A homemade motorcycle stopped on the M25 was described by a traffic officer as the "most unusual vehicle" he had apprehended in 26 years.
It was spotted just before junction 21 for the M1 in Hertfordshire at about 20:30 BST on Sunday.
The officer tweeted that it was "all checked and in order" but he was "not convinced" he knew what it was.
A police spokeswoman said the vehicle was approved and registered by the DVLA.
Another roads policing officer called the vehicle a "homemade electric motorcycle" which was "made from bits of balsa wood and duct tape".
Sgt Stephen Andrews, from the Bedfordshire, Cambridgeshire and Hertfordshire Roads Policing Unit, said: "This is certainly not a vehicle that is seen very often on our roads but after road side inspection we couldn't find anything that would prevent the rider to continue his journey.
"The vehicle was keeping up with other traffic and didn't cause any obstruction to other road users.
"The owner made sure that he fulfilled all the safety regulation as well as keeping the insurance, MOT and tax in date."
A spokeswoman for the force said the vehicle was "registered as a motorbike with a 'shell' around it made mostly out of fibreglass and plastic".