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Publication Number: FHWA-HRT-04-135
Date: December 2005

Enhanced Night Visibility, Volume IV: Phase II—Study 2: Visual Performance During Nighttime Driving in Rain

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CHAPTER 3—RESULTS

Results included in this report are based on statistically significant effects at an alpha symbol = 0.05 level except where otherwise stated. In main effect graphs, means with the same letter are not significantly different based on the Bonferroni post hoc test. Bars above and below the means indicate standard error.

OBJECTIVE MEASUREMENTS

An ANOVA was performed on the objective measurements taken during the Smart Road portion of the study. The model for this portion of the study was a 12 (VES) by 3 (Age) by 7 (Object) mixed factorial design. ANOVA summary tables were obtained for both objective dependent measurements (table 6 and table 7). A total of 2,509 observations were obtained from the experiment for each objective measurement. When drivers were not able to detect and recognize an object, a value of 0 was assigned. Several main effects and interactions were considered significant (table 8).

ANOVA results showed no significant differences between the three age groups in terms of detection distances as seen below:

  • Younger age group: mean = 60.3 m (198 ft), standard error (SE) = 0.8 m (2.7 ft).
  • Middle-aged group: mean = 58.8 m (193 ft), SE = 0.8 m (2.7 ft).
  • Older age group: mean = 58.8 m (193 ft), SE = 0.9 m (2.9 ft).

The results also showed no differences between the recognition distances of the three age groups as follows:

  • Younger age group: mean = 53.0 m (174 ft), SE = 0.8 m (2.5 ft).
  • Middle-aged group: mean = 52.1 m (171 ft), SE = 0.8 m (2.5 ft).
  • Older age group: mean = 51.2 m (168 ft), SE = 0.8 m (2.7 ft).
Table 6. ANOVA summary table for the dependent measurement: detection distance.
Source DF SS MS F value P value  
TOTAL 2508 16126389.6       
Between
Age 2 11699.1 5849.5 0.20 0.8187  
Subject/Age 27 783683.3 29025.3     
 
Within
VES 11 495073.4 45006.7 14.49 <0.0001 *
VES by Age 22 46920.7 2132.8 0.69 0.8524  
VES by Subject/Age 297 922560.2 3106.3     
 
Object 6 9460693.4 1576782.2 726.7 <0.0001 *
Object by Age 12 26194.2 2182.9 1.01 0.4458  
Object by Subject/Age 162 351506.1 2169.8     
 
VES by Object 66 240227.8 3639.8 1.86 <0.0001 *
VES by Object by Age 132 325313.9 2464.5 1.26 0.0279 *
VES by Object by Subject/Age 1771 3462517.4 1955.1     


Table 7. ANOVA summary table for the dependent measurement: recognition distance.
Source DF SS MS F value P value  
TOTAL 2508 13806193.9       
Between
Age 2 14070.7 7035.4 0.24 0.7856  
Subject/Age 27 780184.4 28895.7     
 
Within
VES 11 420087.0 38189.7 13.93 <0.0001 *
VES by Age 22 28862.3 1311.9 0.48 0.9789  
VES by Subject/Age 297 814502.9 2742.4     
 
Object 6 7907939.2 1317989.9 728.05 <0.0001 *
Object by Age 12 29927.3 2493.9 1.38 0.1814  
Object by Subject/Age 162 293269.2 1810.3     
 
VES by Object 66 167976.5 2545.1 1.46 0.0104 *
VES by Object by Age 132 258250.7 1956.4 1.12 0.1722  
VES by Object by Subject/Age 1771 3091123.4 1745.4     


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 x x
VES by Object by Age x  
VES by Object by Subject/Age    

The main effects of and interactions between VES and object were significant (p < 0.05) for both detection and recognition. The VES, object, and age interaction was significant (p < 0.05) only in terms of detection distances (figure 14 through figure 25).

The HLB headlamp is the most commonly available VES, making its experimental results a baseline measure. It is important to compare the results of other VESs 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; it does not necessarily represent all halogen lamps currently on the market.

