Investigations by NBC-owned stations across the country have found school bus drivers running red lights in four of the nation’s largest school districts — districts that serve more than 2 million kids.

In New York City, WNBC found drivers at several bus companies had accumulated more than 6,000 speeding and red light violations in just two years. In Miami and nearby Broward County, WTVJ found hundreds of school bus drivers racked up traffic tickets while driving school buses but continued to drive — one even on a suspended license.

And just last month in Texas, an investigation by KXAS revealed that school bus drivers in the Dallas metro area had racked up nearly 500 tickets in a two-and-a-half-year span, and found traffic camera videos that showed buses racing through red lights.

Related: Chattanooga Mom Says She Complained About Driver in Deadly Crash

In some cases bus drivers were even caught on camera running past other school buses that had their stop arms extended for loading or unloading students.

An NBC News investigation of traffic violations led Dallas County Schools to suspend more than 200 drivers. KXAS

"We expect our school bus drivers to perform at a higher standard honestly than the rest of drivers driving in public," said Deborah Hersman, head of the National Safety Council, which offers driver training programs. "They are charged with carrying our most precious cargo — our children."

Click Here for the KXAS Investigative Series About Dallas County Schools

Hersman, a former board member and chairman of the National Transportation Safety Board, said school bus company managers need to tackle traffic tickets and unsafe driving immediately — and re-train employees.

Said Hersman, "If there's dangerous driving behavior out there, if there's bad habits that are out there they need to be addressed. And it needs to be done quickly. Sometimes it might mean discipline in very severe situations."

KXAS found that Dallas County Schools, an agency that provides services in 12 area school districts, had never disciplined any of the drivers caught on video running red lights.

Instead, the agency spent $80,000 of taxpayer money to pay the tickets. The drivers walked away with no punishment, until KXAS contacted the school board.

Dallas County Schools told KXAS that 13 school bus drivers have been fired and 229 drivers will be suspended without pay for five to nine days as a result of the investigation — about 10 percent of the agency's drivers.

"It is inexcusable of a bus driver to ever do that. They know better," said Dallas County Schools' Board President Larry Duncan.

He said two senior managers have also been demoted because of failure to follow procedures and lack of oversight.

"You found a problem. We owned it and we fixed it. We are taking the steps necessary — both systemic and personnel — so that this will not and cannot happen again," said Duncan.

On Monday, a single-vehicle school bus crash in Chattanooga, Tennessee, killed five children and injured more than 30 others. Bus driver Johnthony Walker, 24, was arrested on five counts of vehicular homicide, reckless endangerment and reckless driving, Chattanooga police said late Monday. An arrest report said Walker was driving "well above" the 30 mph speed limit.

Experts note, however, that school buses are still the safest way for kids to get to school. Despite Monday’s tragedy, even in crashes kids are rarely hurt badly — often because school buses are so big.