Aichi Prefecture tops the list of prefectures in total carbon dioxide emissions from cars, but Tokyo is even worse when it comes to the amount released from a car per kilometer, the Land, Infrastructure and Transportation Ministry said.

In its first estimates on carbon dioxide emissions from passenger vehicles by prefecture, the ministry said cars release 3.12 million tons a year in Aichi, followed by 2.93 million tons in Tokyo and 2.81 million tons in Hokkaido.

Prefectures with more vehicles and longer travel distances led the list, while prefectures in western Japan with smaller populations were often at the bottom, including Tottori Prefecture at 370,000 tons and Kochi Prefecture at 420,000 tons.

Reflecting congested traffic in and near big cities, Tokyo topped the list in the amount of carbon dioxide released from a vehicle per kilometer at 0.193 kg, followed by Okinawa Prefecture at 0.178 kg and Osaka Prefecture at 0.177 kg.

The ministry calculated the estimates based on the 1999 traffic census that recorded the number of passenger vehicles and their speeds on highways and national and prefectural roads.

"Vehicles release more carbon dioxide when traffic congestion causes poor fuel efficiency," the transport ministry said. "The government must draft a policy to ease traffic jams to counter global warming."

The ministry said that upgrading three circular expressways around Tokyo alone is likely to cut carbon dioxide emissions by 2 million to 3 million tons, which is equivalent to absorbing carbon dioxide by planting trees in an area about three to four times the size of Tokyo's 23 wards.