NASHVILLE, Tenn.  — The fatal crash in Chattanooga Monday was the third serious school bus crash in Tennessee in just four days.

The other two happened Friday in Middle Tennessee when one person was killed after crashing into a bus in Rutherford County.

Hours later, Chester County students were hurt when their bus rolled on Interstate 65 near Briley Parkway in Madison.

Now parents are asking questions about school bus safety.

According to the most recent data from the National Highway Traffic Safety Administration, approximately 450,000 public school buses transport 23.5 million children to and from school and school-related activities every year, and on average, six children on those buses are killed in crashes each year.

One question many people are asking is, “Why aren’t there seat belts on school buses?” Six states, California, Florida, Louisiana, New Jersey, New York, and Texas, do require seat belts on school buses.

We’ve all seen tragedies when people don’t wear seat belts in cars, trucks and SUV’s, but representatives from the National Highway Traffic Safety Administration say large school buses are heavier and distribute crash forces differently than passenger cars and trucks.

They say when your child is in a bus, they’re protected by the “closely-spaced seats that have energy-absorbing seat backs.”

The National Transportation Safety Board studied ways to improve school bus safety and found, “funds used to purchase and maintain seat belts might be better spent on other school bus safety programs.”

In 2015, the Tennessee General Assembly considered a bill that would require seat belts on school buses. It didn’t pass because advocates couldn’t prove seat belts would actually save lives, and because of cost.

Adding seat belts to existing school buses is estimated to cost around $12-$15,000 per vehicle.