Not a guarantee but it feels like customers on the right side of the road generally want to go forward. Much like in real life though not every passenger gets the unspoken rules, but the strategy still seems to work enough of the time. So I'll personally pick up red and yellow "short trip" passengers headed both ways in an area, and then when it starts looking sparse, I'll pick up a green "long trip" passenger on the right side of the road to take me into the next area.