Because it does go faster. Well, not always, but more often than your own lane is faster.

Before I tell you why this is, let’s consider a simpler scenario for a moment. You’re driving in a congested single lane road. For the sake of this thought experiment, assume that the road is made of two equal parts. Through half of the length of the road the traffic moves relatively fast, but through the rest of the length the traffic moves slower. Let’s call the fast part A and the slow part B. Then you’ll spend more time at part B than at part A. If for example the traffic moves twice as fast in part A than in part B, you’ll spend 1/3 of the driving time in part A and 2/3 of the time at part B.

So even though the slow and the fast parts are equal in *length*, they are not equal in *time*. And this is true no matter how the slow and fast parts are distributed throughout the road. The fast and slow parts could be interleaved for example, so that for the first km the traffic goes fast, the second km the traffic is slow, and so on; in the end the result is the same: you spend more time in the slow parts.

Now back to the original question. The road has two lanes (let’s call them X and Y) and they’re both made of “fast” segments and “slow” segments. It’s safe to assume that the lanes are symmetric, in the sense that the total length of segments where X is faster than Y is equal to the total length of segments where Y is faster than X. (Even if the lanes aren’t identical, the probability of you choosing to enter each one of them is 50:50, so statistically speaking the assumption holds).

Let’s say that you only stay in one lane throughout the whole road. Then by the assumption above, the length of the road where your own lane moves faster than the other lane is equal to the length of the road where your lane moves slower. But remember that if you move through a road composed of a slow part and a fast part of equal lengths, you’ll spend more time at the slower part. Consequently, you spend more time at the segments where the other lane is faster! Just like in the first example, even though the two types of segments are equal in length, they’re different in time.

So next time you’re in a traffic jam and wondering why the cars in the other lane go faster, remember that Murphy doesn’t hate you; it’s just that you drove fast through the fast parts.

What happens if you move between lanes? It depends on your timing and on the distribution of the fast and slow segments along the road. For example, it seems to be the case that if a segment along the lane is slow, it says nothing about the probability that the next segments is slow (the events are statistically independent). If the segments are short, then by the time you change the lane the traffic could already move in a different pattern and you gain nothing. In that case, it would seem to you that the lane became slower when you moved to it, when in fact you just experience the same effect as staying in the original lane.