Flickr / woodleywonderworks Twenty three million kids nationwide ride the school bus every day, and the bulk of them are making the trip without a seat belt.

So why do cars come with seat belts and most school buses don't?

It comes down to cost and design.

Large school buses don't need seat belts because the seats are specifically designed to absorb the impact if a child was thrust forward in a crash: They are close together, high and padded, explains CBS affiliate in Minneapolis. This safety system is called compartmentalization.

Moreover, the cost to install seat belts outweighs the safety benefits.

A study done by the Alabama State Department of Education found that it would cost between $32 and $38 million to install seat belts on all the state's buses, while only saving one life, the station reports.

The federal government only requires seat belts in small buses that weigh less than 10,000 pounds where having a seat belt would affect safety.

The greatest risk isn't riding the bus to school, it's approaching and leaving the bus, according to the National Highway Traffic Safety Administration.