Is it permissible for us to combine the Dhuhr and Asr prayers in a capital city congested with cars, such that traversing a distance of one hundred meters takes a full hour, and we might not be able to return to the mosque after the Dhuhr prayer to perform the Asr prayer?

The fear of missing the congregational prayer in the mosque is not considered a valid excuse for combining the two prayers of the same time (Dhuhr and Asr, and Maghrib and Isha). If it is possible to catch the congregational prayer in the mosque, that is preferable. However, if that is not possible due to congestion, then the prayer should be performed congregationally at home, even if there are only two worshippers. If congregational prayer is not possible, then one should pray individually. The aforementioned combining of prayers is not permissible in these situations.