Why Did That Semi Driver Do That?

Updated on January 22, 2019

Everyone who has every driven a car has, as some point, asked the question “Why did that trucker do that?!?” I know for many years before I got my CDL license, I asked that question to the ether while driving, often shouting it out loud in frustration as it seemed that the semi was purposely trying to mess with me.

Although there ARE truckers out there who delight in seeing a “four-wheeler”— truckers' jargon for anything other than a big rig—have to slam on his brakes because of a semi, most of us out there are actually trying our best to keep EVERYONE on the highways as safe as possible. I know that at driving school my instructor would start the day off with a reminder that I was now the professional, and the main part of my job was to get myself and all those around me on the roadways home safe and sound.

Here are a few things that many four-wheelers don't know or don't think about when they encounter a semi in their daily routines.

1. Limited Driving Time

Truck driver working hours are dictated and regulated by the Department of Transportation, or DOT for short. They are allowed to work 14 hours from the time they start their day, and can drive for 11 hours within that 14 hour window. Once that 11-hour driving mark is reached, it doesn't matter where they are they are required to stop and shut down for 10 hours at the closest “safe/legal” location. In other words, anywhere that the truck can legally be parked and the driver and his load will be safe while taking a break: a rest area, truck stop, etc.


2. Expanded Field of Vision

Part of the training when learning to drive a semi is a rule commonly called “Get the Big Picture” in the Smith System or “Expand Your View” in MegaSafe7. What this means is that truckers are taught to look far down the road ahead to try and pick out things that will affect the flow of traffic as early as possible.

A fully loaded truck (80,000 lbs or more) traveling 65mph can take nearly two football fields to stop on clear, dry pavement, so the driver will avoid emergency stops as much as possible. That truck driver sitting 8' above above the road surface can see a LONG way down the road. Most truck drivers, when they see something affecting traffic ahead, will slow down and engage their hazard lights, both to be sure that four-wheelers see them and in an attempt to let other drivers know that there is something ahead to be aware of.

3. Stopping Distance

Big rigs take a LONG time to accelerate and decelerate, even in optimal conditions, and, as mentioned earlier, do not stop on a dime in an emergency. That big gap between the semi and the car in front of it is not there for cars to zip in and out of, it's there to allow a safe stopping distance if something happens suddenly. And a trucker hauling liquid has an even harder time, since the fluids in the tanker move with every action the driver takes, even after the truck itself has stopped moving.

This action is called “surge,” and it is not uncommon, after a sudden hard braking action, for the surge to actually move a truck another few feet, even through held brakes. On slippery or wet surfaces that movement can be doubled or tripled. Thus a car that cuts in front of a tanker, forcing him to slam on the brakes instead of slowly easing into them, can cause the truck to rear-end that car, even after stopping.

4. Time Is Money!

Most truck drivers are paid by the mile, not hourly or salaried, so they are only making money when that truck is moving as fast as legally possible. That trucker doesn't want to be slowed down any more than you do, nor does he want to spend time in a miles-long “parking lot” on the interstate.

5. Stop on a Dime?

Contrary to popular belief, air brakes do not mean faster braking! They mean a safer system that can handle the force of bringing 40 tons of material to a stop over and over without failing. In an emergency the semi is going to take a LOT longer than any car out there to shut down; give him room!

6. If They Can't See You...

As mentioned earlier, truck drivers have a vastly improved sight distance in front of them, but the opposite is true to the sides and behind. A car directly behind a big rig, following closely, is literally invisible to the driver. A rule of thumb is that if you cannot see the trucker's mirrors from behind the semi, then he cannot see you.

There is also a “blind spot” on the right side from near the front of the trailer to the nose of the truck, despite all those extra mirrors. Trucker drivers have what is called a scanning pattern, checking every few seconds on both side of the truck, but it is easy to miss a car sliding into one of those blind spots.

Being Considerate

So the next time you encounter an 18-wheeler on the roadways, take a moment and think about some of the factors that go into driving a semi, and try to be considerate of that trucker. Every one of them may not notice, but those that do will appreciate even the smallest act of courtesy from a four-wheeler.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.


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