Wimol Kaewchoo's eyes, shaded behind a dark pair of sunglasses, do not only watch for traffic mischief on the highway. They are also an intelligent scanner for signs of criminals behind the wheel.

The detective instinct is in Pol Sen Sgt Maj Wimol's veins. Sometimes, he wears a drug suppression detective officer's hat. At other times, he takes on the role of an agent investigating human trafficking.

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His common sense, street smarts and hard work have paid off on several occasions.

Over the past couple of years, Pol Sen Sgt Maj Wimol - who has been trained as a highway police officer, not as crime suppression officer - has intercepted marijuana traffickers four times by using just his eyes and his wits.

The total weight of the drugs he has helped to seize in roadside arrests on Phetkasem Highway in Chumphon amounts to more than a tonne.

''I've not been trained [in crime suppression], nor I have been tipped off to catch those criminals,'' the 43-year-old highway police officer said.

''My key technique is observation and I don't think I will just do my traffic control duties. Highway policemen can suppress crime, too.''

Pol Sen Sgt Maj Wimol's good work has been recognised by his superiors, including acting Highway Police chief Somyos Promnim, and earned him the respect of his colleagues.

Good observation allows him to differentiate between a criminal suspect and a normal driver or passenger.

He says he cannot explain how to judge who might be a drug trafficker. His instinct and common sense ''just come into play''.

Usually, a drug trafficking operation is carried out by one or two people. And, normally, when one or two people travel together in a car on a highway, they carry at least one piece of luggage. So, if they don't have any luggage, there is a good reason to suspect them.

He said another hint is drug traffickers almost always plug their mobile phones to a car charger. He said they need to be reached by phone at any time and cannot afford to lose communication with accomplices while they're on the road just because their cell phone batteries run out.

When stopped by police, a drug trafficker's hand might shake harder than an innocent driver who is only slightly nervous when encountering a traffic police officer.

''They seem to tremble from head to toe,'' Pol Sen Sgt Maj Wimol said. ''Even on the fourth or fifth request to check their driving licences, their hands shake hard enough to notice.

''And when I refuse to take his bribe, a criminal suspect usually keeps offering a higher sum of money, much higher than a normal driver would try to bribe a traffic police officer.''

Some drug traffickers, however, are much more sophisticated. They might intentionally drive over the speed limit to get themselves arrested for speeding and then bribe officers to be freed. This tactic is used to divert authorities' attention from illegal goods that might be in the vehicle.

Pol Col Somyos, who supervises the Highway Police unit, supports Pol Sen Sgt Maj Wimol's point of view that highway police should also take a role of crime suppression in addition to their main duty to ensure road safety for motorists.

''Highway police have to do everything, including pulling a dead dog hit by a car off the road,'' Pol Col Somyos said.

All criminals _ from human traffickers to drug runners _ need to use the roads and it is the duty of the Highway Police forces stationed at 207 checkpoints across the country to track them down. (see details in graphic)

Pol Sen Sgt Maj Wimol summarised guidelines for other highway police officers to follow.

The first sign of an irregularity might be the driver apparently travelling in the wrong direction. If a truck driver who has come from the Thai-Lao border area says he transports dried chillies from Laos to provinces in Thailand, he could be a drug trafficker, Pol Col Somyos said. He said dried chillies usually are transported in the reverse direction from northeastern provinces to Laos.

The second hint is an unusual weight of the vehicles. If a truck is heavier than it should be _ given the type of products the driver claims to be stored in the truck _ the vehicle might carry war weapons.

The third warning sign is the licence plate. Cars from provinces in the upper Northeast or the far North that are heading to the South should be checked thoroughly for drugs, especially marijuana which is in high demand in southern provinces.

The fourth tip is to keep asking the driver about his travel plans and if he gives a vague answer, he deserves to be suspected of deceiving police about his real route. For example, drivers who say they are travelling from the North to the South yet embark on an unusual route to the Northeast before entering the Central region, would come under suspicion.