Life-threatening motor vehicle crashes in bright sunlight

Abstract

Bright sunlight may create visual illusions that lead to driver error, including fallible distance judgment from aerial perspective. We tested whether the risk of a life-threatening motor vehicle crash was increased when driving in bright sunlight.This longitudinal, case-only, paired-comparison analysis evaluated patients hospitalized because of a motor vehicle crash between January 1, 1995 and December 31, 2014. The relative risk of a crash associated with bright sunlight was estimated by evaluating the prevailing weather at the time and place of the crash compared with the weather at the same hour and location on control days a week earlier and a week later.The majority of patients (n = 6962) were injured during daylight hours and bright sunlight was the most common weather condition at the time and place of the crash. The risk of a life-threatening crash was 16% higher during bright sunlight than normal weather (95% confidence interval: 9-24, P < 0.001). The increased risk was accentuated in the early afternoon, disappeared at night, extended to patients with different characteristics, involved crashes with diverse features, not apparent with cloudy weather, and contributed to about 5000 additional patient-days in hospital. The increased risk extended to patients with high crash severity as indicated by ambulance involvement, surgical procedures, length of hospital stay, intensive care unit admission, and patient mortality. The increased risk was not easily attributed to differences in alcohol consumption, driving distances, or anomalies of adverse weather.Bright sunlight is associated with an increased risk of a life-threatening motor vehicle crash. An awareness of this risk might inform driver education, trauma staffing, and safety warnings to prevent a life-threatening motor vehicle crash.

Level of evidence: Epidemiologic Study, level III.

Figure 1

Relative risk of a life-threatening motor vehicle crash associated with bright sunlight according to time of day and accompanied by equivalent nighttime hours of clear sky. X-axis shows time grouped in consecutive 3-hour segments that span full 24-hour interval and center on noon as midpoint. Y-axis shows relative risk of a life-threatening motor vehicle crash calculated by comparing crash days to control days. Horizontal line for null effect. Solid circles indicate estimate and vertical bars indicate 95% confidence intervals. Square brackets for count of total crashes during each time segment. The odds ratio is a valid estimate of relative risk because the baseline risk of a crash is low (<1%) for an average day. The boundary zones separating day and night are imprecise and vary by season. Main findings show an increased risk during daylight hours with no consistent patterns during nighttime.

Figure 2

Relative risk of a life-threatening motor vehicle crash associated with bright sunlight and other daytime weather. X-axis shows weather condition starting with bright sunlight and ending with the pair of high barometric pressure (above 100 kPa) and low barometric pressure (below 99 kPa). Y-axis shows relative risk of a life-threatening motor vehicle crash comparing crash days and control days. Horizontal line for null effect. Solid circles indicate estimate and vertical bars indicate 95% confidence intervals. Square brackets for count of total crashes for each condition. Main findings show increased risk during bright sunlight, corresponding decrease during cloudy weather, additional increases during rain and snow, and no significant change with high or low barometric pressure.