What is the most crucial hand signal to agree on between the driver and helper when backing up?

When backing up a vehicle, the most crucial hand signal to agree upon between the driver and the helper is the signal to stop. This is vital for safety as it ensures that the driver is clearly informed to cease all movement immediately. Given that the driver has limited visibility when reversing, the helper plays a critical role in directing them and preventing potential accidents.

A stop signal allows the driver to halt before any potential collision occurs, whether it's with objects, pedestrians, or other vehicles. Establishing a clear stop signal is fundamental to managing the operation safely, especially in busy or tight environments where the risk of obstacles is higher.

The other options serve specific purposes during reversing, such as indicating when to slow down or pause, but none carry the same level of urgency as the command to stop. Stopping is the most important action that can prevent accidents and ensure safety is maintained throughout the backing-up process.