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Publication Number: FHWA-HRT-04-145
Date: December 2005

Enhanced Night Visibility Series, Volume XIV: Phase III—Study 2: Comparison of Near Infrared, Far Infrared, and Halogen Headlamps on Object Detection in Nighttime Rain

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CHAPTER 4—DISCUSSION

This chapter begins with an explanation of stopping distance and continues with the calculation of stopping distances for each VES at various speeds plus tabulated detection distances for each object. The discussion continues with a summary of the performance of each of the VESs and comparisons to both the baseline HLB VES (a readily available system) and the FIR VES (the system that outperformed the others in detecting pedestrians in the clear driving condition in the previous study (ENV Volume XIII). The summaries for the different VESs also contain more detailed observations about the performance of each VES. The chapter concludes with some general comments about results of the clear weather study versus results in the rainy weather study, and the results of the near (active) versus far (passive) IR systems.

As mentioned in chapter 2, the aiming protocol used for this study resulted in a deviation in the location of maximum intensity from where it typically is for the HLB VES. Details about this deviation are discussed in ENV Volume XVII, Characterization of Experimental Vision Enhancement Systems. As a result of the headlamp aiming, the detection and recognition distances likely increased for the HLB configuration. It is important to consider the results presented in this study in the context and conditions tested. If different halogen headlamps or aiming methods had been used, different results might have been obtained.

STOPPING DISTANCES

While these detection and recognition distances provide an indication of the advantages of one system over another, they fail to describe completely potential safety benefits or concerns based on VES use. With a limited number of assumptions, the VES-specific detection distances in rainy weather conditions can be compared with various speed-dependent stopping distances, which can help determine how easy it is to out-drive a system. In other words, when are the increased detection distance advantages of a particular system overridden by an increase in vehicle speed resulting from a driver’s unfounded sense of security?

Collision-avoidance research dealing with different aspects of visibility suggests that time-to-collision is an important factor in enhancing driving safety.(9) For consistency, time-to-collision is presented as distance-to-collision (or stopping distance) for direct comparisons to the detection distances in this current study. Stopping distance is the sum of two components: (1) the distance needed for the braking reaction time (BRT) and (2) braking distance (table 8). Braking distance is the distance that a vehicle travels while slowing to a complete stop.(10) The results from driver braking performance studies suggest that the 95th percentile BRT to an unexpected object scenario in open-road conditions is about 2.5 s. (See references 11, 12, 13, and 14.) The braking distances in table 8 are calculated using the equation shown in figure 22.

Equation. Braking distance approximation. Click here for more detail.

Figure 22. Equation. Braking distance approximation.

The equation in figure 22 assumes an acceleration (g) of 9.8 m/s2 (32.2 ft/s2), a final speed of zero, a coefficient of friction (f) between the tire and the pavement of 0.35, and a straight, level roadway
(gradient, G=0 percent).

Table 8. Stopping distances needed for a wet roadway.
25 mi/h 35 mi/h 45 mi/h 55 mi/h 65 mi/h 70 mi/h
Speed (ft/s) 37 51 66 81 95 103
BRT in terms of Distance (ft) 92 128 165 202 238 257
Braking Distance (ft) 60 117 193 289 403 468
Stopping Distance (ft) 151 245 358 490 642 724

The calculations shown in table 8 represent a simple and ideal condition, but they allow for some visualization of the capabilities of VESs. These stopping distances can be used as a measure of the capability of VESs to provide enough time to detect, react, and brake to a stop at different speeds, but with some caveats. First, in this study, distances were obtained while drivers were moving at approximately
16 km/h (10 mi/h), and their ability to detect objects will not necessarily remain the same as speeds increase. Second, systems that produce stopping distances close to those in table 8 or longer stopping distances could produce less effective results when conditions worsen (e.g., standing water, worn tires, or downhill condition).

Table 9 through table 12 present VES and object combinations with mean detection distances that might compromise sufficient stopping distances. (In these tables, an "X" means the stopping distance might be compromised). Note that the detection distances in tables 9 through 12 for each VES and object combination were collected while the drivers were traveling in a controlled manner (e.g., within a specified speed range) and thus assume that such distances translate to all of the speeds listed.

