The Federal Motor Carrier Safety Administration (FMCSA) has a Safety Violation Hotline where you can report unsafe driving by trucks and buses. You have two options to report aggressive driving to FMCSA. You can either call the Safety Violation Hotline at 1-888-dot-saft (1-888-368-7238) between 10:00 am and 6:00 pm Monday-Friday Eastern Time or you can fill out the safety violation reporting form found on their website.
IMPORTANT: When encountering an aggressive driver, stay calm, keep your distance from that driver, do not return gestures and remember not to take it personally.
According to the "Smooth Operator" site, each law enforcement agency has specific requirements and procedures for reporting traffic safety violations. It is up to each agency to respond to file reports.