Sag Vertical Curve Design Criteria for Headlight Sight Distance (2013) / Chapter Skim
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56 CHAPTER 6 VISIBILITY EXPERIMENTS Two human-subjects experiments were conducted to determine the effects of headlamps and sag vertical curves on visibility. The first experiment, called the Smart Road study, examined the effects of varying types of headlamps.
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57 Independent Variables Several independent variables were manipulated or controlled for this experiment. They are listed below.
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58 Figure 21. Determining detection distance.
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59 minimum of 20/40 was required for participation. Participants were allowed to wear corrective lenses if they indicated that they normally wear them while driving.
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60 Figure 22. Primary sag vertical curve on the Smart Road.
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61 of the tangent lines were calculated to determine the algebraic change in grade while the horizontal distance between the two tangent points was used to determine the length of the curves. Table 25 summarizes this information.
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62 Figure 25. Example of Targets used in the Visibility Experiment.
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63 Table 26. Headlamps Headlamp Abbreviation Description Original Vehicle Photo HHB Standard Halogen High Beam 2000 Ford Explorer HLB Standard Halogen Low Beam 2000 Ford Explorer VOLHID1 Visually Optically Aligned Left HID 2001 Mercedes S500 VOLHID2 Visually Optically Aligned Left HID 2003 Mercedes E320 VORHID Visually Optically Aligned Right HID 2003 Lincoln Navigator VORHAL Visually Optically Aligned Right Halogen 2003 Lincoln Navigator The test vehicles were also equipped with data acquisition systems (DAS)
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64 experimenter, as well as network data such as speed and distance. Detection distance was determined by analysis of this data.
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65 Participants drove 2 laps with each headlamp for a total of 12 laps. Starting from the headlamp changing area, participants drove through the flat area and across the large curve.
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66 Table 27. Headlamp Presentation Orders Order 1 Order 2 Order 3 Order 4 Halogen Low Beam VOR Halogen VOR HID VOL HID 1 Halogen High Beam VOL HID 2 Halogen High Beam VOR Halogen VOL HID 1 Halogen Low Beam Halogen Low Beam VOR HID VOL HID 2 Halogen High Beam VOR Halogen Halogen High Beam VOR HID VOL HID 1 VOL HID 2 Halogen Low Beam VOR Halogen VOR HID VOL HID 1 VOL HID 2 Not all objects were presented in every curve.
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67 curve was left to each participant's discretion, and was generally between 25 and 35 mph. Because speed may have an impact on detection distance, vehicle speed was used as the covariate in each of the three analyses.
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68 Figure 27. Mean detection distance by headlamp.
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69 A significant interaction was found between curve and age. Younger participants had significantly longer detection distances than did older participants on the flat roadway, and in every curve.
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70 Figure 31 shows the significant interaction of headlamp and curve. The mean detection distance for the flat roadway was significantly longer than for any of the sag curves for every headlamp except for the HHB.
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71 viewed with a high contrast background (the opposite side of the curved road) , whereas the pedestrian had a low contrast background when viewed on the flat road (sky)
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72 significant interaction found here for light source was most likely due to a confounding effect of beam pattern. Figure 33.
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73 Figure 34. Mean detection distance by curve and pattern.
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74 A separate set of analyses was performed for the detection of speed limit signs, which only appeared on the flat roadway. The only significant factor for all three analyses was age.
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75 Public Road study. Specifically the VOLHID2 and VORHID headlamps were selected so that a direct comparison of beam pattern could be made without the potential confound of light source.
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76 Figure 37. Determining detection distance.
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77 Table 32. Visual Acuity Scores Visual Acuity (Snellen)
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78 Figure 38. Test route (Source: Google)
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79 Table 34. Sag Vertical Curve Measurements Road Name Road Type Design Speed (mph)
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80 Table 35. Headlamps Headlamp Abbreviation Description Original Vehicle Photo VOL Visually Optically Aligned Left HID Mercedes E320 VOR Visually Optically Aligned Right HID Lincoln Navigator The test vehicles were also equipped with DASs.
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81 participants read aloud the number on any speed limit sign they encountered as soon as they could read it. This task was used only to keep participants active and alert during long stretches between sag vertical curves.
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82 Table 37. ANCOVA Results for the Headlamp Analysis Source DF Type III SS Mean Square F Value Pr > F Sig Age 1 2940.6471 2940.6471 0.27 0.6108 Headlamp 1 11246.658 11246.658 3.51 0.0766 Age*
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83 Figure 39. Mean detection distance by curve.
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84 Figure 40. Mean detection distance by curve length.
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85 correlation was found where a larger K value tends to result in longer detection distances. The two residential curves which had mean detection distances which were significantly shorter than flat roadway had the lowest KR values.
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