NEW incentives are being considered to persuade minibus and van drivers to switch from diesel to petrol. Government officials yesterday released figures intended to fend off criticism of the proposed five-year changeover. An extension of the conversion period for small school buses and private vehicle fleets was being considered, said Planning, Environment and Lands Branch deputy secretary, Tony Cooper. Operators say conversion within five years will be costly, because their 'well-maintained vehicles' can be used for up to 10 years. Minibus and taxi operators have also said operating costs will increase significantly with the change. They have been calling for a rethink of the plan and better monitoring of low quality diesel vehicles. Responding to the demand for the first time, Mr Cooper said more stringent inspections would be costly and disruptive to the public. 'To be effective in minimising [particles] emissions, a diesel vehicle would have to be inspected for every 10,000 kilometres it travels, which practically means monthly maintenance for high mileage vehicles.' However, a pressure group formed by the operators said the fuel switch was not the only option. Spokesman Leung Charn-keung, who believed the Government was trying to 'patronise' some of the operators by extending their conversion period, said all affected vehicles should benefit from the extension. According to the Government's figures, diesel is used by about one-third of vehicles in Hong Kong and constitutes two-thirds of the total mileage. Diesel vehicles account for about 98 per cent of the suspended particles emissions from road traffic. The polluting particles, which can cause respiratory illnesses and cancer, have consistently exceeded the air quality objective by up to 25 per cent. The situation would not improve to acceptable levels until 2002 even with the switch plan, the Government said. Concerning the claim that petrol was related to a high concentration of benzene, the Government said roadside benzene level was only about 45 per cent of the limit set by UK experts. And the level is expected to drop to about 20 per cent of the limit when vehicles without catalytic converters are phased out. Mr Cooper said operating cost estimates for the proposed scheme would be closely monitored and adjusted.