VES by Object by Age Interaction

For the significant three-way interaction VES by Object by Age (figure 14 through figure 25), there were no marked differences between VES configurations in terms of detection distances. On average, all detections were less than 100 m (328 ft); a few levels of the interactions stood out as the ones that caused the significant difference. For example, the five UV–A + HLB configuration increased detection distances of pedestrians and cyclists with white clothing for older drivers up to 36 percent. This increase in detection for older drivers was less than 26.5 m (87 ft) farther than detection distances of HLB alone, but it was the biggest difference for this interaction.

The other levels of the three-way interaction did not show differences with a meaningful improvement; the detection distances for low-contrast objects (i.e., parallel pedestrian, black clothing; perpendicular pedestrian, black clothing; and tire tread) under all age by VES combinations were less than 51.8 m (170 ft). Age did not seem to follow any trends on this particular three-way interaction. Overall, the different UV–A + HLB configurations resulted in the best detection distances for all objects under the different age groups.

Bar graph. Results for the interaction: VES by Object by Age for IR–TIS. Click here for more detail.
Figure 14. Bar graph. Results for the interaction: VES by Object by Age for IR–TIS.


Bar graph. Results for the interaction: VES by Object by Age for HLB–LP. Click here for more detail.
Figure 15. Bar graph. Results for the interaction: VES by Object by Age for HLB–LP.


Bar graph. Results for the interaction: VES by Object by Age for HOH. Click here for more detail.
Figure 16. Bar graph. Results for the interaction: VES by Object by Age for HOH.


Bar graph. Results for the interaction: VES by Object by Age for HHB. Click here for more detail.
Figure 17. Bar graph. Results for the interaction: VES by Object by Age for HHB.


Bar graph. Results for the interaction: VES by Object by Age for five UV–A + HLB. Click here for more detail.
Figure 18. Bar graph. Results for the interaction: VES by Object by Age for five UV–A + HLB.


Bar graph. Results for the interaction: VES by Object by Age for three UV–A + HLB. Click here for more detail.
Figure 19. Bar graph. Results for the interaction: VES by Object by Age for three UV–A + HLB.


Bar graph. Results for the interaction: VES by Object by Age for hybrid UV–A + HLB. Click here for more detail.
Figure 20. Bar graph. Results for the interaction: VES by Object by Age for hybrid UV–A + HLB.


Bar graph. Results for the interaction: VES by Object by Age for HLB. Click here for more detail.
Figure 21. Bar graph. Results for the interaction: VES by Object by Age for HLB.


Bar graph. Results for the interaction: VES by Object by Age for five UV–A + HID. Click here for more detail.
Figure 22. Bar graph. Results for the interaction: VES by Object by Age for five UV–A + HID.


Bar graph. Results for the interaction: VES by Object by Age for three UV–A + HID. Click here for more detail.
Figure 23. Bar graph. Results for the interaction: VES by Object by Age for three UV–A + HID.


Bar graph. Results for the interaction: VES by Object by Age for hybrid UV–A + HID. Click here for more detail.
Figure 24. Bar graph. Results for the interaction: VES by Object by Age for hybrid UV–A + HID.


Bar graph. Results for the interaction: VES by Object by Age for HID. Click here for more detail.
Figure 25. Bar graph. Results for the interaction: VES by Object by Age for HID.

VES by Object Interaction

The significant difference (p < 0.05) for the VES by Object interaction under both detection and recognition distances appears to be mainly the result of the object contrast levels: black (low contrast) versus white (high contrast) objects (figure 26 through figure 29).

In general, the HLB performed as well as or better than the other VESs for the detection and recognition of high-contrast objects (figure 26 and figure 28). The only exception was the HLB with five UV–A, which enhanced drivers’ detection and recognition of the white-clothed pedestrians and cyclist. However, with five UV–A + HLB the detection of white-clothed pedestrians and nonmotorists was less than 15.2 m (50 ft) farther away than with HLB (12 to 18 percent farther on average), and recognition was less than 12.2 m (40 ft) farther away (12 to 17 percent farther on average). On the other hand, the detection and recognition distances with HID were significantly shorter than with HLB for the cyclist and the perpendicular pedestrian with white clothing (10 to 12 percent and 17 percent closer to the object, respectively). Overall, detection and recognition distances with the IR–TIS were not different from HLB.

