For several years now Canada has required all new cars sold to have daytime running lights. Any time the car is running the headlights are on, but the taillights and other lights are off. You have to turn on these other lights from the dashboard at night. Studies seem to indicate that having the headlights on during daylight hours reduces the number of multiple vehicle accidents (although there has been some controversy about people forgetting to turn on their other lights at night -- a mistake that causes extra accidents, and a good example of the "law of unintended consequences"!).
The US has not adopted this law, but if it did they would definitely consume gasoline. Headlights require power, and a car's engine produces power using gasoline. If you make a few assumptions, it is possible to estimate how much gas the law would consume.
A typical headlight bulb uses about 55 watts; sometimes the daytime running lights run at a lower wattage so they use a little less power. Let's say the daytime running lights use 100 watts since there are two bulbs.
To calculate the energy used, we need to figure out how much time people will spend with their lights on. According the to NHTSA, vehicles in the US drove 2,560 billion miles in 1997. We need to make a guess at the average speed people drive including stops in order to figure out how much time people spent driving their cars. Let's guess 30 mph, which means each mile takes two minutes. That makes 5,120 billion minutes or 85.3 billion hours. Now if each car normally drives at night about half the time, that means that the daytime running lights would be on 42.6 billion hours a year. Multiplying by the 100 watts we get 4,260 billion watt-hours or 4.26 billion kilowatt-hours. The U.S. uses about that much electricity nationwide in 12 hours.
Now we need to figure out how much electrical energy we can get out of a gallon of gas. A gallon of gas contains about 60 kilowatt-hours of chemical energy, but this energy has to go through two conversion processes before we can use it in a light bulb. First the chemical energy must be turned into mechanical power by the engine of the car. Car engines don't do this very efficiently -- only about 25% of the chemical energy can be turned into mechanical power, and the rest is wasted as heat. After the engine gets done with our gallon of gas we have 15 kilowatt-hours left.
Now the alternator on the car has to turn the mechanical power from the engine into electrical power. The alternator does this a lot better than the engine, but it is still only about 70% efficient. In the end we get about 10.5 kilowatt-hours of electrical energy out of a gallon of gas.
To calculate how many gallons of gas this is, you can divide the 4.26 billion kilowatt hours of energy that the daytime running lights consume each year by the 10.5 kilowatt-hours of energy each gallon of gas yields. If daytime running lights were on all the vehicles in the U.S., we would burn an extra 406 million gallons of gas each year. That's only a couple gallons for each vehicle, but in total it is more than all of the vehicles in the country burn in a day. At $1.50 a gallon, that's $600 million per year. Looking at it another way, an extra 8 billion pounds of Carbon Dioxide would be added to the atmosphere by this law.
It's an interesting question because it shows how a simple idea like, "let's have everyone turn on their headlights all the time" can have a real cost when you try to implement it! Whether the benefit is worth the cost is an important question in almost any public policy decision.
Here is an interesting link:
US Department of Energy fuel economy Site