See this summary from Die Hard Wiki about Die Hard with a Vengeance:
McClane has Jerry drive him to the tunnel as Zeus went to Yankee Stadium. As Jerry was driving, he gives McClane a tour of tunnel 3. Then they stopped when they notice a truck blocking their path. Jerry honks his horn and the truck still doesn't move. McClane finds this suspicious and decides to investigate. He takes Jerry's hardhat and jacket, disguising himself as an aqueduct security guard and makes his way to the truck. Inside the truck, Nils and Klaus wait for McClane to get close enough so they can kill him. Unfortunately for them, McClane beats them to the draw and fires several shots into the door of the truck, killing them both.
While watching the movie, I didn't understand what was the role of this truck and why did it stop? And why does John McClane kill the drivers without identifying them?
We can assert that the terrorists' trucks were the last to enter in the tunnel, so John McClane thinks that this truck can only be one of the terrorists. But I think this assumption is weak. What if it was a normal driver in the truck?