How to encourage people to move to the back of the bus?

The diagram below shows the layout of the public transportation buses in my city:

What this 2-dimensional diagram cannot show properly is that the rear area of the bus is raised about 2 feet above the floor of the rest, those 2 stairs being necessary to connect the sections. That seating area at the rear of the bus is obviously important. It can hold about 15 people seated and probably an additional 10 standing in the aisle.

The trouble is that when the bus becomes crowded and runs out of seating space in the front area, people do not move to the rear area. Though it is only a few steps away up the stairs, they cease to move. I suspect that it might be because the stairs are painted yellow to warn riders of their presence. Passengers are content to pool in the space in front of the rear doors. If people continue to board, they fill the front aisle and then the open space behind the driver and front doors (often crowding that area to particular extremes). The result looks something like this, where circles represent people:

Though the driver has a button that causes a pre-recorded voice to announce "Please move to the rear of the bus", people do not obey it and the driver must yell to tell people to move to the back, and then they do not make a proper effort and the rear is less crowded than it could be. How can the bus be better designed to encourage people to occupy the rear by default?