The government is planning to spread the so-called idling-stop campaign from buses, taxis and other commercial vehicles to the general car-owning population to save fuel and reduce greenhouse gas emissions.
Studies have found that switching off the engine when a vehicle is stopped for more than a very short time results in a considerable reduction in fuel use and emissions.
Vehicles equipped with so-called automatic idling-stop devices are on the increase, and the government extends subsidies to firms that use such vehicles.
But there are no such measures for privately owned cars, which greatly outnumber vehicles for business use.
The Energy Conservation Center of Japan, affiliated with the Ministry of Economy, Trade and Industry, carried out a three-week test in summer 2002 using two cars of the same model. One had an automatic idling cutoff and the other did not.
The test run covered about 3,700 km from Cape Soya in Hokkaido to Cape Sata in Kagoshima Prefecture.
The results of the test showed large reductions in the consumption of fuel and emissions of carbon dioxide, center officials said.
On open roads between cities there was not much difference between the two vehicles in fuel-saving because stops were few. But the vehicle with the idling-prevention device still used 3.4 percent less gasoline than the other car.
In cities, the saving was 13.4 percent.
Between cities, the time vehicles spent stopped was 7.9 percent of the running hours, compared with 25.9 percent in cities, the test found.
A center official said that as the saving on fuel was large, "Operators of buses running on regular routes in cities as well as those of home-delivery trucks and taxi companies have begun to actively introduce the practice."