In which week of September, the total number of vehicles passing through the toll plaza was the second highest?

Let me calculate the total number of vehicles for each week by summing buses, trucks, and cars.

Week I:

$28.3 + 15.4 + 40.2 = 83.9$ (hundreds)

Week II:

$24.5 + 12.3 + 36.1 = 72.9$ (hundreds)

Week III:

$20.4 + 14.2 + 42.9 = 77.5$ (hundreds)

Week IV:

$24.4 + 24.4 + 39.8 = 88.6$ (hundreds)

Ordering from highest to lowest:

Week IV: 88.6

Week I: 83.9

Week III: 77.5

Week II: 72.9

Therefore, Week I had the second highest number of total vehicles passing through the toll plaza in September.