IMPACT OF MODERN HEADLAMPS ON THE DESIGN OF SAG VERTICAL CURVES A Thesis Proposal by Madhuri Gogula Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE December 2005 Major Subject: Civil Engineering
INTRODUCTION When designing a road for night driving, it is important to consider the visibility of the road and other objects on it. A driver must be able to see the path (s)he is traveling on to maintain control over the vehicle and stop in time to avoid hitting any object on the road. Crash statistics show that 42 percent of all crashes and 52 percent of fatal crashes occur at night and during other degraded visibility conditions (1). Studies have also shown that the nighttime crash rate is about three to four times the crash rate during the day (2). This is relatively high considering the number of vehicle miles traveled is less during night. Intoxication and fatigue are some of the factors that account for this high nighttime crash rate (3). Even when considering only non-alcohol related crashes, the nighttime fatal crash rate is twice the daytime crash rate (2). Some studies have also identified that crashes during night are higher on unlit roads compared to roads with street lighting (3). Keeping this in view, it is reasonable to assume that poor visibility contributes to nighttime crashes (2). A driver requires sufficient visible length of roadway, at least equal to the safe stopping distance, to allow him to stop safely and avoid collisions with other vehicles or obstructions. One of the design criteria for sag vertical curves is based on the sight distance provided by vehicle headlamps. This design criterion for sag curves was developed based on the requirements and standards of sealed beam headlamps, which are rare in modern vehicles. Modern headlamps have different performance characteristics than sealed beam headlamps as evidenced by recent research on sign retroreflectivity that shows there has been considerable reduction in the amount of light reaching roadway signs. This has been attributed to the change in the amount of light produced above the horizontal axis of headlamps (4, 5). This change in headlamps might also impact the amount of light reaching the roadway on a sag curve. Therefore it is appropriate to examine the criteria used for sag vertical curve design. PROBLEM STATEMENT Drivers should have a clear view of the road they are driving on. When this condition is not fulfilled it results in increased workload. This makes the task of driving more complex, potentially reducing safety. Amongst the different factors that contribute to increase in number of crashes occurring at night, roadway visibility is a key factor. The formula used to determine the length of a sag vertical curve is based on the length of roadway that is visible due to the upward divergence of the headlight beam. However, considering the changes occurring in the headlamp beam pattern, modern headlamps have less light directed upward, reducing roadway visibility on sag curves. A study examining the change in the amount of light reaching the road due to changed headlamp design would make it possible to determine the adequacy of the design formula currently being used. Degraded headlamp lenses also have an adverse affect on the amount of light emitted from headlamps. Modern headlamps are made of hard plastic and are more susceptible to degradation compared to sealed beam headlamps that have lenses made of glass. This is because hard plastic is prone to yellowing, fogging, etc. caused by different factors like acid rain, condensation, and high heat. Degraded headlamp lenses might have a significant impact on the amount of light emitted. If this is true, it would be necessary for the design criteria to account for headlamp degradation. 1
OBJECTIVES The objectives of this research are listed below: Compare the illuminance produced by sealed beam headlamps on sag curves to that produced by modern headlamps. Perform this task for both theoretically calculated values and values measured in the field. If appropriate, recommend changes to the design criteria to accommodate modern headlamps. Compare the revised design formula with the other formulas used for determining the minimum length of sag curves, like the comfort criteria and drainage criteria, to see if they become the governing formulas. Evaluate the impact of headlamp lens degradation on the illuminance produced by a headlamp. Determine whether this change is significant enough to be considered in the design criteria. BACKGROUND Knowledge of lighting terminology is useful when studying nighttime roadway design conditions. The following lighting terms are commonly used when designing highways for nighttime driving. Luminous intensity (I): The amount of light produced by headlamps in a particular direction. S.I. unit: candelas, (cd). English unit: lumens, (lm). Illuminance (E): The amount of light