With respect to the low-contrast objects (figure 27 and figure 29), HLB was either better or no different than other VESs. For the parallel pedestrian with black clothing and the tire tread, there was no significant difference between HLB and all the other VESs for either detection or recognition distances. When drivers used HLB, they were able to detect and recognize the child’s bicycle farther away than when they used the HID, HHB, or the HLB–LP (25 to 27 percent, 36 to 41 percent, and 27 to 28 percent farther, respectively). The detection and recognition distances for the perpendicular pedestrian with black clothing were farther away with HLB than with IR–TIS, HID, HID with any of the UV–A configurations, or the HLB–LP.

Across all objects, the halogen baseline configuration allowed drivers to detect and recognize objects sooner than did its HID counterpart. Depending on the type of object, the HLB allowed object detection ranging from 5.8 to 13.7 m (19 to 45 ft) farther away (15 percent farther for low-contrast objects and 17 percent farther for high-contrast objects, respectively) than the HID.

Bar graph. Results on detection distances for the VES by Object interaction for pedestrians and cyclist with white clothing. Click here for more detail.
Figure 26. Bar graph. Results on detection distances for the VES by Object interaction for pedestrians and cyclist with
white clothing.


Bar graph. Results on detection distances for the VES by Object interaction for pedestrians with black clothing and other objects. Click here for more detail.
Figure 27. Bar graph. Results on detection distances for the VES by Object interaction for pedestrians with black clothing
and other objects.


Bar graph. Results on recognition distances for the VES by Object interaction for pedestrians and cyclist with white clothing. Click here for more detail.
Figure 28. Bar graph. Results on recognition distances for the VES by Object interaction for pedestrians and cyclist
with white clothing.


Bar graph. Results on recognition distances for the VES by Object interaction for pedestrians with black clothing and other objects. Click here for more detail.
Figure 29. Bar graph. Results on recognition distances for the VES by Object interaction for pedestrians with black clothing
and other objects.

VES Main Effect

VESs were significantly different from each other (p < 0.05) in terms of the detection and recognition distances. Post hoc analyses showed that the HLB provided detection and recognition distances that were significantly longer than the IR–TIS, HID, and HLB–LP VESs by approximately 6.1 m (20 ft). The HLB distances were significantly less than those provided by the five UV–A + HLB VES by 6.7 m (22 ft) (figure 30); however, the magnitude of these differences was relatively small, representing only 10 percent of the HLB performance levels.

Bar graph. Bonferroni post-hoc results for the main effect: VES. Click here for more detail.
Figure 30. Bar graph. Bonferroni post-hoc results for the main effect: VES.

Object Main Effect

Type of object was also significant for both detection and recognition distances. Post hoc test results showed three distinct groups: white clothing, black clothing, and ground-level objects (figure 31). This suggests that overall the contrast rather than the motion of the object (or lack of) caused the observed differences. The high-contrast objects (i.e., pedestrians and cyclist with white clothing) were detected and recognized from farther away than were the other objects. The detection distances for the tire tread and child’s bicycle were statistically different (p < 0.05) from the other objects; they were detected farther away than were black-clothed pedestrians but closer than were pedestrians with white clothing. The detection distances for pedestrians wearing black clothing were the closest to the actual object, and recognition distances were either as close (parallel pedestrian wearing black clothing) or closer (perpendicular pedestrian wearing black clothing) than the tire tread’s recognition distance. The child’s bicycle was detected and recognized farther away than were the pedestrians with black clothing and the tire tread.

Bar graph. Bonferroni post-hoc results for main effect: Object. Click here for more detail.
Figure 31. Bar graph. Bonferroni post-hoc results for main effect: Object.

SUBJECTIVE MEASUREMENTS

An ANOVA was performed to analyze the subjective measurements taken on the Smart Road. The model for this portion of the study was a 12 (VES) by 3 (Age) factorial design. ANOVA summary tables were generated for each of the seven subjective statements (table 9 through table 15), and significant main effects and interactions were summarized (table 16).

