SACRAMENTO, Calif. — The California Air Resources Board is delaying its off-road diesel engine emissions rules to give truck operators more time to green their fleets.

CARB, widely considered the toughest environmental regulatory body on the continent, said a 50 percent drop in construction activity since the rules were introduced three years ago has led to significantly lower particulate emissions levels than first thought.

Plus, the battered California economy is making it difficult for off-road equipment operators to comply with the requirements to purchase newer, cleaner trucks or retrofit existing equipment.

In December, the agency said it would consider loosening similar rules for highway trucks and buses for many of the same reasons.

The off-road regs were slated to take effect this year. They required tractors, forklifts, bulldozers and other types of diesel-powered equipment to be replaced or retrofitted over the next 15 years. Large fleets have until 2020, while fleets with fewer than 20 vehicles have until 2025.

According to the Associated Press, the board hasn’t decided on a new implementation date, but staff has been directed to consider how to better meet the state’s emissions targets by 2014, which is also when federal off-road emission rules take affect.

With about 200,000 fewer construction vehicles in California than the board had initially thought, CARB admitted it had overestimated pollution levels by 50 to 80 percent.