative to the vehicle as a threshold. Traffic safety measures were examined from the perspective of the traffic accident situation before and after sunset and traffic visibility condition. 2003
4. RESULTS 4.1 Definition of Accident Types The first vehicle concerned and the second vehicle concerned were recorded in the traffic accident database of Chiba Prefecture, Japan. All accidents were classified into the five accident types of heavy vehicle, car, two-wheeled motorized vehicle, bicycle and pedestrian, as shown in Table 1. Accident Type Heavy vehicle Passenger car Two-wheeled motorized vehicle Bicycle Pedestrian Table 1. Definitions of accident types Definition Heavy vehicle alone, Heavy vehicle to heavy vehicle or passenger car Passenger car alone, Passenger car to passenger car Accident not involving a pedestrian or a bicycle, The first and/or second vehicle concerned is a motorized two-wheeled vehicle Accident not involving a pedestrian, The first and/or second vehicle concerned is a bicycle The first or second person concerned is a pedestrian 4.2 Traffic Accident Analysis Focusing on Time Slots 4.2.1 Analysis of traffic accident rate per time slot and accident type The solar zenith angle at accident locations varies with season and time of day. The frequency of occurrence of solar zenith angle for vehicle drivers is not constant for each 10-degree range. Traffic accidents in Chiba Prefecture were analyzed in this paper, so the solar zenith angle at the Chiba Prefectural Headquarters was defined as the representative value for Chiba Prefecture. Solar positions at various locations in Chiba Prefecture at the same time do not differ greatly. The frequency of occurrence of solar zenith angles for vehicle drivers at Chiba Prefectural Headquarters for one year was calculated. To apply Formulae (1) to (9), solar positions every two minutes at the Chiba Prefectural Headquarters were calculated. As shown in Table 2, the calculated solar zenith angles were aggregated to determine the frequency of occurrence of solar zenith angle. Figure 3. shows traffic accident rates (numbers/hour) for total accidents and each accident type. The axis of abscissa is the solar zenith angle and the axis of ordinates represents traffic accident rate. The method for calculating traffic accident rates according to solar zenith angle is shown as Formula (11). The number of traffic accidents during a 5-year period (An) and the cumulative hours in a year for each range of solar zenith angle (ZA) were calculated, and traffic accident rates (number/hour, Ar) were estimated to use these data. Solar zenith angles were classified according to whether the sun was east or west, and the solar zenith angle was rounded off to the nearest 10 degrees; the 25 degrees in Figure 3 means that the solar zenith angle is between 20 and 30 degrees. Where, Ar=An/ZA/5 (11) 2004
Fatal Accident Rate FA r (Numbers/Hour) Journal of the Eastern Asia Society for Transportation Studies, Vol.11, 2015 Ar: Traffic accident rate according to cumulative hours of each range of solar zenith angle (numbers/hour) An: Number of traffic accidents according to each range of solar zenith angle (2007-2011) ZA: Cumulative hours of each range of solar zenith angle at Chiba Prefecture Headquarters in a year Solar Zenith Angle (Degrees) Cumulative Hours for Each Solar Zenith Angle Table 2. Cumulative hours in a year for each range of solar zenith angle at Chiba Prefectural Headquarters East West Total Percentage Cumulative Percentage Cumulative of Hours for of Hours for Cumulative Each Solar Cumulative Each Solar Hours Zenith Angle Hours Zenith Angle Percentage of Cumulative Hours 15 96.4 1.1 96.2 1.1 192.6 2.2 25 167.8 1.9 167.9 1.9 335.7 3.8 35 218.1 2.5 218.1 2.5 436.2 5.0 45 276.9 3.2 276.4 3.2 553.3 6.3 55 400.0 4.6 399.9 4.6 799.9 9.1 65 383.2 4.4 383.2 4.4 766.5 8.7 75 333.7 3.8 333.5 3.8 667.2 7.6 85 321.5 3.7 321.5 3.7 643.0 7.3 95 321.9 3.7 322.3 3.7 644.2 7.4 105 335.4 3.8 335.1 3.8 670.5 7.7 115 388.0 4.4 388.7 4.4 776.6 8.9 125 403.8 4.6 403.4 4.6 807.3 9.2 135 272.5 3.1 272.5 3.1 545.0 6.2 145 210.8 2.4 210.8 2.4 421.6 4.8 155 159.5 1.8 160.1 1.8 319.5 3.6 165 90.4 1.0 90.5 1.0 180.9 2.1 Total 4380.0 50.0 4380.0 50.0 8760.0 100.0 6.0 Heavy Vehicle Car Two-wheeled Vehicle Bicycle Pedestrian Total 5.0 4.0 3.0 2.0 1.0 0.0 165 155 145 135 125 115 105 95 85 75 65 55 45 35 25 15 15 25 35 45 55 65 75 85 95 105 115 125 135 145 155 165 Sun is in East (High Sun) Bef After (Low Sun) ore Sun set Night Solar Zenith Angle( ) Figure 3. Accident rate for each accident type according to solar zenith angle 2005