Table 9. ANOVA summary table for the Likert-type rating for detection.
Statement 1: Detection
Source DF SS MS F value P value  
Between
Age 2 24.0 12.0 0.82 0.4499  
Subject/Age 27 394.4 14.6     
 
Within
VES 11 244.0 22.2 14.86 <0.0001 *
VES by Age 22 32.4 1.5 0.99 0.4800  
VES by Subject/Age 297 443.3 1.5  
TOTAL 359 1138.2       


Table 10. ANOVA summary table for the Likert-type rating for recognition.
Statement 2: Recognition
Source DF SS MS F value P value  
Between
Age 2 26.8 13.4 0.87 0.4320  
Subject/Age 27 418.1 15.5     
 
Within
VES 11 217.3 19.8 13.01 <0.0001 *
VES by Age 22 36.8 1.7 1.10 0.3439  
VES by Subject/Age 297 450.9 1.5     
TOTAL 359 1149.9       


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  
Between
Age 2 30.8 15.4 1.29 0.2922  
Subject/Age 27 323.2 12.0     
 
Within
VES 11 297.5 27.0 18.83 <0.0001 *
VES by Age 22 26.8 1.2 0.85 0.6655  
VES by Subject/Age 297 426.6 1.4     
TOTAL 359 1104.9       


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  
Between
Age 2 13.2 6.6 0.70 0.5074  
Subject/Age 27 256.9 9.5     
 
Within
VES 11 223.0 20.3 13.25 <0.0001 *
VES by Age 22 34.8 1.6 1.03 0.4240  
VES by Subject/Age 297 454.6 1.5     
TOTAL 359 982.5       


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  
Between
Age 2 21.0 10.5 0.80 0.4580  
Subject/Age 27 352.9 13.1     
 
Within
VES 11 230.6 21.0 13.56 <0.0001 *
VES by Age 22 38.3 1.7 1.13 0.3167  
VES by Subject/Age 297 459.1 1.5     
TOTAL 359 1101.9       


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  
Between
Age 2 11.9 5.9 0.39 0.6794  
Subject/Age 27 407.9 15.1     
 
Within
VES 11 262.1 23.8 15.73 <0.0001 *
VES by Age 22 39.5 1.8 1.18 0.2599  
VES by Subject/Age 297 450.0 1.5     
TOTAL 359 1171.4       


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  
Between
Age 2 10.0 5.0 0.30 0.7428  
Subject/Age 27 446.8 16.5     
 
Within
VES 11 245.1 22.3 14.77 <0.0001 *
VES by Age 22 44.1 2.0 1.33 0.1513  
VES by Subject/Age 297 448.0 1.5     
TOTAL 359 1193.9       


Table 16. Summary of significant main effects and interactions for the Likert-type rating scales.
Source 1 2 3 4 5 6 7
Between
Age              
Subject/Age              
 
Within
VES x x x x x x x
VES by Age              
VES by Subject/Age              

To understand drivers’ ratings of the various VESs in terms of safety and comfort, the results of all seven statements for every VES were sorted by ascending mean rating. Drivers rated the five UV–A + HID configuration as the most likely to help them detect and recognize objects sooner. The IR–TIS fared the worst on these same statements, obtaining a neutral rating. In general, HIDs received better rankings than did HLBs on statements relating to farther detection and recognition distances, effectiveness in lane-keeping assistance, less visual discomfort, and overall perception of safety. A list of all statements and mean ratings for each VES is presented next.

  • 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 + HID 1.93
    Three UV–A + HID 2.10
    Hybrid UV–A + HID 2.27
    Hybrid UV–A + HLB 2.37
    HOH 2.37
    HID 2.47
    Five UV–A + HLB 2.53
    Three UV–A + HLB 2.60
    HLB 2.77
    HLB–LP 3.27
    HHB 4.03
    IR–TIS 4.87

  • 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 + HID 1.90
    Three UV–A + HID 2.07
    Hybrid UV–A + HLB 2.37
    HOH 2.47
    Hybrid UV–A + HID 2.47
    HID 2.50
    Three UV–A + HLB 2.63
    Five UV–A + HLB 2.67
    HLB 2.73
    HLB–LP 3.30
    HHB 3.97
    IR–TIS 4.73

