Road safety advocates are calling on authorities to enforce road safety regulations more strictly after 13 people were killed when the pickup truck they were riding on overturned Sunday morning.

Nikorn Chamnong, a member of the House of Representatives' transport committee and a road safety advocate, said officials should enforce road rules when they spot risky road user behaviour. "Officers must pay extra attention," he said.

"If they see more than six people on the tray of a pickup truck that is exceeding the speed limit, they should stop the vehicle right away."

Early Sunday morning, a pickup truck carrying 18 people overturned in Samut Prakan's Bang Phli district -- killing 13 students from a technical college in Si Sa Ket and seriously wounding the driver, police said.

Mr Nikorn said existing laws prohibit passengers from riding at the back of pickup trucks. "However, due to the lack of efficient public transportation networks, many officers often hesitate to enforce the law," he said.

As such, Mr Nikorn urged officers to follow the "middle path" when enforcing the law. For example, Mr Nikorn said that a maximum of six people should be allowed to sit on the tray, as having too many passengers at the rear will affect the truck's centre of gravity and handling.

"They must not sit on the edge and the driver must limit the truck's speed to 80 kilometres per hour on highways and lower in residential areas," he said.

The National Council for Peace and Order had tried in 2016 to ban people from riding on the back of pickup trucks, especially during long holidays such as New Years' Day and Songkran, but the ban was then revised to allow a maximum of six passengers to ride on the tray of the truck, following a widespread backlash.

Deputy police spokesman Kissana Phathanacha­roen told the Bangkok Post  police officers were often accused of preying on travellers whenever they tried to enforce the law.