Stop light problem

Problem: https://brilliant.org/practice/probability-is-everywhere/?p=2

I understand the desire to assume that because a car is there before you that you will have to wait less time, but without knowing if the light is controlled by time or by sensors, it seems wrong to assume that the car makes any difference.

The solution states that because the car is there you can assume that you are later than normal, but that seems flawed. What if that driver only drives on that road every other day? Or what if a road elsewhere is under construction and that driver had to take this route instead?

Is there something I'm missing here?

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Math	Appears as
Remember to wrap math in `\(` ... `\)` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$