Table 9. Detection distances by types of object and potential detection inadequacy when compared to stopping distance at various speeds: FIR.
Type of Object Detection
(ft)
151 ft at 25 mi/h 245 ft at 35 mi/h 358 ft at 45 mi/h 490 ft at 55 mi/h 642 ft at 65 mi/h 724 ft at 70 mi/h
Pedestrian, Left, Denim Clothing 155   X X X X X
Pedestrian, Right, Denim Clothing 172   X X X X X
Pedestrian, Left Turn, Left 127 X X X X X X
Pedestrian, Left Turn, Right 180   X X X X X
Pedestrian, Right Turn, Left 201   X X X X X
Pedestrian, Right Turn, Right 139 X X X X X X
Pedestrian, Dynamic 169   X X X X X
Tire Tread 86 X X X X X X


Table 10. Detection distances by types of object and potential detection inadequacy when compared to stopping distance at various speeds: NIR 1.
Type of Object Detection
(ft)
151 ft at 25 mi/h 245 ft at 35 mi/h 358 ft at 45 mi/h 490 ft at 55 mi/h 642 ft at 65 mi/h 724 ft at 70 mi/h
Pedestrian, Left, Denim Clothing 269     X X X X
Pedestrian, Right, Denim Clothing 251     X X X X
Pedestrian, Left Turn, Left 254     X X X X
Pedestrian, Left Turn, Right 247     X X X X
Pedestrian, Right Turn, Left 346     X X X X
Pedestrian, Right Turn, Right 243   X X X X X
Pedestrian, Dynamic 232   X X X X X
Tire Tread 80 X X X X X X


Table 11. Detection distances by types of object and potential detection inadequacy when compared to stopping distance at various speeds: NIR 2.
Type of Object Detection
(ft)
151 ft at 25 mi/h 245 ft at 35 mi/h 358 ft at 45 mi/h 490 ft at 55 mi/h 642 ft at 65 mi/h 724 ft at 70 mi/h
Pedestrian, Left, Denim Clothing 237   X X X X X
Pedestrian, Right, Denim Clothing 242   X X X X X
Pedestrian, Left Turn, Left 217   X X X X X
Pedestrian, Left Turn, Right 211   X X X X X
Pedestrian, Right Turn, Left 281     X X X X
Pedestrian, Right Turn, Right 137 X X X X X X
Pedestrian, Dynamic 216   X X X X X
Tire Tread 93 X X X X X X


Table 12. Detection distances by types of object and potential detection inadequacy when compared to stopping distance at various speeds: HLB.
Type of Object Detection
(ft)
151 ft at 25 mi/h 245 ft at 35 mi/h 358 ft at 45 mi/h 490 ft at 55 mi/h 642 ft at 65 mi/h 724 ft at 70 mi/h
Pedestrian, Left, Denim Clothing 180   X X X X X
Pedestrian, Right, Denim Clothing 181   X X X X X
Pedestrian, Left Turn, Left 140 X X X X X X
Pedestrian, Left Turn, Right 198   X X X X X
Pedestrian, Right Turn, Left 217   X X X X X
Pedestrian, Right Turn, Right 187   X X X X X
Pedestrian, Dynamic 192   X X X X X
Tire Tread 92 X X X X X X

To provide an overview of each system’s performance, graphics are provided for each VES. Figure 23 depicts some of the general graphics used in the representations of specific VESs. The VES-specific representations, figure 24 through figure 27, depict the detection performance for each of the pedestrian scenarios and the obstacle scenario (tire tread). Pedestrian icons facing straight down on the diagram (e.g., Static Pedestrian, Left) were presented on straight road segments. Pedestrian icons angled with the road (e.g., Pedestrian, Left) were presented on the curved road segment. Each graphic is intended to give an overall impression rather than precise comparisons.



Diagram. Graphics for detection distances. Click here for more detail.

Figure 23. Diagram. Graphics for detection distances.

Where patterns or items of interest are identified for specific VESs in figure 24 through figure 27, the reader is encouraged to refer to table 19 through table 24 (presented subsequently) to investigate the information in more detail. Additionally, while reading the following Discussion sections, the graphics provide a quick comparison of the discussed results. Each graphic includes an icon representing mean detection distance for a given scenario. The mean-detection-distance scale is located on the left side of the diagram. On the right side, the approximate stopping distance required for given speeds is shown. Where an icon is below a given speed, the stopping distance (where required) may be insufficient for the given speed.

Diagram. FIR mean detection distances. Click here for more detail.
Figure 24. Diagram. FIR mean detection distances.

