As many have pointed out, the drag force scales with vˆ2. Therefore the power to overcome it scales with vˆ3, so it becomes negligible at low velocities.

Furthermore, very little drag comes form the front. You can have some curvature on the edges of the front side, which will reduce the drag, or it may actually generate suction, that compensates the drag of the vertical part perpendicular to the travel direction, so in total there is no drag resulting from the front side.

Large part of the drag comes from the back side of vehicles. This is becuse the air that passed the bus gets into a larger cross sectional area behind the bus, so it fills that space, and its velocity decreases. But there is shear on the surface, friction force, resulting from viscosity, which slows the airflow further down. Plus, the pressure is increasing in the direction of the airflow, because of the decreasing velocity. The air cannot follow along the surface of the body, because its momentum is decreased by the friction (viscosity), so it cannot go opposite of the pressure increase. So it leaves the surface, this is called separation. This is what you can see on the photos by /u/bart2019 In the separation bubble the pressure is low, and this basically pulls the bus back, increasing the drag and consumption.

In case of a bus, the friction force on the long body can have a significant effect, too. So this has to be taken into account as well.

For some photos and more info see these slides: http://www.ara.bme.hu/oktatas/letolt/Vehicleaerodyn/Vehicleaerodyn.pdf

Source: PhD student, department used to do research on the aerodynamics of buses A test from the old times: http://galeria.vezess.hu/files/752/084/000/84752/84752_702309_1000x700.jpg