falling on a unit area of the roadway. S.I. unit: lux, (lx). English unit: foot-candles, (ft-c). Luminance (L): The amount of light reflected from the roadway. S.I. unit: candelas/m 2, (cd/m 2 ). English unit: foot-lamberts, (ft-l). The amount of light needed on the road for the safe operation of a vehicle depends on a number of factors like target reflectivity, contrast, etc. (6). Also, the illuminance on an unlit road depends on the geometric design of the road (i.e. presence of sag curves, etc.), luminous intensity, and position of the headlamps. Thus, it is difficult to set standards dictating the minimum amount of light needed on the road at night. There are no specific standards set for the amount of illuminance required on the road. However, the Illuminating Engineering Society of North America (IESNA) has recommended average illuminance values for road lighting based on the type of pavement. In addition to the headlight illumination from vehicles, they recommend fixed lighting to be provided for more distinct visibility of the roadway and traffic (7). Table 1 shows the average illuminance values for different types of roadways. 2
Table 1. Average Illuminance Values (lux) Pavement Classification Illuminance Road and Area Classification R2 and Uniformity Ratio R1 R4 R3 Eavg to Emin Freeway Class A 6 9 8 3 to 1 Freeway Class B 4 6 5 Commercial 10 14 13 Expressway Intermediate 8 12 10 3 to 1 Residential 6 9 8 Commercial 12 17 15 Major Intermediate 9 13 11 3 to 1 Residential 6 9 8 Commercial 8 12 10 Collector Intermediate 6 9 8 4 to 1 Residential 4 6 5 Commercial 6 9 8 Local Intermediate 5 7 6 6 to 1 Residential 3 4 4 Where, R1: Portland cement, concrete road surface R2: Asphalt road surface (60% gravel) R3: Regular asphalt road surface R4: Asphalt road surface with smooth texture. Design Criteria for Sag Vertical Curves Geometric design standards provide guidance to engineers designing highways. They also aid in developing safe and economic solutions while meeting the requirements of the highway users. These design guides state the different criteria for determining the sag vertical curve length. Amongst these the headlight sight distance is an important factor to be considered (8). Figure 1 shows the headlight sight distance available on a sag curve. The following equations are used to determine the length of sag vertical curves based on the available headlight sight distance. When S is less than L, 2 AS L = 200[2.0 + S(tan1 )] When S is greater than L, 200[2.0 + S(tan1 )] L= 2S A Where, L: length of sag vertical curve, ft 3
S: light beam distance, ft. This is taken equal to the Stopping Sight Distance for a given speed. A: algebraic difference in grades, percent Figure 1. Headlight Sight Distance on a Sag Vertical Curve (9) Tan 1º in the formula represents the upward divergence of the light beam from the longitudinal axis of the vehicle and accounts for the additional visible length provided on the roadway (8). The sag curve formulas were developed based on the beam pattern of sealed beam headlamps that were widely used through the 1980s. The 1954 AASHO geometric design policy featured these formulas (10) and they have remained same since then. Recent studies show that there has been considerable change in the beam pattern of headlamps (4). A review of the literature shows that the formulas used to design sag curves have not been revised over time, neither have any studies been taken up to examine their adequacy in relation to the changing headlamp patterns. Figure 2 shows the difference in the beam patterns of sealed beam and modern headlamps, it can be seen that the beam pattern varies significantly above the horizontal cutoff and the amount of light appears to be reduced in modern headlamps. Figure 2. Sealed Beam Headlamp Beam Pattern and Modern Headlamp Beam Pattern (11) 4
Several studies have observed the change in the amount of illuminance from headlamps and have expressed the need to improve driver visibility in order to address the related safety issues (12). From a study conducted in 2001, the researches at the University of Michigan s Transportation Research Institute have determined that the pedestrian fatality rate at night is four times higher than the rate during the day. They partly attributed this high fatality rate to the insufficient amount of light on the roadway produced by modern High Intensity Discharge (HID) and halogen lights (12). It was also observed that HID lamps have a sharper horizontal cut off beam pattern. This could further reduce the portion of lighted highway on sag curves. Degradation of Headlamp Lenses Casual observance of headlamp lenses indicates that they degrade rather quickly. This causes light scatter in headlamps, resulting in lesser illuminance on the road. Sealed beam headlamps have comparatively lesser degradation because of their glass lenses, while modern headlamps are