In general, stopping distances are insufficient for the FIR system except at speeds less than 48 km/h (30 mi/h) for all objects. Stopping distances are compromised at speeds of only 40 km/h (25 mi/h) for the pedestrian on the left side of a left curve, the pedestrian on the right side of a right curve, and the tire tread. The FIR system produced detection distances similar to, but always less than, the HLB in all of the tested scenarios, though not significantly so. It follows that the FIR system also underperformed the NIR systems in all scenarios. The FIR detection distances were, in fact, significantly lower than those of the NIR 1 system for the detection of all pedestrian scenarios except the pedestrian on the right side of both the left and right curves (LFtrnRT and RTtrnRT). Note that detection distances were not significantly different for these two scenarios between any of the VESs. Table 13 and table 14 illustrate some of the differences between the FIR system results and those of the other VESs. These tables include information similar to that provided for the HLB baseline, but the percentage differences comparisons are made to the FIR system. (An asterisk indicates a significant difference.)

Table 13. Percentage differences from FIR: detection distances by VES and object.
VES BlueLF BlueRT PedDyno LFtrnLF LFtrnRT RTtrnLF RTtrnRT Tire
NIR 1 * 74 * 46 * 38 * 99 37 * 72 75 −7
NIR 2 * 53 * 41 28 71 17 40 −1 8
HLB 16 5 14 10 10 8 35 7


Table 14. Percentage differences from FIR: recognition distances by VES and object.
VES BlueLF BlueRT PedDyno LFtrnLF LFtrnRT RTtrnLF RTtrnRT Tire
NIR 1 * 91 * 61 * 40 * 138 56 70 58 −16
NIR 2 * 54 * 51 20 78 46 27 −3 5
HLB 30 14 17 32 33 16 29 6


Diagram. NIR 1 mean detection distances. Click here for more detail.
Figure 25. Diagram. NIR 1 mean detection distances.

For the NIR 1, all pedestrian scenarios were associated with detection distances close to or higher than required stopping distances at speeds in the 56 km/h (35 mi/h) range. Detection of one scenario, the pedestrian on the left side of a right curve, could allow sufficient stopping distance at a speed of nearly 72 km/h (45 mi/h). The NIR 1 system provided better overall performance (shorter detection distances) than the HLB and the FIR system in all scenarios except the tire tread scenario, when it performed worse (longer detection distances) than any other system, though not significantly. The NIR 1 also performed better than or similar to the NIR 2 system in several scenarios. The differences between the NIR 1 system and the HLB, as well as the FIR, are generally significant. The exceptions, as mentioned earlier, are the scenarios with pedestrians on the right side of curves, for which detection distances are longer for NIR 1, but not statistically significant. Table 15 and table 16 illustrate some of the differences between the NIR 1 system results and those of the other VESs. These tables include information similar to that provided for the HLB baseline, but the percentage differences comparisons are made to the NIR 1 system. (An asterisk indicates a significant difference.)

Table 15. Percentage differences from NIR 1: detection distances by VES and object.
VES BlueLF BlueRT PedDyno LFtrnLF LFtrnRT RTtrnLF RTtrnRT Tire
FIR * −42 * −31 * −27 * −50 −27 * −42 −43 8
NIR 2 −12 −4 −7 −14 −14 −19 −44 17
HLB * −33 * −28 −17 * −45 −20 * −37 −23 15


Table 16. Percentage differences from NIR 1: recognition distances by VES and object.
VES BlueLF BlueRT PedDyno LFtrnLF LFtrnRT RTtrnLF RTtrnRT Tire
FIR * −48 * −38 * −29 * −58 −36 −41 −37 18
NIR 2 * −20 −6 −15 −25 −7 −25 −39 25
HLB * −32 * −29 −17 * −44 −15 −32 −18 25
Diagram. NIR 2 mean detection distances. Click here for more detail.
Figure 26. Diagram. NIR 2 mean detection distances.

Similar to the NIR 1 system, pedestrian detection distances for NIR 2 were acceptable compared to required stopping distances at speeds in a range around 56 km/h (35 mi/h). The detection of some pedestrian scenarios allows adequate stopping distance up to slightly above 56 km/h (35 mi/h), and others are acceptable only at speeds slightly below 56 km/h (35 mi/h). The exception is the pedestrian on the right side of a right curve, which is not detected at an acceptable distance, even at a speed as low as 40 km/h (25 mi/h). Although similar to NIR 1 values, the NIR 2 system effectively decreases the allowable speed for adequate stopping distance for every pedestrian scenario, compared to the same scenarios using the NIR 1 system. With the exception of the tire tread, the NIR 2 tended to perform close to or below the NIR 1, but better than the other two systems. In ENV Volume XIII, the NIR 2 VES was found to have a generally lower performance than the other VESs in clear weather. Thus, the improvement in relative performance in this study indicates the potential benefits of NIR technology. Table 17 and table 18 illustrate some of the differences between the NIR 2 system results and those of the other VESs. These tables include information similar to that provided for the HLB baseline, but the percentage differences comparisons are made to the NIR 2 system. (An asterisk indicates a significant difference.)

