Motorists are required by law to send their petrol-driven cars for vehicle inspection biennially if the car is between three and 10 years old, and annually if it is more than 10 years old.

For taxis that run on diesel, the requirement is every six months, regardless of the age of the taxi.

One of the tests checks exhaust emission levels of the vehicle using a probe inserted into the exhaust pipe to collect a gas sample for measurement.

That raises few questions on its accuracy.

First, what methodology is used to test cars of different ages? A three-year-old petrol car would not have much emission compared with that of a car over five years old. Are the pass criteria the same for cars of all ages? What about diesel-driven taxis?

Second, the test cycle does not reflect true performance on the road. So there may be a big gap between the test result and real emission on the road.

According to the International Council on Clean Transportation, which researches such matters, fuel consumption is on average 40 per cent higher on the road, and nitrogen oxide emissions seven times higher than the gentle test cycle in inspection centres.

Third, are private-hire cars subject to the same stringent test schedules as taxis?

Francis Cheng