BOSTON — Bald tires and overloading of passengers were among the violations found as authorities stopped 149 vans and station wagons carrying infants and toddlers to Boston day care centers and preschools during a recent early-morning investigation.

In the crackdown on so-called 7-D pupil transport vehicles, Massachusetts Registry of Motor Vehicles (RMV) inspectors and Boston Police officers set up nine safety checkpoints throughout the city as part of a joint investigation dubbed "Operation Guardian Angel."

Police issued 123 violations with $5,350 in fines that included driving without the required 7-D license, vehicle equipment defects such as bald tires, overloading passengers and not properly securing children in car seats or seat belts.

"The RMV has many responsibilities, but none more important than keeping this most vulnerable population safe,” said Registrar Rachel Kaprielian, who participated in the operation. “We need to assure parents that their children are riding in safe vehicles with properly trained drivers. But parents can help by being vigilant and asking tough questions."

This is the 11th time since 2002 that the RMV and Boston Police have conducted the safety checkpoints. In 2008, teams checked 191 vehicles and wrote 206 violations. Twenty-two operators were cited for driving without the 7-D license.

Drivers of 7-D vehicles must undergo a criminal background check to apply for a special license. 7-D vehicles, which are usually cargo vans and station wagons, must also undergo special inspections twice a year to ensure that safety features are working properly.

Five drivers in the operation last month were cited for not holding a 7-D license, and their companies were charged with allowing improperly licensed operators behind the wheel. Fifty-six violations were issued for equipment defects.