Improvement of Visibility for Night Driving, With
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IMPROVEMENT OF VISIBILITY FOR NIGHT DRIVING Richard N. Schwab, Federal Highway Administration; and Roger H. Hemion, Southwest Research Institute The research documented in this report confirms that at night on rural highways most motorists overdrive their headlights. Studies conducted to determine visibility of various highway vision targets and the effect of various lighting configurations on driver performance are summarized. Means by which night visibility may be improved are discussed. It is concluded that (a) a polarized headlighting system is the most promising system likely to solve the night visibility problem; (b) the system is technically and economically feasible in regard to today's vehicle popula tion; (c) the system would be advantageous in terms of improved visibility with less glare for motorists; and (d) the results of the use of such a system would be increased vehicular control, safety, and comfort and probably improved traffic flow and utilization of highways at night. A public test and evaluation of polarization in an isolated location is rec ommended prior to nationwide implementation to determine more precisely the benefits, side effects, and operational problems. •FOR many years the Federal Highway Administration, through in-house and contract research, has been studying methods of improving the visual environment for night driving to provide more effective and safer use of our roadway network (1, 2, 3, 4, 5). Two methods are currently employed for illuminating the roadway at nighl-=ti.xed--=source "public" or "street" lighting and vehicular-mounted headlights. The fixed-source illu mination systems are comparatively expensive and therefore are usually considered applicable to the higher volume urban areas. At the present time only a small fraction of the total surfaced road and street mileage has some form of fixed illumination. Although urban driving now accoW1ts for approximately half of all travel and is in creasing its relative share, the absolute magnitude of travel on the other parts of the highway network is also increasing. One estimate suggests that the magnitude of travel on the two-lane rural highway system in the United states will increase about 50 percent within the next 20 years (6). Obviously, then, vehicular-moW1ted illumina tion must still be relied on for a large portion of night driving. The purpose of this paper is to summarize some 15 recent research and develop ment studies having two broad objectives: (a) to develop an understanding of the effects on traffic dynamics of glare and restriction in visibility during darkness, especially on two-lane rural highways and (b) to recommend for public trial a system of vehicular illumination that would be expected to provide an improved visual environment for night driving. The remainder of this paper will discuss the nature of the night visibility problem and then describe a series of studies designed to meet the two broad objectives stated. As a result of these studies, it would appear that a polarized headlighting system might be a promising solution. Therefore, past objections to the polarized system are out lined and means for overcoming them are suggested. NATURE OF THE PROBLEM Headlighting design is currently based on a compromise between the need for adequate illumination of the road ahead and the need to avoid "dazzling" the eyes of the oncoming Sponsored by Committee on Visibility and presented at the 50th Annual Meeting. 1 2 drivers with "glare" light. During the past half century a number of modifications have been made in the control of headlight beam configuration. These changes in in tensity, beam pattern, and aiming have improved the visual environment for night driv ing. Changes in beam configuration alone, however, cannot possibly provide adequate lighting to enable the driver to operate his vehicle safely during many common night driving situations because of the nature of the necessary design compromises. This is especially true on two-lane highways when meeting another vehicle. Because of the extremely small angular separation between the approaching vehicles and the variation in possible roadway geometry, it is necessary to limit radically the luminous output in the upper left quadrant of the headlight beam to keep glare out of the eyes of oncom ing motorists. Some luminance must be aimed in this direction, however, so that the driver can see things approaching from the left and can negotiate left-hand curves. Also, the designer is limited by physical considerations, such as filament size, in de termining how sharp this cutoff should be. Therefore, the possibilities of developing improved visual environments by beam configuration changes are extremely limited. The improved capability of seeing at night that comes with reduced glare can be expected to reduce the frequency of accidents. This is perhaps the major benefit. There are, however, many other aspects of nighttime driving that can be expected to benefit from better visibility of the road scene. Increased time will be available for driver decision-making because of the increased distances at which road obstacles can be detected. It should be possible to maintain safely somewhat higher driving speeds, with consequent increased traffic flow and greater utilization of highways dur ing off-peak nighttime hours. Although an improved headlighting system might produce an increase in night speed, this appears unlikely considering the small and directionally variable shifts in speeds encountered after installation of fixed illumination systems. As a result of elimination of glare, there would be a reduction in stress, a reduced sensation of "tunnel" driving with its pressures for more exact control of lateral posi tion, and an extension of the night driving capability of older drivers whose glare ad aptation responses have deteriorated. BEAM USE STUDY To provirlP h::irkernnnrl nn thP PrlPnt nf thP. prnhlPm and data for benefit-cost analyses, a nationwide survey of headlight beam use was conducted (7) at 17 test sites in 15 states. Each site was 1,200 ft long and located, in most instances,-on rural two-lane unlighted highways of moderate traffic volumes. Three sites differed from these: Site 15 was a four-lane freeway with a median 50 ft wide; site 16 was a two-lane, high-volume rural road carrying a large proportion of recreational traffic; and site 17 was a suburban two-lane road with overhead lighting. One of the rural two-lane sites (site 7) was ob served under two conditions-snowy winter and clear spring weather. Observers at each end of the test site recorded the type of vehicle and headlight configuration on an event recorder chart with the vehicle position information. The instrumentation package automatically recorded time and position of the passage of individual vehicles in normal traffic on the same recorder chart. The observations showed that during normal nighttime, clear weather conditions over 75 percent of drivers were using their headlights improperly, i.e., using low beams when high beams should have been used because they were neither meeting another vehicle nor following closely behind one. To obtain sufficient data within a reasonable study time, the criterion employed in selecting sites was that they have an average daily traffic volume of 3,000 to 5,000 ve hicles. This results in minimum volumes at night of approximately 20 vehicles per hour. Therefore, the observed beam use patterns must be extrapolated to apply to lower volume roads not studied and prorated among all roads in order to estimate the beam use pattern on all rural highways. It was theorized that the average driver is not conscious of traffic volume as such but is aware of the time intervals between meetings with opposing vehicles. When this time interval is consistently "too short," the driver will no longer bother to switch back to high beam between meetings and will drive continuously on low beam. To 3 evaluate this theory and determine the effect of traffic volume on beam use when there was no other vehicle in sight, traffic volumes were recorded for each 15-min interval of the study. The intervals were classified by volume in increments of 10 vehicles per hour, and the percentage of high-beam use was plotted as shown in Figure 1. As can be seen, 50 percent high-beam use did not develop until the traffic volume had dropped to less than 30 vehicles per hour, i.e., when the driver sees an opposing ve hicle at intervals greater than 2 min. It should be noted that if a driver in moving traffic meets 30 vehicles per hour, the traffic volume counted at a fixed position along the roadway will be 15 vehicles per hour in one direction, provided that all traffic is uniformly distributed and moving at the same average speed. If traffic volumes in each direction are equal, the two-way traffic volume will be 30 vehicles per hour or the same volume as met by the moving observer. Thus, if these assumptions are met, the traffic volume in Figure 1 may be interpreted in two ways: (a) the number of opposing vehicles that a moving driver meets or (b} the two-way traffic volume counted at a fixed station. The confidence bands bounding the regression line as shown in Figure 1 indicate a wide variation in beam use from one observation to another at any specific traffic vol ume. However, Figure 1 does provide a reasonable method for making a projection because the mean values for all observations at any given volume grouping fall closely about the regression line. For benefit-cost and similar purposes, the mean value is of principal interest rather than beam use for any individual observation. The fraction of high-beam use for each volume grouping was multiplied by the fraction of nighttime travel occurring in that volume category.