As I understand it, gas/petrol prices are down for two reasons:
1) Actual demand is down. Not so much driving-type of demand (thought that is down too), but industrial demand - industrial plants, construction, all the big gas/petrol using activities are way down in this worldwide economic slump.
2. Anticipated demand is down. People think that the economy will be in the pits for a long time, they anticipate that demand will be down for a long time.
When the economy improves, demand will go up, and so will the price of gas/petrol.
I heard it explained this way: What is the value of a gallon of gas to a non-industrial society? Zero. So, the worse the economy becomes, i.e., the more we approach a non-industrial society, the lower the price of gas will be.