How was the car being "herded"?

The bridge was blocked, and the car was forced to move forward.... the sides were covered.

In a minute or two he knew that they were, because the opposite end of the narrow, covered bridge was blocked. They were being herded into a cul-de-sac and the tactical situation was changed and their plan useless. There would be no field of fire from the side ports of the truck. The fight would have to be made entirely from front and rear. He said, "Keep going. "They had to drive right into it. If they stopped short of the bridge and jumped out to make their fight at a distance then the highwaymen could shoot and run. They had to get in close.