In the weeks leading up to the national election last year, the normally lax enforcement of traffic laws in Phnom Penh practically halted altogether.

The pattern continued for months, when the city turned into a staging ground for opposition members protesting the poll. The government was blamed for using the traffic police, or lack thereof, to please a discontented population.

But lost in the political argument was the main side effect of the free-for-all on city roads: Helmet use among motorbike drivers plummeted about 20 per cent from July through September of last year, according to a new study, during a year in which serious injuries from accidents were on the rise.

The findings were part of a larger effort over the past four years by the US-based Johns Hopkins International Injury Research Unit, which focused on researching helmet use and drunken driving. With the help of Handicap International, research teams observed motorists at various locations in Phnom Penh and four provinces, doing fresh monitoring every two months.

“Where enforcement has been reduced, the number of people wearing helmets” has also gone down, said Joe Weber of UK-based Consumers International.

Weber, speaking yesterday at the Cambodiana Hotel during a media briefing on the amended Traffic Law, said the figures “indicate the importance of the enforcement work that has been undertaken”.

The study also shows that many drunken drivers received a pass as enforcement was “strongly impacted due to elections”, and it recorded abysmally low rates of helmet use among passengers.

Ear Chariya, an independent road safety expert who worked at Handicap International when the study was being conducted, said there is probably a “link” between the invisible enforcement and an increase – from 5,349 in 2012 to 5,671 in 2013 – of serious injuries, defined as ones that require a hospital visit that lasts at least six days.

Phnom Penh Municipal Traffic Police chief Chev Hak denied that his officers had backed off from the streets.

“We do it both daytime and nighttime to enforce the Traffic Law,” he said.