made of plastic. Modern plastic headlamp lenses suffer from yellowing, fogging, etc. These are caused by various factors like acid rain, condensation, and high heat. The reduction in the illuminance due to light scatter has not been considered in the research pertaining to the design of sag vertical curves. Considering the rate at which modern headlamp lenses degrade, it might be necessary to incorporate a factor accounting for the degradation of lenses in the design formula. WORK PLAN The following tasks describe the work plan to be followed for the research. Task 1: Perform Literature Review A preliminary literature review looked into various aspects involving the crash statistics during nighttime, the changes in the headlamp beam patterns, and the design criteria of sag vertical curves. Further review of the literature will look into the following: The relationship between nighttime visibility and illuminance, The basis for using the 1 degree upward angle in the formula of sag curves, Standards which reflect the minimum amount of illuminance needed by nighttime drivers, All other factors which determine the minimum length of a sag curve, like comfort criteria, drainage conditions, etc., Different headlamps and headlight beam patterns (i.e. vehicle lighting trends) to understand the changes occurring in the modern headlamps, The impact of different factors like, headlamp height, aim, degradation of lenses, etc. on the amount of illuminance produced. Task 2: Obtain Photometric Data for Different Headlamps Photometric data consists of luminous intensity values at regular intervals on the headlamp. These data are used to determine the luminous intensity at the required points on a headlamp. Table 2 represents a sample array of photometric data for a CARTS (sealed beam) headlamp. The top row consists of horizontal headlight angles (Hh) at 0.5-degree increments and the first column consists of vertical headlight angles (Hv) at 0.5-degree intervals. Photometric data for different headlamps is usually available from engineering firms involved in headlamp testing. The researcher will approach these firms to obtain photometric data required 5
for the study. The researcher will use the photometric data representative of the sealed beam lamps and modern headlamps for the analysis. This photometric data will be used in the determination of illuminance at different points on the roadway. Table 2. Luminous Intensity (cd) Values for a Sealed Beam Headlamp Horizontal Headlight Angles, Hh (degrees) -2-1.5-1 -0.5 0 0.5 1 1.5 2 3 262 294 311 325 336 360 369 375 379 2.5 325 346 351 374 405 422 453 436 441 2 387 408 419 443 507 540 544 574 608 1.5 464 501 558 594 651 742 797 878 821 1 533 613 665 752 813 924 1032 1090 1091 0.5 644 761 876 1060 1099 1297 1682 1771 1688 0 961 1114 1286 1793 2310 3010 3640 4400 4440 Vertical Headlight Angles, Hv (degrees) -0.5 1679 2050 2500 3470 5050 7350 9370 10200 11920-1 2900 3320 4190 6180 8960 12850 16370 17490 19130-1.5 4560 4810 6740 9060 12640 15300 18320 20300 21500-2 5040 6140 7480 9410 11850 13850 16590 19000 19780-2.5 5780 6600 7030 8390 9790 11070 11960 13140 13790-3 5200 5690 6440 6950 7310 7970 8080 8820 9650 Task 3: Calculate and Compare Theoretical Illuminance Values for Different Headlamps The illuminance at a point where the 1-degree upward light beam meets the road (headlight sight distance) will be determined using a spreadsheet developed for this purpose. The headlight sight distance is taken equal to the stopping sight distance at a particular speed on a highway. At this distance on the road, the researcher will use the following formula to determine the illuminance at different points across the width of the roadway, at intervals of two feet, as shown in Figure 3. I *10.764 E = 2 S cos( n + 1 ) Where, E : Illuminance (lux) I : Luminous Intensity (candela) S : horizontal distance (feet) n : Grade of tangent at the required point (-ve for downgrade, +ve for upgrade) The researcher will calculate the illuminance values at the required distance for the left and right headlamps independently and determine the total illuminance at a point by adding these values. This procedure is going to be performed for both sealed beam and modern headlamps. The computed values will be compared to identify any significant difference in illuminance values. Based on this information, the researcher will propose revisions to the formulas being used in the design of sag vertical curves. 6
Figure 3. Profile and Plan View of a Sag Curve 7
