It increases as the square of air resistance compared to traveling at low but constant speeds.

But I find that when I'm driving at lower speeds I'm constantly stopping and starting for traffic.

Around where I live there are many roads posted at 80km/h (50mph) with traffic lights every km or so. So it's accelerate to 80 for a couple minutes, then come to a complete stop, then accelerate back to 80. That's clearly what swallows up most of the range.