We’ve all been there. One moment we’re minding our own business, travelling down the highway on our way to satisfy some temporary urge. Suddenly we notice a vehicle appear to our right, and he wants to merge into our lane.  We know that the merging driver wants to enter our lane, but he has limited time and space to decide exactly how to proceed. Now we’re in a precarious situation with a number of potentially fatal possibilities. Here’s a series of images below depict a common outcome:

Figure 1. The first image shows a dark sedan reaching the merge lane, while a white pickup truck is proceeding in the right lane.

Figure 2. Noticing the sedan, the driver of the pickup begins to move into the left lane.

Figure 3. Realizing that the pickup is moving into the far lane, the driver of the sedan begins to merge.

Figure 4. The sedan continues merge uninhibited now that the pickup is out of the way.

Figure 5. The sedan’s merge is nearly complete.

Figure 6. The merge is now complete. Both vehicles continue travelling safely down the highway.

Most find the events detailed above fairly unremarkable. In fact, they might even say that it was a great example of a driver etiquette and caution on the part of the pickup driver, but the truth is nothing of the sort. Noticing that the sedan was travelling alongside him, the driver of the pickup was faced with three basic options:

  1. Slow down and let the sedan merge in front of him.
  2. Accelerate in order to allow the sedan to merge behind him.
  3. Continue at the same speed.

The driver of the sedan also has three similar options:

  1. Slow down speed and attempt to merge behind the pickup.
  2. Accelerate in order to merge ahead of the pickup.
  3. Continue to merge at the same speed.

It’s kind of like playing a game rock-paper-scissors, with each driver trying to anticipate the other’s next move. Here’s a table showing the possible results:

Pickup Choice Sedan Choice
Slow down Accelerate Continue at Same Speed
Slow Down  Crash  No crash  No crash
Accelerate  No crash  Crash  No crash
Continue at Same Speed  No crash  No crash  Crash

As we can see, 3 of the 9 possibilities result in a crash, which is alarming. This is why drivers often choose the secret fourth option: moving into another lane. Of course, this is only possible when there’s another lane in which to move, but doing so circumvents the merging problem entirely. However, it has a serious drawback.

When the pickup driver decides to move over and allow the sedan to merge, he is entering into a lane that could already have traffic in it. This creates another table of possibilities based on the number and speed of vehicles in the left lane. If there is a vehicle in the left lane, then this vehicle and the pickup are now in a nearly identical situation as were the pickup and the sedan. And of course, many highways have more than two lanes, which means that should the driver in the left lane choose to move over, it could result in a third encounter.

The problem here is not that moving out of the right lane to accommodate merging traffic is inherently unsafe, but that it attempts to resolve a crisis by merely shifting the problem to another party. According to traffic regulations, it is the duty of the merging vehicle to match the speed of highway traffic and select a position in which to merge. In the aforementioned case, the pickup driver alleviates the driver of the sedan from this responsibility by taking it upon himself to merge into the left lane. While this act may seem prudent and selfless, it actually has some serious consequences. Here are a few things wrong with this behavior:

  1. It forces additional vehicles into a merging scenario, each of which could result in a crash.
  2. It slows down traffic in the passing lane.
  3. It fosters an expectation for highway traffic to yield to merging drivers.

The third point is more difficult to measure, but its effects are likely the most serious. By yielding to merging traffic, the rules of the road are obscured, and merging drivers come to mistakenly believe that they have the right-of-way. This, in turn, results in a greater number of dangerous merging scenarios, which means more crashes. The whole point of traffic laws like those governing highway merging is to clearly indicate which party has priority, so that we don’t feel like we’re playing rock-paper-scissors.

Now changing lanes to make room for merging traffic is not always unwise. If done with caution and well in advance of the merging lane, it’s a great way to avoid a potentially hazardous situation. Just make sure you’re not merging to avoid merging.