China's traffic authority said on Thursday more vehicles than usual are expected to be on the highways over the extended break because of the country's toll-free holiday policy.

The authority said road safety remains the top concern during the upcoming annual travel boom.

A total of 660 million passengers will travel by road and water during the longer-than-usual eight-day Golden Week holiday, which combines the National Day holidays and Mid-Autumn Festival and starts on Sept 30.

An average of 80.9 million passengers are estimated to use the roads every day during the period, up 8.9 percent compared with last year, said ministry spokesman He Jianzhong at a press conference in Beijing.

"This year, the holiday traffic will see an increase in private vehicles for self-driving trips, spurred by the lifting of tolls on highways," He said.