I guess Taliban attacked Department of Traffic following the below aims:
1. Department of Traffic have recorded various information regarding the cars that Taliban have been using.
2. This department have tracked what kinds of car, with what plate, color made by what countries and where they have enter through in Afghanistan.
3. Department of Traffic has recently passed new law on plates, license and other stuff to not only control traffic jam, but also find terrorists much better.
Since we don't have electronic professional storing and recording systems, we might have lost bulks of information and records. That's why Taliban/insurgents targeted Department of Traffic, although it is said that they were aimed to fire on Border Police Department (that I doubt).
Please share it if you think I evaluate well.