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Publication Number: FHWA-HRT-04-136
Date: December 2005

Enhanced Night Visibility, Volume V: Phase II—Study 3: Visual Performance During Nighttime Driving in Snow

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CHAPTER 3—RESULTS

OBJECTIVE MEASUREMENTS

A total of 240 observations were obtained from the experiment for each of the objective measurements, detection and recognition distances. An ANOVA was performed on both objective measurements using a 4 (VES) by 2 (Age) by 3 (Object) mixed factorial model. The ANOVA summary tables for both detection distance (table 6) and recognition distance (table 7) indicate that only the main effects for VES and object were significant for detection and recognition distances. The interactions and the main age effect were not significant at p < 0.05 (table 8).

The mean and standard error (SE) detection distances of the two age groups were as follows:

  • Younger age group: Mean = 60.7 m (199 ft), SE = 2.1 m (7 ft).
  • Middle-aged group: Mean = 58.8 m (193 ft), SE = 1.8 m (6 ft).

The mean and SE recognition distances were as follows:

  • Younger age group: Mean = 56.7 m (186 ft), SE = 2.1 m (7 ft).
  • Middle-aged group: Mean = 53.6 m (176 ft), SE = 1.8 m (6 ft).
Table 6. ANOVA summary table for the dependent measurement: detection distance.
Source DF SS MS F value P value
TOTAL 239 1201614.61      
Between
Age 1 2462.85 2462.85 0.33 0.5714
Subject/Age 18 133398.01 7411.00    
 
Within
VES 3 78260.57 26086.86 13.51 <.0001 *
VES by Age 3 8128.74 2709.58 1.40 0.2518
VES by Subject/Age 54 104264.09 1930.82    
 
Object 2 501349.91 250674.95 71.44 <.0001 *
Object by Age 2 2614.38 1307.19 0.37 0.6916
Object by Subject/Age 36 126324.28 3509.01    
 
VES by Object 6 12586.14 2097.69 1.03 0.4085
VES by Object by Age 6 12746.63 2124.44 1.05 0.4002
VES by Object by Subject/Age 108 219479.03 2032.21    

 

Table 7. ANOVA summary table for the dependent measurement: recognition distance.
Source DF SS MS F value P value
TOTAL 239 1215616.69    
Between
Age 1 5869.12 5869.12 0.61 0.4459
Subject/Age 18 173962.99 9664.61  
 
Within
VES 3 60887.44 20295.81 10.87 <.0001 *
VES by Age 3 10399.35 3466.45 1.86 0.1480
VES by Subject/Age 54 100852.77 1867.64  
 
Object 2 481380.77 240690.38 73.94 <.0001 *
Object by Age 2 2787.11 1393.56 0.43 0.6550
Object by Subject/Age 36 117183.20 3255.09  
 
VES by Object 6 16970.24 2828.37 1.34 0.2447
VES by Object by Age 6 17810.23 2968.37 1.41 0.2177
VES by Object by Subject/Age 108 227513.45 2106.61  

 

Table 8. Summary of significant main effects and interactions.
Source Significant
Detection
Significant
Recognition
Between
Age    
Subject/Age    
 
Within
VES x x
VES by Age    
VES by Subject/Age    
 
Object x x
Object by Age    
Object by Subject/Age    
 
VES by Object    
VES by Object by Age    
VES by Object by Subject/Age    

The post hoc results for the significant main effects are shown as graphs in figure 9 and figure 10. Standard error bars are provided with the means, and means with the same letter in their grouping were not considered significantly different (based on the Bonferroni post hoc test).

The HLB headlamp is the most commonly available VES, making its experimental results a baseline measure; therefore, it is suggested that a comparison of the results of other VESs be made to results obtained for the HLB in the following descriptions of the significant results. Note that this is only one halogen headlamp type and beam pattern, and the results are not necessarily representative of all halogen headlamps currently on the market.

VES Main Effect

The VES main effect for both detection and recognition distances can be seen in figure 9. For detection distance, the supplemental UV–A showed a significant benefit over the baseline HLB. The average detection distance for the five UV–A + HLB configuration was 66.1 m (217 ft), and the detection distance for the hybrid UV–A + HLB was 62.2 m (204 ft). The detection distance for the HLB was 59.4 m (195 ft). Detection distances for all the VESs were significantly different than that for the HID configuration (51.2 m, 168 ft), which performed the worst. HID performed similarly on recognition distances, with both UV–A + HLB pairings and the HLB alone having significantly greater distances. The recognition distances for HLB and the HLB with supplemental UV–A were not significantly different from each other.