  • 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 + HID 2.03
    Three UV–A + HID 2.07
    HOH 2.30
    Hybrid UV–A + HID 2.33
    HID 2.33
    Hybrid UV–A + HLB 2.63
    Three UV–A + HLB 2.77
    Five UV–A + HLB 2.87
    HLB 3.00
    HLB–LP 3.53
    HHB 4.37
    IR–TIS 5.10

  • 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
    Five UV–A + HID 2.07
    Hybrid UV–A + HID 2.37
    HID 2.37
    Three UV–A + HID 2.43
    Three UV–A + HLB 2.70
    Hybrid UV–A + HLB 2.70
    HOH 2.70
    Five UV–A + HLB 2.83
    HLB 3.07
    HLB–LP 3.23
    HHB 4.17
    IR–TIS 4.93

  • 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
    Five UV–A + HID 1.53
    HOH 1.73
    Three UV–A + HID 1.80
    HID 1.80
    Three UV–A + HLB 1.97
    HLB 2.03
    Five UV–A + HLB 2.20
    HLB–LP 2.30
    Hybrid UV–A + HLB 2.40
    Hybrid UV–A + HID 2.40
    HHB 3.70
    IR–TIS 4.33

  • 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 + HID 1.90
    Three UV–A + HID 1.93
    HOH 2.30
    Hybrid UV–A + HID 2.30
    Hybrid UV–A + HLB 2.40
    HID 2.43
    Three UV–A + HLB 2.53
    HLB 2.67
    Five UV–A + HLB 2.87
    HLB–LP 3.17
    HHB 4.07
    IR–TIS 4.93

  • 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 + HID 1.73
    Three UV–A + HID 1.90
    HOH 2.20
    Hybrid UV–A + HID 2.20
    HID 2.27
    Hybrid UV–A + HLB 2.33
    Three UV–A + HLB 2.43
    HLB 2.60
    Five UV–A + HLB 2.67
    HLB–LP 2.80
    HHB 3.90
    IR–TIS 4.77

Post hoc test results were graphed for ease of interpretation (figure 32 through figure 38). Type of VES had the only significant effect on statements 1 through 7 (table 9 through table 16).

In statement 1, “This vision enhancement system allowed me to detect objects sooner than my regular headlights,” a significant difference (p < 0.05) was observed between the IR–TIS configuration and all other configurations except HHB. IR–TIS received a mean rating of 4.87 (i.e., above “Neutral” with a tendency toward “Disagree”), while the HLB baseline received a mean rating of 2.77 (figure 32). Statements 2 through 7 followed a grouping pattern similar to that of statement 1 (figure 33 through figure 38).

Bar graph. Bonferroni post-hoc results on the ratings evaluating detection for the main effect: VES. Click here for more detail.
Figure 32. Bar graph. Bonferroni post-hoc results on the ratings evaluating detection for the
main effect: VES.


Bar graph. Bonferroni post-hoc results on the ratings evaluating recognition for the main effect: VES. Click here for more detail.
Figure 33. Bar graph. Bonferroni post-hoc results on the ratings evaluating recognition for the
main effect: VES.


Bar graph. Bonferroni post-hoc results on the ratings evaluating lane-keeping assistance for the main effect: VES. Click here for more detail.
Figure 34. Bar graph. Bonferroni post-hoc results on the ratings evaluating lane-keeping
assistance for the main effect: VES.


Bar graph. Bonferroni post-hoc results on the ratings evaluating roadway direction for the main effect: VES. Click here for more detail.
Figure 35. Bar graph. Bonferroni post-hoc results on the ratings evaluating roadway direction
for the main effect: VES.


Bar graph. Bonferroni post-hoc results on the ratings evaluating visual discomfort for the main effect: VES. Click here for more detail.
Figure 36. Bar graph. Bonferroni post-hoc results on the ratings evaluating visual discomfort
for the main effect: VES.


Bar graph. Bonferroni post-hoc results on the ratings evaluating overall safety for the main effect: VES. Click here for more detail.
Figure 37. Bar graph. Bonferroni post-hoc results on the ratings evaluating overall safety
for the main effect: VES.


Bar graph. Bonferroni post-hoc results on the overall rating for the main effect: VES. Click here for more detail.
Figure 38. Bar graph. Bonferroni post-hoc results on the overall rating for the main effect: VES.

 

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