You have just passed a large truck on the highway and want to move back into the right lane. When is it safe to do so?

Why D is correct

A tractor-trailer has four blind zones, and the one beside the cab is big enough to swallow a passenger car whole. The reliable test is simple: if you cannot see the driver's mirrors, he cannot see you, and moving over on that basis is asking him to react to a car appearing from nowhere. Waiting until both of his headlights show in your own centre mirror puts you far enough ahead for your whole vehicle to be visible to him. There is also a stopping-distance reason to give him room — a loaded truck needs dramatically more distance to slow, so cutting in close removes his ability to brake while leaving him no other option.

Why the other options are wrong

A. As soon as the truck's cab appears level with your rear bumper

Cab level with your bumper means you are beside him, which is precisely inside the blind zone. If he needs to change lanes or react to debris, you are in the one place he cannot possibly know about.

B. Immediately, before the truck has a chance to speed up

Moving over quickly is what creates the emergency rather than avoiding it. Trucks cannot slow like cars, so the vehicle you cut in front of may be physically unable to avoid you no matter how hard the driver tries.

C. When the truck driver flashes his headlights at you

A flash of the headlights is courtesy, not clearance, and many drivers find it ambiguous. Making your lane change depend on someone else's signal means depending on a convention not everyone uses or reads the same way.

On the road

Passing on an interstate with a tailwind, you clear the tractor and drift back a touch early. The driver never saw you and has nowhere to go with a fully loaded trailer behind him. Waiting the extra two seconds until the whole cab fills your mirror costs nothing and removes that entirely.

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