Bar graph. Bonferroni post-hoc results on detection and recognition distances for the main effect: VES. Click here for more detail.
Figure 9. Bar graph. Bonferroni post-hoc results on detection and recognition
distances for the main effect: VES.

Object Main Effect

The post hoc results suggest that the clothing color (white versus black) determined the significant differences for the object main effect (figure 10). The detection distances of both the perpendicular and parallel pedestrians wearing white clothing—69.869.8 m (229 ft) and 69.2 m (227 ft), respectively—were significantly greater than the 39.9-m (131-ft) detection distance of the perpendicular pedestrian wearing black clothing. This suggests that, overall, the clothing color rather than the motion of the object caused the observed differences. These trends were also followed by the recognition distances, where the perpendicular and parallel pedestrians wearing white clothing—65.2 m (214 ft) and 64.3 m (211 ft), respectively—outperformed the 36.0-m (118-ft) detection distance for the perpendicular pedestrian wearing black clothing.

Bar graph. Bonferroni post-hoc results on detection and recognition distances for the main effect: object. Click here for more detail.
Figure 10. Bar graph. Bonferroni post-hoc results on detection and recognition
distances for the main effect: object.

SUBJECTIVE MEASUREMENTS

An ANOVA was performed to analyze the subjective measurements using a 4 (VES) by 2 (Age) mixed factorial model. ANOVA summary tables were generated for each of the seven subjective statements, shown in table 9 through table 15. No significant difference was found for any of the statements. Missing values for statement 1 (one response), statement 5 (one response), and statement 6 (two responses) resulted in total degrees of freedom (DF) for these statements that were different from those of statements 2, 3, 4, and 7.

Table 9. ANOVA summary table for the Likert-type rating for detection.
Statement 1: Detection
Source DF SS MS F value P value
TOTAL 78 222.4    
Between
Age 1 3.6 3.6 0.79 0.3868
Subject/Age 18 82.8 4.6  
 
Within
VES 3 14.5 4.8 2.20 0.0989
VES by Age 3 5.0 1.7 0.75 0.5245
VES by Subject/Age 53 116.4 2.2  

 

Table 10. ANOVA summary table for the Likert-type rating for recognition.
Statement 2: Recognition
Source DF SS MS F value P value
TOTAL 79 242.0    
Between
Age 1 7.8 7.8 1.67 0.2132
Subject/Age 18 84.4 4.7  
 
Within
VES 3 13.4 4.5 1.83 0.1524
VES by Age 3 4.2 1.4 0.58 0.6323
VES by Subject/Age 54 132.1 2.4  


Table 11. ANOVA summary table for the Likert-type rating for lane-keeping assistance.
Statement 3: Lane-keeping assistance
Source DF SS MS F value P value
TOTAL 79 197.0    
Between
Age 1 0.1 0.1 0.01 0.9196
Subject/Age 18 96.0 4.8  
 
Within
VES 3 3.1 1.0 0.59 0.6213
VES by Age 3 4.1 1.4 0.78 0.5121
VES by Subject/Age 54 93.9 1.7  


Table 12. ANOVA summary table for the Likert-type rating for roadway direction.
Statement 4: Roadway direction
Source DF SS MS F value P value
TOTAL 79 198.8    
Between
Age 1 0.2 0.2 0.04 0.8510
Subject/Age 18 99.1 5.5  
 
Within
VES 3 2.1 0.7 0.41 0.7499
VES by Age 3 4.1 1.4 0.79 0.5042
VES by Subject/Age 54 93.3 1.7  


Table 13. ANOVA summary table for the Likert-type rating for visual discomfort.
Statement 5: Visual discomfort
Source DF SS MS F value P value
TOTAL 78 202.6    
Between
Age 1 7.5 7.5 1.93 0.1822
Subject/Age 18 70.3 3.9  
 
Within
VES 3 13.1 4.4 2.10 0.1111
VES by Age 3 1.3 0.4 0.20 0.8951
VES by Subject/Age 53 110.4 2.1  


