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Publication Number: FHWA-HRT-04-139
Date: December 2005
Enhanced Night Visibility Series, Volume VIII: Phase II—Study 6: Detection of Pavement Markings During Nighttime Driving in Clear Weather
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CHAPTER 4—DISCUSSION AND CONCLUSIONS
For this research, a series of questions was established to evaluate VES types and pavement markings. First, which VES from the 11 chosen provided the longest pavement marking detection distances? Second, which pavement marking material was the most visible with all the VESs? Third, does the addition of UV–A headlamps to the baseline headlamps affect pavement marking visibility?
As mentioned in chapter 2, Methods, of this study, the aiming protocol used resulted in a deviation in the maximum intensity location from where it typically is for some headlamp types. Details about this deviation are discussed in ENV Volume XVII, Characterization of Experimental Vision Enhancement Systems. As a result of the headlamp aiming, it is likely that the detection distances increased for the HLB configurations and HOH and decreased for the HHB VES. 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.
The first research question in this study evaluated which of the VESs offered the greatest beginning and ending detection distances.
For the beginning detection distance only, the hybrid UV–A + HLB provided the longest detection distance. In addition, HLB, three UV–A + HLB, five UV–A + HLB, and HOH had longer detection distances compared to the HID, HID + UV–A configurations, HHB, and HLB–LP. The shortest beginning detection distance was obtained by the HLB–LP, a common sedan headlamp configuration.
The five UV–A + HLB had the longest ending detection distance overall; however, it was not significantly different than the HLB, hybrid UV–A + HLB, three UV–A + HLB, or HOH VESs, all of which had significantly longer ending detection distances when compared to the other VESs. The poorest performing VES with the shortest ending detection distance was again the HLB–LP, or common sedan headlamp configuration.
For both beginning and ending detection distances, the UV–A + HLB configurations and HLB (baseline) provided consistently longer detection distances, and no significant differences were found among them. The HLB VES appears to provide sufficient visibility without the addition of the UV–A. In contrast to the HID beam pattern, the HLB, HOH, and HHB VESs all had a straight-ahead beam pattern that did not contain a sharp cutoff, resulting in an increase in visibility down the road.(23)
The HID configurations generally had the shortest detection distances and were significantly shorter than the HLB configurations. As with the HLB configurations, the addition of UV–A did not significantly increase detection distance over the baseline HID VES.
When comparing the detection distances of all the configurations to the recommended pavement marking preview time of 3.65 s (i.e., at 40 km/h or 25 mi/h, about 41 meters or 134 ft) as suggested by Schnell and Zwahlen, all the VES designs, even the VES with the lowest detection distances (HLB–LP), provided adequate visibility for that benchmark.(16) A reduction in pavement marking visibility as speed is increased,(7) however, implies that the VES configurations with greater detection distances may be more beneficial at greater speeds.
The HLB–LP provided the lowest detection distances of all the VESs, perhaps resulting in part from the low profile of both the vehicle and its headlamp. When riding higher in a vehicle, a driver is more likely to obtain longer visibility distances because of the perspective rather than the specific headlamp configuration in use.
In summary, the overall best-performing VES is the HLB configuration. The addition of the UV–A headlamps to the HLB and HID headlamps did not significantly increase the detection distances obtained in the data. Within the confines of this research, it is not known how well other HLB headlamps would perform given the same evaluation technique but with a different beam pattern, with a different aiming strategy, or at higher speeds.
In addition to the VES configurations tested, a series of pavement marking materials were also evaluated, which included fluorescent paint, fluorescent thermoplastic, and a two-part liquid system. For the beginning detection distances, the fluorescent thermoplastic had the longest detection distances, followed by the liquid system. The detection distances for both the fluorescent thermoplastic and the liquid system were significantly longer than that of the fluorescent paint, but neither had statistically different detection distances when compared to each other. Similar findings occurred for the ending detection distances, with the liquid system having the farthest but not significantly longer detection distance than the thermoplastic. Again, both the thermoplastic and the liquid system had significantly longer detection distances than the fluorescent paint. The detection distance results are somewhat similar to other research (for example, Nitzburg et al., 1998) that found that thermoplastic outperformed other pavement marking materials.(24) However, the current research found no differences between the fluorescent thermoplastic and the nonfluorescent liquid system.
Both materials, the liquid system and the thermoplastic, outperformed the fluorescent paint and hence they are recommended for increased night visibility. An interesting note is that all three materials exceeded the minimum visibility benchmark suggested by Zwahlen and Schnell.(16) These researchers suggested a minimum 3.65-s preview time to allow drivers to see pavement markings at a speed of 40 km/h (25 mi/h). The detection means for the three pavement marking materials for both the beginning and ending detection distances exceeded the approximately 41-m (134-ft) distance (i.e., 3.65-s visibility distance at 40 km/h or 25 mi/h). It is important to remember that these results are for a specific lower speed only; thus, all pavement marking materials tested in this research may be used effectively on lower-speed roadways. The visibility requirement on higher-speed roadways involves additional research that is beyond the scope of the current project.