Fatal Accident Rate FA r (Numbers/Hour) Journal of the Eastern Asia Society for Transportation Studies, Vol.11, 2015 0.05 0.045 0.04 0.035 0.03 0.025 0.02 0.015 0.01 0.005 0 Heavy Vehicle Car Two-wheeled Vehicle Bicycle Pedestrian Total 165 155 145 135 125 115 105 95 85 75 65 55 45 35 25 15 15 25 35 45 55 65 75 85 95 105 115 125 135 145 155 165 Sun is in East (High Sun) Bef After (Low Sun) ore Sun set Night Figure 4. Fatal accident rate for each accident type according to solar zenith angle As shown in Figure 3, traffic accident rates before sunset and after sunset are higher than any other time slots, with the rate for 95 degrees, i.e. after sunset, being the highest. The rate for 85 degrees is lower than that for 75 degrees. It may be that traffic accidents caused by sun glare tend not occur before sunset because the presence of topographic features means that the sun has already effectively set. Figure 4 shows fatal traffic accident rates (numbers/hour) for total accidents and each accident type. The axis of abscissa is solar zenith angle and the axis of ordinates is traffic accident rate. The method for calculating fatal accident rates according to solar zenith angle is shown as Formula (12). The number of fatal accidents for the 5-year period (An) and cumulative hours in a year (ZA) for each range of solar zenith angle were calculated, and the fatal traffic accident rate (number/hour, FAr) were estimated from these data. Total fatal accident rates after sunset are much higher than for any other time slot. This tendency is attributed to the very high rate of fatal pedestrian accidents after sunset. It is very important to carry out traffic safety measures aimed at preventing pedestrian accidents after sunset. FAr = FAn/ZA/5 (12) where, FAr: Fatal accident rate according to cumulative hours of each range of solar zenith angle (numbers/hour) FAn: Number of fatal accidents according to each range of solar zenith angle (2007-2011) ZA: Cumulative hours of each range of solar zenith angle at Chiba Prefecture Headquarters in a year 4.3 Analysis of Pedestrian Accidents Solar Zenith Angle( ) 4.3.1 Pedestrian accidents analyzed with respect to weather conditions Figure 5 shows pedestrian accident rates (numbers/hour) for total accidents and each accident type. The axis of abscissa is solar zenith angle and the axis of ordinates represents traffic 2006
Pedestrian Accident Rate PA r (Numbers/Hour) Journal of the Eastern Asia Society for Transportation Studies, Vol.11, 2015 accident rate. The method for calculating pedestrian accident rates according to solar zenith angle is shown as Formula (13). The number of pedestrian accidents for the 5-year period (PAn) and cumulative hours in a year (ZA) for each range of solar zenith angle were calculated, and pedestrian accident rates (numbers/hour) were estimated from these data. The pedestrian accident rate after sunset is much higher than for any other time slot, with the rate for 95 degrees, i.e. after sunset, being the highest. The rate for 85 degrees is lower than that for 75 degrees. It may be that traffic accidents caused by sun glare tend not to occur before sunset because the presence of topographic features means that the sun has already effectively set. 1.2 Fine Cloudy Rain/Foggy/Snowy Total 1 0.8 0.6 0.4 0.2 0 165155145135125115105 95 85 75 65 55 45 35 25 15 15 25 35 45 55 65 75 85 95 105115125135145155165 Sun is in East (High Sun) Bef After (Low Sun) ore Sun set Night Solar Zenith Angle( ) Figure 5. Pedestrian accidents rate for each weather condition according to solar zenith angle PAr = PAn/ZA/5 (13) where PAr: Pedestrian accident rate according to cumulative hours of each range of solar zenith angle (numbers/hour) PAn: Number of pedestrian accidents according to each range of solar zenith angle (2007-2011) ZA: Cumulative hours of each range of solar zenith angle at Chiba Prefecture Headquarters in a year 4.3.2 Analysis of Pedestrian Accidents with Respect to Vehicle Travel Direction Pedestrian accidents in which the first vehicle concerned was a bicycle, a pedestrian or unknown (e.g., hit and run) were excluded. Figure 6 addresses the 10,602 pedestrian accidents for which the sun is in the west. According to this analysis, many of the vehicle travel directions for the first vehicle concerned in these pedestrian accidents are straight through (stop) and right-turning. Pedestrian accidents in which the travel direction of the first vehicle concerned is straight through (stop) or right-turning were extracted. Figure 7 and Figure 8 show the ratio of pedestrian travel direction for pedestrian accidents. In general, vehicles tended to hit pedestrians crossing from the left side in daytime and to hit pedestrians crossing 2007