Table 3 and Figure 4 show the illuminance values obtained from preliminary calculations performed using the CARTS 50 (sealed beam headlamp) and UMTRI US Low 2001 (modern headlamp) photometric data, for: A: 4 percent and S: 730 ft at a speed of 70 mph. Table 3. Comparison of Illuminance Values for the CARTS (Sealed Beam Headlamp) and UMTRI (Modern Headlamp) Profiles Illuminance (lux) Distance to left of right edge line, d (ft) CARTS 50 (Sealed Beam Headlamp) Left Headlight Right headlight Total UMTRI Low US 2001 (Modern Headlamp) Left Headlight Right headlight Total Change in Illuminance (lux) % Change 0.00 0.02 0.02 0.04 0.01 0.01 0.02-0.02-41.08 2.00 0.02 0.02 0.04 0.01 0.01 0.02-0.01-39.44 4.00 0.02 0.02 0.03 0.01 0.01 0.02-0.01-37.83 6.00 0.02 0.02 0.03 0.01 0.01 0.02-0.01-36.36 8.00 0.02 0.02 0.03 0.01 0.01 0.02-0.01-35.31 10.00 0.02 0.02 0.03 0.01 0.01 0.02-0.01-34.50 12.00 0.02 0.01 0.03 0.01 0.01 0.02-0.01-33.22 Comparison of Illuminance Illuminance (lux) 0.040 0.030 0.020 0.010 0.000 0-2 -4-6 -8-10 -12 Offset (ft) CARTS 50 UMTRI US Low 2001 Figure 4. Graph Showing Comparison of Illuminance Values Task 4: Measure and Compare Illuminance Values Obtained from Field Tests In order to provide a comparison to the theoretical values, the researcher will measure actual illuminance values in the field using 4 cars; 2 sporting the sealed beam headlamps (pickup trucks) and 2 having modern headlamps (Ford Taurus 2003). Locating a sag vertical curve in the field, with all the required characteristics and controlled conditions is difficult. Even if such a curve is located, closing the road for taking observations is not practical. For the field study, the researcher will simulate similar conditions representing sag vertical curves at Riverside campus. The set up consists of a rack on which the illuminance meter heads are fixed at predetermined 8
points and a car located at the calculated distance. The researcher will measure the illuminance at these points located across the width of the road at different heights based on the distance being considered. The researcher will repeat this procedure with different vehicles and will apply correction factors to the obtained illuminance values to account for field variations if determined necessary. Some of the correction factors that might be applicable are correction for voltage, correction for dirt on the lenses, etc. The researcher will compare the illuminance values measured in the field at specific points for sealed beam and modern headlamps to observe any significant difference in these values. The researcher will also compare the measured illuminance values with the theoretical values determined in Task 3. This will show how the theoretical values obtained from measurements taken under ideal conditions might vary from the field measurements. Task 5: Study the Impact of Headlamp Degradation on Luminous Intensity Degraded headlamp lenses scatter light. A study examining the degradation of headlamp lenses would help understand the impact this might have on the illuminance produced. As a part of this thesis, the researcher will attempt to measure this change in illuminance. For this task a test setup consisting of a laser pointer and illuminance meter will be arranged. The laser pointer will be held by a bracket in place of the bulb of a headlamp and the illuminance meter will be mounted in front of the headlamp. The researcher will measure the illuminance at different points for 6 headlamps, 3 with new and 3 with degraded lenses. Based on the results, the researcher will have an indication of the potential decrease in the illuminance. If this reduction is significant, it might be necessary to incorporate a factor accounting for the degradation of lenses in the design formula. Task 6: Summarize Findings Based on the results from the illuminance comparison study (theoretical and field study), the researcher will determine whether the formula used in the design of sag vertical curves provides sufficient length when used with modern headlamps. If the researcher determines that the formulas are inadequate, she will suggest revisions to the formula. These revisions could result in a change of the 1-degree value used in the formula. To address the changed beam pattern in modern headlamps, it is likely that a smaller upward divergence angle of the light beam will be appropriate. The researcher will examine the results from the degradation of headlamp lenses study to understand its impact on the amount of light produced. Based on these results, the researcher will give recommendations regarding the necessary changes to be made to the design formula. This will potentially result in the inclusion of a factor in the design formula, accounting for the degradation of lenses. POTENTIAL BENEFITS OF THE RESEARCH Decreased visibility of the traveled path on sag curves would result in less available stopping sight distance, potentially increasing the number of accidents. A proper understanding of the changes headlamps have been going through and the resulting changes in the light reaching the road is necessary. This research will help understand and identify the necessary changes to the formula used in the sag vertical curve design. 9
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