The pavement markings used in the research were 2 months old when the data collection took place, and the florescent properties may have degraded in that time frame. This degradation may have affected the results for the UV–A configurations; however, the degradation of the material reflects normal weathering encountered on all roadways over time.
In summary, both the thermoplastic and liquid system provided longer detection distances when directly compared to a fluorescent paint design. However, all three pavement marking materials provided the minimum visibility benchmark requirements for a specific low speed.(16)
Significant interactions were found between VES and the pavement marking materials for both the beginning and ending detection distances. When looking at the beginning mean detection distances for each headlamp and pavement marking, the fluorescent paint did not perform well for the HID configurations, HHB, and the HLB–LP. Because of the poor performance of the UV–A supplemental VES (discussed in more detail later), fluorescent paint is not recommended for implementation.
In fact, based on the results of the current research, recommending a specific pavement marking and VES combination is not advised. All three pavement marking materials provided beginning detection distances longer than 84 m (275 ft) when viewed with HLB configurations. Conversely, all three pavement markings had beginning detection distances close to or less than 76 m (250 ft) when viewed with the HID configurations. These results likely result from the interaction between the VES and pavement marking material used. For example, with the HID configurations, perhaps the physical property of the pavement marking (e.g., bead pattern) is less compatible with the increased blue light emitted by the HID headlamps. Another possibility is that the sharp cutoff of the HID headlamps send less light down the road, or that their higher foreground luminance makes it difficult to detect relatively low-luminance markings at greater distances.
The ending detection distances in combination with the various VESs were generally lower overall in comparison to the beginning distances. When reviewing the means for the HLB configurations, all pavement marking visibility distances were around 76 m (250 ft) or less. Visibility performance dropped to around 61 m (200 ft) or less for all three pavement marking materials for the HID configurations.
The results of the post hoc tests for the VES main effects indicated no statistical difference between the HLB and the HLB with UV–A. There also was no statistical difference between the HID and the HID with
This research study focused on three age groups: 18 to 25 years, 40 to 50 years, and 60 years and older. A marginal age effect was found for the beginning detection distance, and a subsequent SNK revealed that younger drivers had longer detection distances than both the middle-aged and older age groups. Furthermore, a significant main effect was found for age with respect to ending detection distance. Again, younger drivers had longer detection distances than both the middle-aged and older drivers. It is likely that visual degradation influenced the detection distances of the older drivers when compared to the younger ones. Furthermore, researchers have also shown that older drivers have longer reaction times than younger drivers, resulting in shorter detection distances.(25)
One limiting factor that was present in the current research was the pavement material on which the pavement markings were applied. Specifically, the fluorescent paint was applied only on concrete; the fluorescent thermoplastic and the liquid system were applied to asphalt. The different pavement materials provide different contrast levels in association with the pavement marking technology. More specifically, concrete generally provides the least contrast under all conditions. From the current data, it is difficult to determine how much the pavement material reduced the visibility of the fluorescent paint material, but the pavement type confound likely resulted in lower averages. The lack of improvement in detection distances when using supplemental UV–A, for both fluorescent materials, indicates that fluorescent paint likely would not be used as a pavement marking. Adding fluorescent material for no meaningful benefit would be cost prohibitive. Future research should take into consideration the type of pavement material onto which the pavement marking is being applied. The type of pavement material may enhance or detract from the overall effect of the pavement marking visibility.
Another limiting factor that should be taken into consideration is that of participants' eye height between the vehicles used. Table 9 shows the vehicle, average participant eye height for that vehicle (measured from the pavement), and the VES configurations associated with each vehicle.
Even between the taller vehicles there was still almost an 8-cm (3-inch) height difference between the SUV and the pickup truck. When comparing similar VES configurations (e.g., halogen) and detection distances for these vehicle types, the detection distance differences were not significant. It should be noted that the poorer detection distances recorded using the HLB–LP likely resulted at least in part from the lower visual perspective of the roadway from the vehicle.
The age of pavement markings also should be taken into consideration when discussing the research limitations. For example, when testing was conducted, the pavement markings were already 2 months old, which may have negatively affected the potential effect of the UV–A headlamps. If the pavement marking material had degraded substantially in that timeframe, perhaps its real-world use would not be cost effective or beneficial.
Finally, the last limitation in this research study concerns headlamp beam pattern and aiming strategy. The sharp cutoff pattern was represented only in the HID configurations; the straight-ahead beam pattern was found in all other visible light headlamps. As discussed previously, the HLB configurations and HOH VES likely had greater detection distances and the HHB likely had shorter detection distances than they would have had if they had been aligned more typically. The differences found between the VESs regarding the pavement marking material visibility may be attributable to differences in headlamp beam pattern and aiming. Caution should be taken when interpreting these results across headlamps with different beam patterns and aiming strategies.
The analysis of the data from this research study provides six conclusions:
Finally, for this research, data were collected in clear weather conditions only. In an effort to more fully examine the effect of pavement markings on visibility, various weather conditions should be explored. For example, Schnell and Ohme commented on the degradation in pavement marking visibility in rain.(26) Researchers also have discussed how precipitation affects the transmissivity of the atmosphere, which in turn could affect the luminance of the pavement marking material.(1)