from the right side at nighttime. The analyses that are shown in Figure 7 and Figure 8 have the same tendency as the general trend in many studies, but the tendency for after sunset is the same as for nighttime, and the tendency for before sunset is for the ratio of vehicles hitting pedestrians crossing from the right side to be slightly lower than for nighttime and slightly higher than for daytime. Staight Through,Stop Right-Turn Left-Turn 100% 103 208 54 219 207 80% 60% 383 692 212 988 1013 40% 20% 983 2069 631 1490 1350 0% (High Sun) (Low Sun) Before After Night Figure 6. Numbers of pedestrian accidents according to vehicle turning direction (The sun is in the west.) 100% Cross from Leftside Cross from Rightside 80% 60% 288 695 240 589 474 40% 20% 382 770 212 360 306 0% (High Sun) (Low Sun) Before After Night Figure 7. Ratio of pedestrian travel direction for pedestrian accidents with straight-through vehicles (The sun is in the west.) 2008
100% Cross from Leftside Cross from Rightside 80% 105 187 65 428 513 60% 40% 20% 207 393 112 401 360 0% (High Sun) (Low Sun) Before After Night Figure 8. Ratio of pedestrian travel directions for pedestrian accidents with right-turning vehicles (The sun is in the west.) 4.3.3 Vehicle Headlight State in Pedestrian Accidents and the Effect of Using Lights from 1 Hour before Pedestrian accidents in which the first vehicle concerned was a bicycle, a pedestrian or unknown (e.g., hit and run) were excluded. Figure 9 addresses the vehicle headlight state for 10,602 pedestrian accidents in which the sun is in the west. For most of the first vehicles concerned, the vehicle lights were off for pedestrian accidents in daytime and were on for pedestrian accidents after sunset. That is to say, even though most of the first vehicles concerned had their lights on after sunset, the pedestrian accident rate after sunset was much higher than for any other time slot. 100% 80% Headlight On Headlight Off, Side Marker or Fog Lamp On 189 Off Unknown 65 60% 40% 1405 2785 606 2316 2330 20% 244 0% 50 148 (High Sun) (Low Sun) Before After Night Figure 9. Vehicle headlight states for pedestrian accidents (The sun is in the west.) 2009
Numbers of traffic accident Journal of the Eastern Asia Society for Transportation Studies, Vol.11, 2015 So, it is important for vehicles to turn on their headlights from 1 hour before sunset. However, according to an analysis of pedestrian accidents after sunset, most first vehicles concerned already had their headlights on after sunset. That is to say, to decrease pedestrian accidents before and after sunset, it is not enough only to recommend the use of lights light from 1 hour before sunset. 4.4 Effect of Sun Glare 4.4.1 Analysis by Solar Azimuthal Angle A solar zenith angle of less than 90 degrees means a daytime accident; one of greater than 90 degrees means a nighttime accident. To show the position of the sun relative to the vehicle travel direction, the difference in azimuthal angle between the sun and the travel direction of the first vehicle concerned was defined as shown in FIGURE 2. If the sun is to the right of the line of travel direction, then the difference in azimuthal angle is greater than 0 degrees; otherwise, the difference is less than 0 degrees. The calculated difference in azimuthal angle is rounded to the nearest 20 degrees. 0 degrees means that the difference is between -10 degrees and 10 degrees. As shown in FIGURE 10, traffic accidents tend to occur at a higher rate when the sun is in front of the vehicle than when the sun is at other positions. As a control, FIGURE 10 shows data for nighttime and unfine weather accidents; the accident numbers are largely independent of the differences in azimuthal angle between the sun and vehicle travel direction. 4,000 time Fine(57,814) Night Fine(20,449) time Except Fine(31,024) Night Except Fine(16,045) 3,500 3,000 2,500 2,000 1,500 1,000 500 0-160-140-120-100-80 -60-40 -20 0 20 40 60 80 100 120 140 160 180 The difference in azimuthal angle between the sun and vehicle( ) Figure 10. Numbers of traffic accidents vs. difference in azimuthal angle between the sun and the vehicle travel direction The vehicle travel direction and the solar azimuthal angle are mutually independent. Because the difference in azimuthal angle between the sun and the vehicle indicates the position of the sun relative to the vehicle travel direction, the frequency of occurrence of the 2010
