Thanks for the valuable contribution above but for clarification, It happened today that this sentence came in my son's English exam. First of all, the context should be clear that it is based on a part of the curriculum taught to grade 9 students in Egypt. In this part, they study the difference between using "might" and "must" in the case of prediction from the known prospective that "might" is used when there is doubt or uncertainty while "must" is used when there is a strong proof that something is going to happen.
Accordingly, the resource used here has tried to contextualize the question mentioning that there is a lot of traffic today as a proof that something is going to happen and it, at the same time, meant not to show any sign of uncertainty or giving a view point in an anticipation such as using I think or doubt, not sure, believe, probably etc. Hence, in his point of view, he is applying the grammatical rule in a perfect sense.
However, the question maker made a mistake that my son who chose "might" and I am not, at all, blaming him, will be the victim for which is that he doesn't have any right sense of the language use and he is dealing with language as Mathematics which is absolutely wrong for the following reasons:
The word today makes it clear that this is an unusual happening that no one knows whether it will continue or not and for how long will it be there. In addition, we have no idea how far the speaker is now from his destination so we have no idea about the level of certainty the speaker means to show. Besides, we are always dealing with regularities not irregularities in such a sense, such as saying roads always have a lot of traffic on Sundays, we must be late. Here we don't have to think about anything that is unusual.
Strong proofs that make "must" the correct answer should not include different interpretations specially when a de-contextualized sentence in an exam is concerned. For example if we say "It cannot be Ali who is knocking the door, he travelled in the morning". Here, the speaker is showing certainty that Ali is not the one knocking the door with no regard to the unusual possibilities that he missed the plane and is coming home back or something unusual happened and he decided not to come back such as that his boss called him and asked him to cancel his trip as they need him in an urgent matter tomorrow morning at work, etc. In another example when we say It must be Ali who is knocking the door, he promised to come at 9am. Here the speaker is showing certainty again as it is 9am now with no regard to the fact that the knocker may be someone else because Ali is running late today due to any reason that the speaker isn't aware of.
In conclusion, as long as the context is absent the test maker needs to understand that he must be very specific and by all means to compensate for this absence by using crystal clear logic not his own in introducing such ideas. Using any of the certainty or uncertainty means would have solved the mystery putting into considerations that using, these questions, changes our kinds from being humans into being machines that are just applying rules rather than trying to understand them and the way they apply to real life, let alone thinking about reasons.
Sorry for not being able to control myself and be brief but I always feel nervous when I see such questions in high stake exams with the full sense of irresponsibility and indifference of the test maker while I doubt he himself understands such constraints.