When you’re in the left lane next to mine on the open highway (typically on interstates or major highways with two or more lanes in each direction, in sparse traffic, usually in the countryside) or in heavier traffic, your speed will usually be greater than mine.
When you have advanced from alongside the rear of my trailer to the trailer midships (middle) section, then to the front of my trailer, then alongside my tractor and finally alongside the front of my tractor, your vehicle’s image has exited my large left mirror completely through its outer (left) edge while remaining in the small convex mirror below it and in the small convex mirror over my tractor’s left front fender, then has almost entirely exited through its outer (left) edge the small convex mirror below my large left mirror while remaining in my tractor’s left fender mirror, then has remained solely in the my tractor’s left fender mirror, and finally has almost entirely exited the fender mirror through its outer (left) edge.
At this point your vehicle is forward of my tractor’s left cab door and alongside its steer (left front) tire and fender. I can see you directly out my left side window, but only the rear part of your vehicle shows in the left fender mirror. While your vehicle is easily visible in daylight, at night or in other limited visibility conditions I depend more heavily on its lights, especially the taillights.
In the event that my speed is greater than yours and I overtake you from behind, I won’t need my mirrors as your vehicle progresses from direct view out my windshield to my left side window.
This zone is slightly less risky than the zone just behind it (alongside my tractor on my left side). You are readily visible and the relative distances our vehicles have to travel to enable me to move left into your lane are even smaller.
This zone poses other risks. True, you are able to see the road ahead of my truck and in wet or snowy weather, water or snow thrown sideways from my truck’s left steer tire is less likely to obscure your view of the road ahead and my view of you. However, if my tractor should have a steer tire blowout, your vehicle may be hit by shrapnel and you are at risk from tractor control problems as I bring the truck to a stop.
Sitting in the front seat of your vehicle, you will be forward of my tractor’s turn signal/emergency flasher light in front of the left cab door. That light may or may not be in your mirrored lines of sight, and the ambient (scattered) light from it is not very noticeable in daylight. You may or may not be able to see the turn signal/emergency flasher light in the left headlight cluster or the ambient light from it out your right side window in daylight. It is generally best to move out of this and other risk zones alongside my truck as soon as it is practical, safe and lawful to do so.
Changing Lanes to My Left
For routine moves to the left for situations such as emergency vehicles or highway maintenance equipment and personnel or other vehicles and people on the right shoulder, I should be able to move over behind you by slowing down while you continue ahead. (Of course, this assumes that no other drivers are alongside me and behind you.)
In an emergency in the right lane and/or shoulder, I prefer that you continue ahead briskly while I brake hard to swerve to the left behind you. (There may not be time for me to signal and you to notice.) If we brake simultaneously, you will block me until you have cleared my trailer tail due to your shorter slowing/stopping distance. Then I may or may not have the time and space to swerve ahead of you. If I do, you benefit by being safely out of my truck’s slowing/stopping path.
This zone is less risky than the zone just behind it (alongside my tractor on my left side), because a truck driver can see your vehicle directly out the cab’s left side window and you have an even shorter distance to advance out of the way in an emergency.
However you are still at risk from shrapnel and tractor control problems in the event of a steer tire blowout, and in daylight you may not instantly notice a truck’s turn signal/emergency flasher lights.
As a general rule, move out of this and all other risk zones around a truck as soon as it is practical, safe and lawful to do so.