The U.S. Department of Transportation announced a requirement for backup cameras in cars in 2014. Kits were sold for cars without the technology. 

Now all cars sold in the U.S. must come with a backup camera. The new standard went into effect on May 1st.

"When a driver shifts a vehicle into reverse, the RVS (rearview video system) shows—either in the dashboard or in a small display in the rearview mirror—an image of the area behind the vehicle. The field of view includes a 10-foot by 20-foot zone directly behind the vehicle. It’s important to remember that rearview video systems are not a replacement for mirrors or turning around to look; rather, they’re an added safety tool for revealing hidden dangers." – National Highway Traffic Safety Administration

According the U.S. DOT, every year on average, more than 200 people are killed and more than 12,000 are injured in back over crashes. Over half of those killed are children 5 and under or adults 70 or over. Rear view cameras can be expected to prevent one in six police-reported backing crashes.