Three of the most important times to check traffic behind you are before:

Three of the most important times to check traffic behind you are before:Backing, changing lanes, or slowing down quickly
It is very important to have this in mind, because errors made when changing lanes are some of the most common causes of automobile accidents. Every driver must use proper lane changing techniques every time you switch from one lane to another.

Joke: i thought it said drake and josh at first answer: probably unsure about this law. depending on how long the time is.