Police are cracking down on truck drivers after an investigation into speed and fatigued drivers in the lower North Island.

They said a recent operation targeting heavy vehicles had uncovered some worrying behaviour.

Nearly 3500 vehicles were stopped at check points in the central and lower North Island. Mostly heavy and commercial vehicles, including long-haul trucks, logging trucks, buses and couriers were pulled over.

Police said data from the survey suggested about 30 per cent of crashes on rural highways were fatigue-related.

About 65 vehicles were caught speeding and police said many commercial drivers were being placed under unreasonable time constraints by employers.