When Your Turn Signal Flashes Quickly, Replace the Bulb
You usually do not see the direction indicators when you are in the car. However, most cars have a subtle way of letting you know when one of your turn signals has burned out: the indicator on your dashboard will flash faster than usual.
As the automotive website 2CarPros explains, the indicator inside your car is designed to operate at a certain electrical resistance. When the bulb burns out, the resistance changes, causing the turn indicator on the dash to flash faster. This is a quick way to find out if you need to change the lights without leaving your car.
Direction indicators are blinking quickly | 2CarPros via Reddit