Table 14. ANOVA summary table for the Likert-type rating for overall safety rating.
Statement 6: Overall safety rating
Source DF SS MS F value P value
TOTAL 77 207.9    
Between
Age 1 0.1 0.1 0.01 0.9156
Subject/Age 18 84.1 4.7  
 
Within
VES 3 9.5 3.2 1.50 0.2243
VES by Age 3 4.2 1.4 0.66 0.5822
VES by Subject/Age 52 110.0 2.1  


Table 15. ANOVA summary table for the Likert-type rating for overall VES evaluation.
Statement 7. Overall VES evaluation
Source DF SS MS F value P value
TOTAL 79 264.4    
Between
Age 1 4.5 4.5 0.78 0.3899
Subject/Age 18 104.6 5.8  
 
Within
VES 3 7.8 2.6 0.99 0.4025
VES by Age 3 5.5 1.8 0.70 0.5546
VES by Subject/Age 54 141.9 2.6  

To understand drivers’ ratings of the various VESs in terms of safety and comfort, the results for all seven statements for every VES were sorted by ascending mean rating. Although not significant, drivers rated the five UV–A + HLB VES as the top configuration that allowed them to detect objects sooner (statement 1), allowed them to recognize objects sooner (statement 2), helped them stay on the road better (statement 3), and made them feel safer (statement 6); it was perceived as being a better VES than their regular headlights (statement 7). The hybrid UV–A + HLB configuration had the top rating for seeing the direction of the roadway (statement 4), and the HLB was rated as the top configuration for not causing any more visual discomfort than the participant’s regular headlights (statement 5).

It is interesting, although not statistically significant, that the HID was rated better than the HLB in all statements other than statement 4 (their averages were equal for statements 6 and 7), conflicting with the objective measurements. Although the subjective results were not significant, the conflicting results do exhibit the difference in driver perception and performance. The HLB was rated as the worst for detection, recognition, staying on the road, and seeing the road direction even though it significantly outperformed the HID on the objective measurements. Although it was the worst performer, the HID was the second-highest rated configuration with respect to feeling safer and being a better configuration than the participant’s regular headlights. Following are the results by statement.

  • Statement 1: This vision enhancement system allowed me to detect objects sooner than my regular headlights (1 = Strongly Agree; 7 = Strongly Disagree).

    VES Mean Rating
    Five UV–A + HLB 2.3
    Hybrid UV–A + HLB 2.7
    HID 3.2
    HLB 3.4


  • Statement 2: This vision enhancement system allowed me to recognize objects sooner than my regular headlights (1 = Strongly Agree; 7 = Strongly Disagree).

    VES Mean Rating
    Five UV–A + HLB 2.3
    Hybrid UV–A + HLB 2.8
    HID 3.1
    HLB 3.4


  • Statement 3: This vision enhancement system helped me to stay on the road (not go over the lines) better than my regular headlights (1 = Strongly Agree; 7 = Strongly Disagree).

    VES Mean Rating
    Five UV–A + HLB 3.0
    Hybrid UV–A + HLB 3.2
    HID 3.4
    HLB 3.5


  • Statement 4: This vision enhancement system allowed me to see which direction the road was heading (i.e., left, right, straight) beyond my regular headlights (1 = Strongly Agree; 7 = Strongly Disagree).

    VES Mean Rating
    Hybrid UV–A + HLB 3.0
    Five UV–A + HLB 3.2
    HID 3.2
    HLB 3.5


  • Statement 5: This vision enhancement system did not cause me any more visual discomfort than my regular headlights (1 = Strongly Agree; 7 = Strongly Disagree).

    VES Mean Rating
    HLB 2.0
    Five UV–A + HLB 2.1
    HID 2.6
    Hybrid UV–A + HLB 3.0


  • Statement 6: This vision enhancement system made me feel safer when driving on the roadway at night than my regular headlights (1 = Strongly Agree; 7 = Strongly Disagree).

    VES Mean Rating
    Five UV–A + HLB 2.4
    HID 3.1
    HLB 3.1
    Hybrid UV–A + HLB 3.3


  • Statement 7: This is a better vision enhancement system than my regular headlights (1 = Strongly Agree; 7 = Strongly Disagree).

    VES Mean Rating
    Five UV–A + HLB 2.2
    HID 2.8
    HLB 2.8
    Hybrid UV–A + HLB 3.1


 

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