The scheme was supposed to end at the New Year, but has been so successful it's been extended to March, by which time Bangkok will be home to a further 500,000 cars.
"The congestion is very bad and next year it's only going to get worse," says Dr Thongchai Panswad, a sustainable transport activist who founded the Thailand Cycling Club to promote travel on two wheels. We need to do a lot of things, but most of all we need to reduce the number of cars."
That is a tough ask in a country where car ownership has exploded alongside the emergence of an affluent middle class. For many, nothing cements their new-found status like a shiny set of wheels.
It's arguably worse than the early-1990s, when Bangkok first became the world's undisputed champion in vehicle congestion, and drivers bought portable urinals called "Ezee-Pee" to avoid getting caught short.
Since then public transport has slowly improved but the number of vehicles has more than doubled. Beleaguered government officials who process registrations are reportedly so inundated with applications they often run out of number plates.
The applications come from people who have pushed the region's population towards 15 million, and its boundaries further afield.
Bangkok's vast urban sprawl now stretches over 2300sq km, with newly built suburbs on the outskirts dominated by another symbol of middle-class affluence - the detached family home.
Most families own at least one car, but usually have no access to public transport.
"We have all these housing estates but no mass transit system to service them," says Panswad. "People have no choice but to get into their car and drive."
Panswad, an emeritus professor in environmental engineering at Bangkok's Chulalongkorn University, is well-versed in the drawbacks to Thailand's love affair with the car.
More than 12,000 lives are lost on the roads annually, including a record 365 over the New Year holiday.
In Bangkok he worries about air quality and the dangers of walking or cycling on the crowded and chaotic streets.
Officially, traffic moves along the city's roads at an average of 18km/h during the morning rush hour, which lasts from 7am until 11am. But most drivers in downtown Bangkok dream of speeds that reach double figures. Their experience involves streets packed with cars, motorcycles, trucks, taxis, tuk-tuks, buses - and even the odd elephant - inching their way around a crippled network. Jams are the norm during the day, and not unusual in the middle of the night either. A typical Bangkok driver is said to spend almost two months of the year in traffic.
Some of the city's 4000 traffic cops have taken part in "laughter therapy" to deal with the pressures of the job. One potential stress is assisting pregnant women in labour stuck in traffic and unable to get to hospital.
To ease peak-hour jams, authorities are reportedly considering proposals to stagger work and school hours.
Against this clogged backdrop, the Government introduced the scheme that gives first-time car buyers a tax rebate of up to 100,000 baht ($3900).
It was particularly popular with young city voters, and the resulting record car sales provided a huge shot in the arm to the local vehicle industry, which suffered losses in the city's devastating floods in 2011.
Ironically, cars contributed to the severity of the floods, as many of the city's once ubiquitous canals - which once helped drain torrential wet season downpours - have been paved over to create roads.
It's a symptom of haphazard infrastructure development that has struggled to keep pace with Bangkok's boom.
Roads still account for just 8 per cent of the city's surface area; compared with up to 30 per cent in Western counterparts.
Despite extending the first-car rebate, the Government insists it wants to reduce the number of private vehicles in Bangkok.
Last week Transport Minister Chadchat Sittipunt gave the green light for construction to start on four extensions to the existing rail network. By 2017, he says, 10 new rail lines will bring mass transit to large areas of the city.
However, the building work would create five years of "extraordinary" traffic snarls and, he said, the Government was also considering the previously unthinkable - charging motorists for using roads in inner Bangkok.
Population overload
Bangkok
• Capital: of Thailand
• Population: 8.2m people in the city area. 14.5m in the wider metropolitan region.
• History: Began as a village on the Chao Phraya River in the 15th century. The present city was founded in 1782.
Thailand
• Population: 67m
• People: Thai 75 per cent, Chinese 14 per cent
• Religion: Buddhist 94.6 per cent, Muslim 4.6 per cent
• History: Known as Siam until 1939
