Using a turn signal to indicate to other motorists that you plan on changing lanes or turning is not option. It is the law to use those signals. However, it turns out that roughly half of all drivers either do not use a turn signal, or do use a signal but then do not turn it off once the maneuver has been made.
The fact that many drivers are not properly using turn signals is a big deal, as according to the Society of Automotive Engineers, turn signal misuse accounts for 2 million motor vehicle accidents a year.
When looking at who is and who is not using turn signals, according to a 2006 study, 57 percent of drivers admitted to not using a turn signal when changing lanes. Among those drivers between the ages of 18 and 24, more than 70 percent said they just don't use signals.
When looking at why people are not using turn signals, one study found that 42 percent of motorists who admitted to not using a turn signal cited not having enough time as the reason. Another 23 percent admitted that time was not an issue rather they were just too lazy to actually use a blinker.
Whatever the reasoning behind not using a turn signal, the issue is that it confuses other motorists and can lead to an accident. For example, without a turn signal, how are other motorists around you supposed to know that you are planning on moving over a lane on the highway? Without this knowledge, this means that others drivers no longer have enough time to respond and adapt to what you plan on doing, which can lead to a car crash.
Source: Fox Business, "Half of drivers don't use turn signals," Mark Vallet, May 4, 2012