It's part of a strict new drink-driving crackdown in Nepal in which the human nose takes the place of a breathalyser, a blood test and even "walking the line" or touching your nose with your forefinger. Breathalysers that were issued didn't work.
'I'd have left my bike'
Thapa, 27, has become one of thousands of road users to fall foul of a "zero-tolerance" policy introduced in the capital, Kathmandu, with police relying on their olfactory nerves to decide guilt. Despite being declared too drunk to drive, he was allowed to continue his journey.
"I would have left my bike and taken a taxi if I was too drunk," he said, "but they caught me just blocks from home." He said he'd had three small glasses of rice wine. "I think a limited amount of alcohol, as in other countries, must be permitted for drivers."
Kathmandu is one of about 20 capital cities worldwide where alcohol is legally available yet drivers must be absolutely sober but breathalysers are scarce and blood tests unavailable so, if a cop thinks the driver has been drinking, he does the "seize the licence, fine the driver" dance.
The crackdown has raised hackles among the city's indigenous Newar community, which says it's an attack on a culture which for hundreds of years has placed alcohol at the centre of its religious and social life.
"Zero-tolerance could hamper our social intricacies," said Arjun Bhandari, who happens to be a leading wine importer, in a recent commentary in the Republica newspaper. "For Newars, every festival is celebrated by offering some alcohol to family and friends. Century-old traditions can't be wiped out overnight without any education or alternative solutions."
Safest roads in world?
Amrit Kansakar, 46, one of 30 000 people forced to attend drink-driving lectures, is a Newar and angry about the law. "Drink-driving alone cannot be blamed for accidents," he said, "Most accidents occur due to a driver's carelessness."
He had been drinking at a relative's funeral.
Bipin Gautam, who runs the talks for boozy drivers, said: "In six months in 2011 there were 253 road accidents but since December there have been only 49."
Which must make Kathmandu one of the world's best for road safety.
Ganesh Rai, deputy inspector general of Kathmandu Traffic Police, said: "The culture factor is just an excuse. We haven't banned drinking. All we have done is ban the drink-driving. Collisions used to occur regularly, especially at night. In most cases the cause was drink driving."
There were fewer than 5000 vehicles in all of Nepal 20 years ago; today there are more than 800 000 in the capital alone.