Good manners are not meant to be kept for special occasions. Good manners should be a part of every aspect of your life. At home, at work and especially when you are behind the wheel where proper driving etiquette equates to staying safe as well.
Passing into another lane without using the corresponding signal light is not only discourteous to others, it's darn right dangerous!And yet, the act of letting another driver (that driver being someone who could slam into your car) know ahead of time that you will be thrusting yourself into their path seems to have fallen unfashionable as of late. What seems to have taken its place is the middle finger salute so many of us feel necessary to dish out to the poor soul who thought it only fair to honk their horn in outrage, letting you know your carelessness put their lives in danger.
(Let's save that cheeky middle finger for something more useful, shall we?)
Always glance over your shoulder, into the direction of the lane you are intending to pass into, to make sure there is no one in your blind spot and then use the signal (specifically designed for this purpose!) to alert drivers of your intent to change lanes BEFORE you actually veer into their path.