Apart from the items pointed out before, consider that frequency isn’t the only quantity to check. Imagine a frequency of 6 min., but with 6 vehicles in the line. In that case, the piece of goods has to wait an average of 3 min. for a vehicle to arrive plus (6 by 6/2 = 36/2) 18 min, travel time. That makes 21 min => that line won’t be chosen…