Nine people were killed in car accidents on Friday as people headed out of cities to enjoy their four-day holiday weekend which began on Saturday, according to the Ministry of Transport.
Forty-eight road accidents, 12 of them involving motorcycles, occurred during the exodus of holidaymakers.
Most accidents happened on roads that run in a straight line.
The ministry said about 80% of accidents were caused by speeding.
The ministry was working closely with traffic agencies to ease congestion on main roads leading out of Bangkok to different regions.
Strict public health regulations were enforced at rest stops.
Chaiwat Tongkamkoon, permanent secretary for transport, said about 2.9 million vehicles travelled into and out of Bangkok on Friday.
Meanwhile, in Nakhon Ratchasima, a British man was killed and a monk seriously injured after a motorcycle with a sidecar they were travelling on was hit by a pickup in Phimai district of this northeastern province yesterday.
The accident occurred about 8.30am at kilometre 48 on the Phimai-Chum Phuang road in tambon Rangka Yai, Phimai district.
Sukanya Kong-on told police she was on the way from Tha Tum district, Surin province, to visit her younger sister in Non Sung district in Nakhon Ratchasima when the pickup she was driving hit the motorcycle with a sidecar from the back.
A British man, 70, identified only as David, who was driving the motorcycle, was killed. Phra Wasant Boonkham, 59, a monk who was riding on the sidecar, sustained broken legs.
Kanchana Boonkham, 41, David's wife, said her husband was taking the monk, her elder brother, from their house at Ban Tabong village in tambon Boat back to Wat Suntarikaram temple at Ban Taban in tambon Rangka Yai when the accident occurred.
The temple is 1km from the spot.
Police were investigating the cause of accident.
In the same province, an elderly man died after being hit by a motorcycle while crossing a road in Phimai district yesterday.
The motorbike's driver and pillion rider were injured.
The accident occurred about 1pm on Phimai-Chum Phuang road at Ban Nong Chik in tambon Boat in Phimai district.
Witnesses told police that a man was walking across the road when he was hit by a motorcycle which was being ridden at high speed.
The man, identified later as Sompong Khomeeklang, 72, from Chum Phuang district, suffered a broken neck and died shortly after.
The man driving the motorcycle and a woman pillion rider were injured.
The two, who were yet to be identified, were admitted to Phimai Hospital for treatment.