More fatal motorbike crashes take place on nights where there is a full moon, research has found.
A study analysed fatal collisions which took place over a 30-year period in the UK, United States, Canada and Australia, comparing the number that took place on the night of a full moon compared to control nights exactly one week before and one week after.
The increased risk was even more accentuated under a supermoon, when a full moon looks bigger than normal because it is at its closest point to earth.
Researchers suggested that momentary distraction is a common contributor to road traffic deaths, with full moons, which occur about 12 times a year, a potential distraction for roads users.
Data from the official US registry of motor vehicle crashes from 1975 to 2014 showed 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.
This 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 when data from the UK, Canada and Australia was analysed.
The study, carried out by the University of Toronto and Princeton University and published in the BMJ, was observational, so no firm conclusions can be drawn about cause and effect, researchers said.
But they added that the findings highlight the importance of constant attention when riding, and that extra care is needed when riding during a full moon.
"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", they added.