An official in Shijiazhuang said it is still uncertain whether the city, one of the most polluted in China, will introduce traffic restrictions based on cars' license plates.

There are plans for the capital of Hebei province to introduce an "odd-even" policy that restricts car use in the city on alternate days, depending on whether their license plates end in odd or even numbers.

For a two-month period around the 2008 Olympic Games, Beijing reduced emissions by using the system.