Traffic Jam

Traffic Jam is a miscellaneous quest in Fallout 4.

Detailed walkthrough
Ask Deb at Bunker Hill for a job. She'll inform the Sole Survivor that some nearby feral ghouls pose a threat to Bunker Hill and that they need to be removed. Go to the National Guard training yard and kill the feral ghouls, and then return to Deb to collect your reward.

When the first building is entered, the player will most likely encounter a few ferals, along with Brotherhood of Steel Knight Astlin, who is dead, starting the quest, The Lost Patrol

In the second there will be more ferals, along with a glowing one, sometimes legendary, depending on difficulty.

If the area had already been cleared previously then the Sole Survivor can directly ask for the reward in the same conversation.

The quest is only offered during the day.

Bugs

 * If you previously cleared the training yard without getting this quest, it may be unavailable to accept or turn-in, blocking Prep School. (Despite the in-game console showing it at stage 200, which is "Report back to Deb")
 * On PC, you can solve this by manually setting the quest stage to 100, and then to 200. (Using the  command) The relevant misc quest entries will be added to the Quests panel, and your next talk with Deb will turn the quest in.