Most complaints focus on drivers who point-blank refuse to take passengers to their destination.
The Department of Land Transport (DLT) reported 23,092 calls to the taxi complaint hotline this year. They include complaints about drivers who turn down passengers (11,216), followed by refusing to turn on meters (3,802), impolite service (3,002), going to the wrong destination (2,361) and careless driving (1,436).
The complaints were made against 19,819 drivers, and the department has cleared 15,992 of the cases.
Drivers can receive fines of 1,000 baht if a first offence is proven, the same fine and a seven-day licence suspension for a second offence, and a fine of 1,000 baht and permanent licence cancellation for a third offence.
A DLT spokesman said the number of complaints has increased each year since the hotline was introduced in 2011, and the rise was due to increased public awareness.
Despite a perception that taxi driver behaviour is as bad as ever, the DLT's Land Transport Supervision Division director Sugree Jarupoom said strict enforcement since March had been effective.
"It can be seen that the numbers [of complaints] have dropped continuously since we strictly enforced the law in March," Mr Sugree said.
"We are surveying many areas and are conducting many campaigns warning drivers about punishments stemming from customer complaints.
"Survey teams who are authorised to enforce the law on taxi drivers operate a daily check during rush hours in main areas such as CentralWorld, MBK, Platinum Mall or Pratunam shopping centres, from where we get most complaints from customers."
Somsak Sakulyuenyong, 50, owns 200 taxis at Hia Dang Garage and said bad drivers were also a concern for his company.
"I have to pay fines of 1,000-1,500 baht on average per taxi should we receive complaints or should they break the traffic rules," Mr Somsak said.
"The driver is actually the one who has to pay these, but it's not his car. If I do not pay, my cars cannot get their licence extended and the business cannot run."
He said it was common for drivers who were found guilty or owe the company money to flee to another company.
"I know only their name, address and driver's licence number but I will never know if the driver applying to my company has received any complaints before," Mr Somsak said.
"I have asked for this kind of information from an official department but I was refused."
Transport Minister Chadchart Sittipunt said a database of taxi drivers in Bangkok is being compiled and will be accessible to government departments and taxi companies and cooperatives.
"Once the system is finished, all blacklisted taxi drivers will be announced to all taxi companies."
Mr Somsak, himself a former taxi driver, understood that passengers' main complaint was being denied service, but asked for understanding.
"Taxi drivers all listen to Jor Sor 100 [a radio station reporting traffic in Bangkok]," he said. "When they realise where there is too much traffic, they would probably refuse to go.
"Sometimes when they are stuck in a traffic jam, their customer leaves them and goes with a motorbike taxi. Sometimes taxis have to drive back from faraway destinations without a customer and it is bad for business."
Taxi driver Somjing Khonman, 53, said it was becoming more difficult to make money, which meant they had to be choosy about which passengers to accept.
"I own my taxi that I have bought from a taxi cooperative, so I have to pay for the LPG myself which currently is 13 baht/kg," he said.
"It is even worse for those renting a taxi at 620 baht per day, as they have to pay about 300 baht for gas only for the 12-hour usage plus the car wash price on a daily basis, so they need to make a minimum of 100 baht per hour."