Before moving into the lane to your left on a multi-lane road, what is the most reliable way to confirm the lane is clear?

Why D is correct

Mirrors leave a wedge of road beside and slightly behind the car that no lens can reach, and that wedge is large enough to swallow a motorcycle or a small car entirely. Turning your head to look directly into the lane you are about to enter is the only action that actually covers it, which is why driving instructors insist on it even in cars with excellent mirror coverage. The correct order is mirror first, signal second, shoulder check third, then move — the glance is a confirmation made immediately before the manoeuvre, not an alternative to the mirrors. Electronic blind-spot systems are genuinely useful backups, but they miss bicycles, they react late to fast-closing vehicles, and they are no defence when a driver merges without looking.

Why the other options are wrong

A. Check the inside rear-view mirror only

The inside mirror shows what is directly behind you, which is exactly the area you are already occupying. It is blind to the lane you are entering, so relying on it alone means merging into traffic you never saw.

B. Rely on the vehicle's blind-spot warning light if it has one

Warning lights are an aid, not a substitute. They can miss cyclists and pedestrians, they are tuned to avoid false alarms, and sensors fail silently. A driver who merges on the strength of a light eventually merges into something it did not detect.

C. Signal for three seconds and then move over

Signalling tells others your intention but does not reveal anything about whether the lane is occupied. Plenty of crashes involve a driver who signalled faithfully for three seconds and then moved into a car that had been beside them the whole time.

On the road

On a two-lane each way arterial, you check the mirror, see nothing, and signal. A compact car has been sitting in your blind spot since the last light — invisible in every mirror. The shoulder check is the only thing that catches it, and it takes about half a second.

Similar questions

• When Are Headlights Required? | CO Permit

  When are you required to have your vehicle's headlights switched on?

• Correct Way to Wear a Seat Belt | CO Permit

  Which of these describes the correct way for an adult to wear a lap-and-shoulder belt?

• Signalling Too Late to Matter | CO

  A driver switches on the turn indicator at the same instant they begin moving the wheel into the turn. Why is this unsafe?

• Hard Braking With ABS: Do Not Pump | CO

  Your vehicle is fitted with anti-lock brakes and something steps into your path. What is the correct way to use them in an emergency stop?

• Holding a Child Instead of a Seat | CO

  A driver places a young child on their lap in the front passenger seat for a short trip and fastens only their own belt. What is wrong with this?