In both cases, which were reported late Wednesday night, police suspect the drivers had dozed off at the wheel.
Around 10 people, including the eight workers, were killed after the pick-up truck they were travelling in collided head-on with an omnibus.
The mishap took place in Villupuram district, around 170 km from Chennai, at around 2.45 am, a police officer told PTI.
Drivers of both the vehicles died, while five others in the pick-up truck escaped, according to the police.
In the other accident which took place an hour past midnight in Thoothukudi district, six people were killed after a mini-passenger van fell into a 20-foot deep culvert.
Though both sides of the road had barriers near the culvert, the van crashed into them before falling into the ditch as the driver apparently slept off, local police said.
Nine occupants of the van, including the driver, survived, police said.
In both cases, the bodies have been sent to nearby government hospitals for a postmortem.