difference in azimuthal angle between the sun and the vehicle travel direction should be the same for any azimuthal angle. The frequency of occurrence of the difference in azimuthal angle for each 20-degree range should be the same as that for any other 20-degree range. In light of this, we can say that sun glare tends to be a factor contributing to accidents during daytime in fine weather, as was found in previous studies. 4.4.2 Analysis by Solar Zenith Angle As shown in Table 2, the calculated solar zenith angles were aggregated to determine the frequency of occurrence of solar zenith angle. Using the combined percentage of time per solar zenith angle, daytime traffic accident rates during fine weather according to solar zenith and azimuthal angle were calculated. Solar azimuthal angle was broken into 90-degree quadrants: Front 90 degrees is the forward quadrant extending 45 degrees left and right of the travel direction, rear 90 degrees is the rearward quadrant extending 45 degrees left and right of a line opposite to the travel direction, and left 90 degrees and right 90 degrees are the respective leftward and rightward quadrants. Moreover, numbers of daytime traffic accidents in fine weather were aggregated according to solar zenith and azimuthal angles. And traffic accident rates (ZAr) according to the frequency of occurrence of solar zenith and azimuthal angle were calculated by using Formula (14). It is difficult to calculate fine weather in daytime by using weather data, because precise weather observation data are not available, so the fine weather rate was calculated by using weather items of daytime accident data. ZAn ZAr = (14) ZA 5 Fr where, ZAr: Traffic accident rate according to the frequency of occurrence of each range of solar zenith and azimuthal angle (accidents / 24 hours of fine weather) ZAn: Number of traffic accidents during fine weather according to each range of solar zenith and azimuthal angle (2007~2011) ZA: Cumulative hours of each range of solar zenith angle at Chiba Prefecture Headquarters in a year Fr: Fine weather rate (accidents during fine weather / all accidents) Figure 11 plots traffic accident rates vs. the frequency of occurrence of the solar zenith and azimuthal angle ranges calculated using Formula (14). When the sun is to the left or right of the first vehicle concerned, traffic accident rates do not increase with increases in solar zenith angle. However, when the sun is in front of the first vehicle concerned, traffic accident rates greatly increase with increases in the angles. When the angles start to exceed 80 degrees, traffic accident rates decrease, mainly because the sun is near the horizon. So, it is assumed that the drivers of the first vehicle concerned were affected more adversely by sun glare when the solar zenith angle was from about 50 to 90 degrees and the sun was in front. In contrast, the drivers of the first vehicle concerned were less affected by sun glare when the sun was near the horizon. 2011
Accident Rate (Numbers/1 hour of Fine Weather) Journal of the Eastern Asia Society for Transportation Studies, Vol.11, 2015 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 The Difference 15 25 35 45 55 65 75 85 (High Sun) Solar Zenith Angle ( ) Front90 Right90 Back90 Left90 (Low Sun) Before Figure 11. Traffic accident rates (accidents / hour of fine weather) vs. solar zenith and azimuthal angle (The sun in in the west.) 5. CONCLUSION According to these accident analyses for before and after sunset, the traffic accident rates for the time slots of before and after sunset are higher than for any other time slots. However, the traffic accident rate before sunset is slightly lower than just before and after this time slot. As a reason for this trend, it may be that more traffic