system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How UPS drivers save gas by avioding left turns.
gpt
UPS drivers drive a large and not very agile box truck.  This means that for a left turn, they must wait, idling the engine, until a rather large gap is open across both directions of the road.  (In the US, the left turn crosses to the far lane.) 

If their route only involved right turns, and it was the same length, then they would use less gas idling, because they would only need ot wait for a gap in the near lane.  Computer software can arrange that, and thus they make left turns only when the distance traveled is significantly less than could be achieved with right turns.  In the US, road networks are dense and one-way roads uncommon, this turns out to be almost never making a left turn.