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You pass an entire community on your daily commute and probably don’t even know it: truck drivers. Our interstate highways are full of long-haul semi trucks and delivery trucks, and they communicate with one another nonstop. Truck drivers use CB radios to check in with one another, drop warnings in Waze, and even use their high-beam headlights to signal one another. But what semi truck drivers mean when they flash their lights at one another can depend on what pattern they flash and which way they are driving.

Truck drivers flash their lights during passing maneuvers

Semi truck drivers navigating a snow-covered interstate highway.
Highway truck traffic | Thomas Kienzle/AFP via Getty Images

One common reason semi truck drivers will flash their headlight high beams, whether at other truck drivers or regular vehicles, is to request a pass. Imagine a semi truck driver is coming down a long hill and using the left lane to pass slower traffic. Then they encounter a slow vehicle camped out in the left lane. The driver doesn’t want to hit the brakes and lose momentum. But they also don’t want to lay on their horn. So instead they will often flash their high beams, hoping the other driver gets the hint and moves to the right.

Many drivers of quick passenger cars will flash their lights for the above reason. But semi truck drivers also often flash their lights when another truck passes them. They do this to let the passing driver know it is safe to merge right.

Why would a semi truck driver signal it’s safe to merge right? There are two reasons. The first is to let the passing driver know there aren’t any small cars or motorcycles in their blind spot. The second is to let the passing driver know that the driver who just got passed isn’t trying to speed up and that there’s plenty of room in the right lane.

There is a final, more rare light flash semi truck drivers will use during a pass. When the driver in the rear flashes their high beams to signal that the passing driver can merge right, the passing driver may flash a “thank you.” How do they do this while driving in the same direction? Many semi truck drivers have wired a running light interrupt switch for this exact reason. Talk about thoughtful!

Truck drivers flash their lights as a warning

A semi truck driver flashing their high beam headlights on the interstate highway at night, a city skyline visible in the backgound.
Semi truck | Luke Sharrett/Bloomberg via Getty Images

Truck drivers–and many other drivers–may also flash their high beams at oncoming traffic as a warning. (This is according to the Mental Floss website).

Drivers can use this signal because they just passed a speed trap, a crash site, or another obstacle that oncoming traffic will soon have to navigate. Whatever the reason, the meaning of this signal is always “watch out!”

Semi truck drivers don’t always have to use this signal. That’s because they can also get on their CB radio and broadcast a warning to all the other truck drivers in their area. But sometimes they have just passed some kind of hazard and don’t have time to reach for the radio. Other times they may want to warn regular vehicles that probably don’t have a CB radio. In these cases, truck drivers often flash a warning to oncoming traffic.

It’s important to pay attention to truck drivers

A row of semi trucks waiting in traffic on the interstate highway.
Truck traffic | Sebastien Bozon/AFP via Getty Images

You can easily cruise down the interstate and forget that there’s a convoy of truckers in the slow lane. But paying attention to semi truck drivers may save you a lot of trouble.

Obviously, it’s important–and often required by law–to merge right when a faster vehicle wants to pass you. Merging right when someone flashes their high beams is a good way to avoid inciting road rage–and to just be polite.

It is also good to pay attention when oncoming traffic flashes its high beams at you. Truckers and other drivers may be trying to warn you of an upcoming obstacle or police officer, so take the hint.

Next, find out why some semi truck trailers have a tiny door set into their main door or learn more about why truck drivers flash their headlights, from a truck driver:

Finally, see how drivers of passenger cars use their high beams to signal others: