Motorcyclists venturing forth under a full moon, beware – the risk of having a fatal crash is higher, researchers have found.

While the reason for the link is not clear the researchers say there could be several possibilities, including that the full moon could distract riders, or make it harder to gauge speed.

“Our study suggests that extra care is needed when riding a motorcycle under a full moon,” said Dr Donald Redelmeier, co-author of the research from the University of Toronto.

The average ride for a motorcyclist “is more dangerous than [for] a drunk driver with no seatbelt who travels the same distance,” he added. “Risk is one of those things that people just can’t sense until it is too late – that’s why we need science about this.”

While motorcyclists face many distractions on the roads, from wildlife to beautiful sunsets, the team say they chose to focus on the full moon because it is large, bright and can suddenly pop into the field of view. Moreover, lunar events can be easily tracked.

Writing in the British Medical Journal, Redelmeier – together with Eldar Shafir from Princeton University – describe how they analysed 40 years of data on motorcycle deaths across the US to explore whether there was a link with full moons.

In total, there were 494 full moons between the start of 1975 and the end of 2014, with 4,494 fatal crashes between the late afternoon and the next morning on those days.

To analyse whether crashes were more common on such nights, the team looked at the number of nighttime crashes exactly one week before and one week after each full moon, when a half moon would be present. That, they note, made sure that the comparisons took into account a host of factors, including the weather, year, day of the week and even changes to road layouts and traffic levels.

Taken together, the results reveal that on average 9.1 crashes took place on nights with a full moon, compared to just over 8.6 when the moon was not full, meaning that there was a 5% greater risk of crashing. The result held regardless of factors including the time of year and decade as well as the motorcyclist’s age, engine size, and whether or not they used a helmet. “5% is a great big deal when it comes to road safety,” said Redelmeier.



When the duo looked at the 65 nights when the moon was in fact a supermoon – a phenomenon where it appears bigger and brighter than a regular full moon – the impact appeared even greater, with a 32% higher risk of a fatal crash compared with similar nights with a half moon and 22% higher risk compared with a regular full moon.

Redelmeier believes are there at least three potential explanations for the link between motorcycle deaths and full moons, including lighting effects produced by the moon that may cause riders to misjudge their speed, or that a full moon means more riders – or other traffic – might take to the roads. “A different possibility is this idea of distraction – that glancing up at a full moon takes the rider’s gaze off the road and creates a moment of inattention that can lead to a loss of control,” he said.

But, the authors note, the study had limitations, not least that it only looked at motorcyclists and did not examine deaths on new moon nights.

David Spiegelhalter, Winton professor for the public understanding of risk at the University of Cambridge, said the study was interesting.

“The typical fatality is a middle-aged man having a head-on collision on a big bike in the country while not wearing a helmet,” he said. “5% more bikers get killed on full-moon nights, and the authors suggest this may be because of distraction. But maybe it’s simply because 5% more of these idiotic ageing juveniles go for a spin on a nice bright night?”