Table 17. Percentage differences from NIR 2: detection distances by VES and object.
VES BlueLF BlueRT PedDyno LFtrnLF LFtrnRT RTtrnLF RTtrnRT Tire
FIR * −35 * −29 −22 −41 −15 −28 1 −8
NIR 1 14 4 8 17 17 23 77 −14
HLB * −24 * −25 −11 −36 −6 −23 37 −1


Table 18. Percentage differences from NIR 2: recognition distances by VES and object.
VES BlueLF BlueRT PedDyno LFtrnLF LFtrnRT RTtrnLF RTtrnRT Tire
FIR * −35 * −34 −16 −44 −31 −21 3 −5
NIR 1 * 24 7 17 34 7 34 63 −20
HLB −15 * −24 −2 −26 −9 −8 33 1

Diagram. HLB mean detection distances. Click here for more detail.
Figure 27. Diagram. HLB mean detection distances.

The HLB provided sufficient detection distances in relation to stopping distances only at low speeds (below 56 km/h (35 mi/h)) for all objects. For the pedestrian on the left side of a left curve and the tire tread, stopping distance could be compromised even at a speed as low as 40 km/h (25 mi/h). Unlike the clear weather performance found in the previous testing, the HLB was surpassed in most of the scenarios by the NIR systems tested. HLB, which was the baseline for the technologies tested, produced significantly lower detection distances than both NIR systems for the static pedestrians on the left and right sides of the straight road. The HLB also had significantly lower detection distances than the NIR 1 system for the pedestrian standing on the left side of both a left curve and a right curve (i.e., LFtrnLF and RTtrnLF). These findings are listed in table 19, which includes percentage differences from HLB detection distances for each of the other three VESs (an asterisk indicates a significant difference). Table 20 lists similar findings for the recognition distances.

Table 19. Percentage differences from HLB: detection distances by VES and object.
VES BlueLF BlueRT PedDyno LFtrnLF LFtrnRT RTtrnLF RTtrnRT Tire
FIR −14 −5 −12 −9 −9 −7 −26 −7
NIR 1 * 50 * 39 21 * 82 25 * 59 30 −13
NIR 2 * 32 * 34 13 55 7 30 −27 1


Table 20. Percentage differences from HLB: recognition distances by VES and object.
VES BlueLF BlueRT PedDyno LFtrnLF LFtrnRT RTtrnLF RTtrnRT Tire
FIR −23 −12 −14 −24 −25 −14 −23 −5
NIR 1 * 47 * 42 20 * 80 18 46 23 −20
NIR 2 18 * 33 2 34 10 9 −25 −1

COMPARISON OF RAIN AND CLEAR CONDITION DETECTION DISTANCES

The participants in this study also took part in the previous IR study in clear weather (ENV Volume XIII). Table 21 to table 27 provide the mean detection distance and standard error for each object in the rain and clear studies. The tables also provide the detection distances in the rain condition as a percentage of the detection distances in the clear condition. The only scenario in which a VES had a higher detection distance in the rain condition than in the clear condition was the FIR with the pedestrian on the left during a left turn (table 21). As discussed in the clear study, this pedestrian likely was not visible with the FIR because of the system’s field of view; therefore, this pedestrian likely was detected with headlamps alone in the clear study as well as in the rain study. The FIR system could not distinguish pedestrians in the rain; therefore, participants may have glanced at the FIR system less and the road more, resulting in slightly longer detection distances with this VES in the rain condition than in the clear condition, although these detection distances are not statistically different. In fact, after considering the relative detection distances of other objects in rain, it appears that detection with the FIR configuration actually was performed with the headlamps alone in the rain condition.

