The role of ambient light level in fatal crashes: inferences from daylight saving time transitions

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Abstract

The purpose of this study was to estimate the size of the influence of ambient light level on fatal pedestrian and vehicle crashes in three scenarios. The scenarios were: fatal pedestrian crashes at intersections, fatal pedestrian crashes on dark rural roads, and fatal single-vehicle run-off-road crashes on dark, curved roads. Each scenario's sensitivity to light level was evaluated by comparing the number of fatal crashes across changes to and from daylight saving time, within daily time periods in which an abrupt change in light level occurs relative to official clock time. The analyses included 11 years of fatal crashes in the United States, between 1987 and 1997. Scenarios involving pedestrians were most sensitive to light level, in some cases showing up to seven times more risk at night over daytime. In contrast, single-vehicle run-off-road crashes showed little difference between light and dark time periods, suggesting factors other than light level play the dominant role in these crashes. These results are discussed in the context of the possible safety improvements offered by new developments in adaptive vehicle headlighting.

Introduction

Over the last decade, vehicle lighting manufacturers have been working to improve the effectiveness of headlighting systems by developing technologies that will allow illumination to be distributed in flexible and dynamic ways that may offer improvements over conventional fixed headlighting. This work has come to be known variously as adaptive headlighting, intelligent lighting, or active headlighting. These solutions are typically applied to situations in which fixed headlighting is limited by design compromises made in order to produce a single acceptable lighting configuration for a variety of driving conditions. For example, fixed headlights mitigate glare to other drivers using a beam pattern designed to keep light away from oncoming vehicles. The solution is optimal for straight, level, and undivided roads. It is unlikely to be optimal on curved, graded, or divided roads.

A key advantage of adaptive headlighting is that the pattern of light distribution can be dynamically and automatically modified to match the changing lighting requirements in the immediate driving situation. For example, current low beam headlamps project the highest intensity light a fixed distance down the road. Depending on the speed of the vehicle, this distance may be inadequate to avoid obstacles in the road (Olson and Sivak, 1983). Some forms of adaptive headlighting involve attempts to remedy the situation by moving the highest intensity light upwards at higher speeds.

Other adaptive lighting solutions apply to different illumination problems. For example, there are adaptive headlighting systems that: adjust illumination to control the veiling luminance caused by atmospheric conditions (e.g. fog, snow); control variation in foreground luminance (a consequence of variation in road surface reflectivity); relevel beams to counteract effects of load distribution or acceleration forces; or adjust overall lighting characteristics to suit a rural, urban, or motorway driving context (Aoki et al., 1997, Damasky and Huhn, 1997, Groh, 1997, Hogrefe and Neumann, 1997, Kobayashi, 1998, Kobayashi et al., 1997, Kormanyos, 1998, Löwenau et al., 1998, Manassero and Dalmasso, 1998, Rumar, 1997, Sivak et al., 1994). Indeed, a wide variety of potential lighting solutions have materialized largely unsupported by empirical assessment of their likely effects on safety.

The purpose of the present study was to derive information about the effects of light on crashes that would be useful in assessing effects of some of these headlighting innovations. To do this, we adopted the following strategy. First, we associated various lighting countermeasures with specific crash scenarios. For example, speed-controlled forward lighting distance might be expected to reduce crashes with objects (in particular, pedestrians) in a high-speed roadway at night. Then we examined the degree to which this type of crash exhibits a strong light-level effect by comparing crash levels under high and low ambient light conditions with most other crash-relevant factors fixed (e.g. exposure, driver fatigue, alcohol impairment). To meet this latter constraint, we compared crashes over the same times of day under dark and light ambient conditions by exploiting Daylight Saving Time (DST) changeover periods. This kind of crash analysis has been done before (Ferguson et al., 1995, Green, 1980, Tanner and Harris, 1956, Whittaker, 1996) and has typically reported a marked vulnerability of pedestrians in darkness, and a similar but somewhat milder effect on crashes not involving pedestrians. But, unlike those earlier studies, the use of more specific crash scenarios in the present study may isolate subpopulations of crashes that exhibit differing degrees of sensitivity to ambient light level, and thus provide insight about the role of light in such crashes.

Section snippets

Method

We selected three types of adaptive headlighting countermeasures to investigate—cornering lamps, curve illumination, and speed-sensitive forward lighting—and identified types of vehicle crashes that each headlighting countermeasure might address. We then selected a crash scenario that would be a reasonable match to each countermeasure. Because of the complexities of possible crash mechanisms, the countermeasures presumably do not perfectly map to crash scenarios—that is, several kinds of

Spring a.m.

Fig. 5 shows the cumulative fatal pedestrian crashes before and after the spring a.m. change to DST during the TW time window over the 11-year period. Some trends consistent with light level are readily apparent. For example, TW shows a decline in crashes from week−8 (39 crashes) to week−1 (eight crashes); at the changeover, when the period is returned to darkness, the crash level rises again (coincidentally, to 39). This pattern mimics the gradual increase in light level in the weeks leading

Discussion

This research provides evidence for a strong effect of light on fatal pedestrian crashes. Estimates drawn from these analyses and other published reports suggest that pedestrians may be 3–6.75 times more vulnerable in the dark than in daylight, depending on the circumstances. In the two pedestrian scenarios investigated, sensitivity to light level was somewhat greater in crash data associated with straight rural high speed roads, where there is less likely to be supplemental lighting at night.

Acknowledgements

Appreciation is extended to the members of the University of Michigan Industry Affiliation Program for Human Factors in Transportation Safety for support of this research. The Affiliation Program currently includes Adac Plastics, AGC America, Automotive Lighting, BMW, Corning, DaimlerChrysler, Denso, Donnelly, Federal-Mogul Lighting Products, Fiat, Ford, GE, Gentex, GM NAO Safety Center, Guardian Industries, Guide Corporation, Hella, Ichikoh Industries, Koito Manufacturing, Libbey-Owens-Ford,

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