accidents occurred by sun glare when the solar zenith angle was 75 degrees. Also, more traffic accidents might have occurred as a result of bad visibility after sunset. The pedestrian accident rate after sunset is higher than that of any other time slot, and the fatal pedestrian accident rate or the pedestrian accident rate under adverse weather after sunset is much higher than for any other time slot. This may be attributable to the difficulty of drivers to detect walking pedestrians after sunset, particularly under adverse weather. In pedestrian accidents after sunset, most vehicles have their headlights on. However, the pedestrian accident rate after sunset is much higher than any other time slot, especially for fatal pedestrian accidents, so to only recommend that headlights be used from 1 hour before sunset is not sufficient to prevent pedestrian accidents. It is very important to implement measures that make pedestrians more visible to drivers when the pedestrian is approaching from the right side. It was showed that sun glare affects traffic accident occurrence. Analyses according to solar zenith and azimuthal angles found that the traffic accident rate increases with decrease in viewing angle. Therefore, it was determined that sun glare was a contributing factor in traffic accidents in which the viewing angle of the sun and the first vehicle concerned was less than 90 degrees. 2012
6. FUTURE DIRECTIONS In many accident analyses that address time slots, the time of day, the season or the day vs. night are used as the time index. In this research, when traffic accidents were analyzed with a focus on traffic visibility condition, the sunset time was found to be an important factor. time should be used for analyses that focus on traffic visibility condition. The period between the time when the sun is dazzling and sunset is not so long, and traffic visibility conditions change drastically during this period. Therefore, traffic visibility conditions should be accurately connected with time slots. It is important to consider the traffic volumes of vehicles and pedestrians. Traffic volumes are high at time slots before and after sunset, so traffic volumes may affect traffic accident rates before and after sunset. To propose improved traffic safety measures, it is very important to quantify the effect of traffic volume. REFERENCES Institute for Traffic Accident Research and Data Analysis (ITATDA) Research Report. (2002), Research on the Effect of Natural Disasters on Traffic Accident Occurrence (in Japanese) Hagita, K., Mori, K.(2011a) Analysis of the Influence of Sun Glare on Traffic Accidents in Japan, Journal of the Eastern Asia Society for Transportation Studies, Vol.9, pp. 1775-1785 Hagita, K., Mori, K. (2011b) Analysis of Traffic Accident Influenced by Sun Glare, Journal of Japan Society of Civil Engineers (D3), pp. 1055-1062 (in Japanese) Hagita, K., Mori, K. (2011c) Analysis of the Influence of Sun Glare on Pedestrian Accidents, Papers of the 31 st Japan Society of Traffic Engineers Annual Meeting CD-ROM, No.7, (in Japanese) Hagita, K., Mori, K. (2011d) Analysis by Vehicle Types of Traffic Accident Influenced by Sun Glare, Summarized Papers, 2011 Society of Automotive Engineers of Japan Annual Congress (Autumn), 12-11, No.8 (in Japanese) Mitra, S. (2008) Investigating Impact of Sun Glare on Transportation Safety, TRB 2008 Annual Meeting CD-ROM, 08-2184. Jurado-Pina, R. and Pardillo-Mayora, J. M. (2009) Methodology to Predict Driver Vision Impairment Situations Caused by Sun Glare, TRB 2009 Annual Meeting CD-ROM, 09-1429. Jurado-Pina, R., Pardillo-Mayora, J. M. and Jimenez, R. Methodology to Analyze Sun Glare Related Safety Problems at Highway Tunnel Exits, Journal of Transportation Engineering, No.6, pp. 545 553, 2010. Daniel, J., Byun, J. and Chien, S. (2009) Impact of Adverse Weather on Freeway Speeds and Flows, TRB 2009 Annual Meeting CD-ROM, 09-2837. Nishida, Y. (2003) Pedestrian Traffic Accident Analysis and Prevention Considering light Condition, IATSS Review, Vol.28, No.1, pp6-13. (in Japanese) Morita, K. (2000) Statistical Analysis of Traffic Accident Rate at Dusk, Journal of Light and Visual Environment, Vol.84, No.8A, pp 507-513. (in Japanese) 2013
Murakami, T. (2010) Methods for Calculating Solar Zenith Angle and Solar Azimuthal Angle, http://www.agr.niigata-u.ac.jp/~muratac/, Accessed Sep. 25. (in Japanese) 2014