Researchers found that the full moon is associated with an increased risk of fatal motorcycle crashes in Britain as well as Australia, the United States and Canada.
They say their findings, published in The BMJ, should encourage motorcyclists to ride with extra care during a full moon and, more generally, to appreciate the power of seemingly minor distractions at all times.
Scientists say momentary distraction is a common contributor to road traffic deaths.
A full moon occurs about 12 times a year, appearing big and bright in the night sky – and is therefore a potential distraction for roads users. The next is on January 2nd.
Professor Donald Redelmeier, of the University of Toronto in Canada, and Professor Eldar Shafir, of Princeton University in the United States, set out to test whether a full moon might contribute to motorbike-related deaths.
They analysed figures from the official United States registry of motor vehicle crashes from 1975 to 2014, and calculated the number of fatal crashes on full moon nights compared with control nights – exactly one week before and one week after the full moon.
A total of 13,029 people were in a fatal motorcycle crash during the 1,482 separate nights – 494 full moon nights, 988 control nights.
The typical motorcyclist was a middle aged man, average age 32, riding a street bike in a rural location who experienced a head-on frontal impact and was not wearing a helmet.
Overall, 4,494 fatal crashes occurred on the 494 nights with a full moon, equal to 9.10 per night, and 8,535 on the 988 control nights without a full moon, equal to 8.64 per night.
That gave an absolute total increase of 226 additional fatal crashes over the study period, meaning for every two full moon nights, there was one additional fatal crash.
Similar results were found after analyses of data from Britain, Australia and Canada.
The increased risk was accentuated under a supermoon.
Of the 494 full moon nights, 65 were a supermoon night – where the moon appears larger and brighter than a regular full moon.
A total of 703 fatal crashes occurred on a supermoon night, equal to 10.82 per night – about two additional deaths on a night with a supermoon.
The researchers said it was an observational study, so no firm conclusions can be drawn about cause and effect.
For example, other distractions and traffic hazards were not taken into account, neither were factors including the prevailing weather.
But they said their findings highlight the importance of constant attention when riding, and that extra care is needed when riding during a full moon.
Professor Redelmeier said: “Additional strategies while riding might include wearing a helmet, activating headlights, scanning the road surface for defects, respecting the weather, being wary of left turning vehicles, obeying traffic laws and forgoing stunts.”