Until you spend time behind the wheel of a car, the concept of "blind spots" can be hard to understand. And even the most experienced driver can forget to perform a shoulder-check when changing lanes from time to time. To understand where your blind spots are, or how big they can be, have a friend help you with the following exercise.

Adjust your mirrors

First, get behind the wheel of a parked car and with the ignition off, adjust the steering wheel, seat, seatback and even the head restraint for yourself as the driver. Next, adjust your driver and passenger side mirrors, if you haven’t already done so. You should just be able to see along the side of your car on each mirror with just a glance of your eyes; you should not have to move your head to do this. To help you adjust this properly, ask your friend to stand at the rear corner of your vehicle, and adjust the corresponding (passenger or driver) side so you can just see your friend in that mirror.

Finally, adjust the rear view mirror so you have the best view possible of what’s behind you. You’ll find that it may not be always possible to see someone standing at the corner of your car through the rear view mirror, but you should be able to see them in your side mirror.

How big is my blind spot?

Now, with your friend starting at the rear corner of your car, ask them to start moving slowly away from the car in a direction perpendicular to the way the car is facing. Tell your friend to stop when you can no longer see them in your side mirror with just a quick glance. Next, ask your friend to turn in the direction that your parked car is facing, and start moving forward slowly. While still facing forward as you would if you were driving, tell your friend to stop as soon as you see her in your peripheral (side) vision.

The area in which your friend was NOT visible is the size of your blind spot on that side. Remember, there are at least two blind spots with each car. Blind spots are shown in red in the illustration below:

Shoulder checking

Blind spots are why it is critical to perform a shoulder-check before you change lanes. Get the feel of shoulder-checking while still in a parked car. With your friend standing in your blind spot, and while keeping both hands on the wheel, perform a quick shoulder check to see if you see your friend. Remember that you only need to know that the blind spot is clear and that it is safe to change lanes in that direction; a quick glance over your shoulder is usually all it takes to keep yourself and others safe. And remember not to pull the steering wheel in the direction of your shoulder check – this is something that will come with practice.

It may not be possible to totally eliminate blind spots in your vehicle. You should always use your vehicle’s turn signal and glance over your shoulder in advance of changing lanes in order to minimize the risk of failing to see a vehicle or pedestrian that may be in your vehicle’s blind spot.

Some vehicles may be equipped with backup cameras or sensors that facilitate identifying objects immediately behind a vehicle and not visible in the rearview mirror. It is always safest to look behind the vehicle before getting inside and to not become overly reliant on backup cameras and sensors.

Be courteous – don’t linger in a blind spot

Now that you know how big your blind spot is, think about other drivers: can they see you if you are in their blind spot? Not likely. And can you be certain that that driver will perform a shoulder check correctly if they need to change lanes? So make it a point not to linger in that area if you think you are in another driver’s blind spot. Take your foot off the gas pedal and allow the car to slow down so that you are no longer in another car’s blind spot.