The potential merit of NIR in rain can be seen by further examination of the NIR 2 results in the clear and rain studies. The detection distances with the FIR in rain conditions ranged from 18 percent to 32 percent of the detection distances in the clear condition; however, for the pedestrian on the left in a left turn scenario, the NIR 2 system showed no detection decrement in rain and a 41 percent longer detection than the FIR. (Both the FIR and the NIR 2 were on the same SUV model and year with the same type of headlamps.) Assuming that the participants driving with the FIR system were using headlamps alone, this result could indicate a potential benefit of NIR. In the clear study, the NIR 2 system had the shortest detection distance for all the objects and VESs that were also included in the rain study (tables 25 to 31). In the rain study, the NIR 2 had the second greatest detection distance in six out of the seven pedestrian scenarios. Assuming that the FIR system provided only headlamps to detect pedestrians, the NIR 2 system indicated an average 30 percent benefit over headlamps alone for these six scenarios. In the remaining scenario, a pedestrian on the right in a right turn, it appears that drivers with the NIR 2 used headlamps only because the short detection distance of the VES was similar to the detection distance with the FIR (table 24). The pedestrian is this scenario may be outside the FOV of the NIR 2 system. (See ENV Volume XIII for further discussion.) The other NIR system, NIR 1, had the longest detection distance in all the pedestrian scenarios in rain conditions, further highlighting the potential benefit of near IR in rain conditions. The NIR 1 system had either the longest or the second longest detection distance for pedestrians in clear condition scenarios that were also included in the rain study.

The halogen lights showed a 50 to 70 percent decrement in detection distance in the rain condition when compared to the clear condition. This decrement, combined with the potential added detection benefit of the NIR system in rain conditions, indicates a possible added safety benefit from including a supplemental NIR system on a vehicle.

Table 21. Detection in rain compared to clear for a pedestrian on the left in a left turn.
VES Clear Mean
(ft)
Rain Mean
(ft)
Clear SE
(ft)
Rain SE
(ft)
Percentage
FIR 98 127 13 27 130
NIR 1 500 254 32 33 51
NIR 2 217 217 29 18 100
HLB 346 140 15 23 40

Table 22. Detection in rain compared to clear for a pedestrian on the left in a right turn.
VES Clear Mean
(ft)
Rain Mean
(ft)
Clear SE
(ft)
Rain SE
(ft)
Percentage
FIR 768 180 101 19 23
NIR 1 682 247 32 32 36
NIR 2 386 211 35 29 55
HLB 412 198 17 15 48

Table 23. Detection in rain compared to clear for a pedestrian on the right in a left turn.
VES Clear Mean
(ft)
Rain Mean
(ft)
Clear SE
(ft)
Rain SE
(ft)
Percentage
FIR 698 201 92 36 29
NIR 1 536 346 50 23 65
NIR 2 372 281 37 44 76
HLB 500 217 23 10 43

Table 24. Detection in rain compared to clear for a pedestrian on the right in a right turn.
VES Clear Mean
(ft)
Rain Mean
(ft)
Clear SE
(ft)
Rain SE
(ft)
Percentage
FIR 434 139 46 45 32
NIR 1 440 243 33 38 55
NIR 2 294 137 21 22 47
HLB 416 187 28 12 45

Table 25. Detection in rain compared to clear for a pedestrian on the left.
VES Clear Mean
(ft)
Rain Mean
(ft)
Clear SE
(ft)
Rain SE
(ft)
Percentage
FIR 851 155 101 14 18
NIR 1 707 269 59 20 38
NIR 2 409 237 64 25 58
HLB 452 180 35 14 40

Table 26. Detection in rain compared to clear for a pedestrian on the right.
VES Clear Mean
(ft)
Rain Mean
(ft)
Clear SE
(ft)
Rain SE
(ft)
Percentage
FIR 894 172 106 19 19
NIR 1 788 251 54 21 32
NIR 2 455 242 51 20 53
HLB 599 181 42 17 30

Table 27. Detection in rain compared to clear for a tire tread.
VES Clear Mean
(ft)
Rain Mean
(ft)
Clear SE
(ft)
Rain SE
(ft)
Percentage
FIR 166 86 18 9 52
NIR 1 152 80 19 12 52
NIR 2 141 93 11 11 66
HLB 186 92 27 13 49

SUBJECTIVE RATINGS

The NIR 1 system received the most favorable ratings overall in the subjective scaled responses. With the exception of the statement "This VES did not cause me any more visual discomfort than my regular headlights," the mean subjective statement responses for NIR 1 were the most favorable among the four VES configurations. In particular, the responses for statements 1 and 2, allowing detection and allowing recognition of objects compared to regular headlamps, were statistically greater for the NIR 1 system compared to the other three systems. These subjective results correspond to the objective results discussed previously, and they demonstrate that drivers subjectively felt the advantage of NIR in the detection and recognition of objects as well.

For the cases in which drivers felt better able to detect and recognize objects while using the NIR 1 system, there were no significant differences in ratings related to the age of the drivers. There was some evidence of more favorable ratings for older drivers when they asked whether systems aided in the determination of road direction (statement 4), but there was no significant difference in these ratings related to VES and no interaction of VES and age for this category.

 

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