It is most dangerous to back up:

Backing up to the right is considered the most dangerous primarily because of visibility limitations. When a driver turns their head to look out the rear window while backing up to the right, their field of vision is obstructed by the vehicle's structure and the angle at which they are turning. This can result in blind spots where pedestrians, cyclists, or other vehicles may not be seen until it is too late.

In many vehicles, the right side has a smaller mirror or may be less visible compared to the left, leading to an increased risk of collision. Additionally, vehicles on the right side may be closer than expected, complicating the driver's ability to gauge distance. Therefore, backing up to the right presents heightened safety risks, making it important for drivers to exercise extreme caution, check mirrors, and possibly even use a spotter when reversing in that direction.

In contrast, backing up straight or to the left generally offers better visibility and control, as drivers can see more clearly through their windows and mirrors without significant obstruction. This is why maneuvering to the right is viewed as a greater hazard in various driving situations.