It was with considerable relief that I stumbled upon a simple method to actually reduce my carbon dioxide (CO2) emissions below zero while driving my car! That is not a typo – it is presently possible to reduce total CO2 emissions while driving an ordinary gasoline powered automobile. Better yet – it is fun and even legal! Like many other environmentalists I sometimes feel guilty when traveling long distances to lobby, protest, or speak at scientific conferences about solutions to the ecological crisis. Although plane travel is less efficient, driving is particularly disturbing for me because I feel as if I am more directly responsible. This feeling of hypocrisy has even discouraged some otherwise active environmentalists from being as effective as possible. This is a shame because if we do not get in our cars and drive to work for ___________ (fill in the blank with your favorite environmental cause) – it simply will not be done as well (or maybe not done at all). The solution to the driving conundrum is technical although it has nothing to do with the latest hybrid technologies or even the fanciest hydrogen fuel cells. In fact you can do it with any kind of automobile. The trick is to strategically drive the speed limit. By strategically driving the speed limit you slow down other automobile drivers and shift their vehicles into more efficient operating regimes. The operating efficiency of modern automobiles is complex. As you accelerate from rest (0 mph) the efficiency of the engine improves because it uses a fixed amount of fuel to power itself and the accessories, and a variable amount of fuel depending on the power required to keep the car going at a given speed. As you accelerate to higher velocities your car overcomes the rolling resistance of the tires and friction in the car’s other components (e.g. friction in the wheel bearings). So in terms of fuel used per mile, the faster the car goes at first, the better use the car make of that fixed amount of fuel required. However, once the speed gets up into the 35-40 mph range each 1 mph increase in speed represents an increase in power required mostly to overcome wind resistance. Eventually, the power required increases more than the efficiency of the engine improves. At this point the mileage starts falling and can drop like a rock after 55 mph as in the graph below.

Strategic speed limit compliance is more than just obeying the law – it is actively ensuring that others do the same. The easiest place to try this out is to set the cruise control to the posted speed limit (or a mph or two less) while driving through a construction zone. On I-80 in Pennsylvania, which I travel on regularly, this is usually 50 mph. There I can easily “stack” up over 50 automobiles. Looking at the graph and estimating that the average unrestrained speed would be 60 mph, each car would get an extra mile for each gallon of gas burned. With 50 autos traveling 10 mph less, this means we get 50 extra miles per gallon collectively. If our little caravan travels 50 miles I would save 1 gallon of gasoline from being burned. Even on a bad day, my Toyota Prius gets more than 50 miles on a single gallon of gas, so I am effectively traveling with no net fuel consumed and thus no net CO2 emissions! On the open highway the results can be even more dramatic because of the rapid decrease in fuel efficiency at speeds over 55mph. For example, by driving parallel to trucks traveling at 65 mph it is quite easy to stack up a dozen cars and SUVs, which would normally be traveling at 75mph or more. Each car that decreases its speed to 65mph is increasing its fuel efficiency by 4.4 mpg. For a hybrid you need to get at least 1 dozen cars stacked up to travel with zero net emissions. If you drive an average car you would need to ensure about 2 dozen automobiles were stacked – again no problem. It should be pointed out here that these estimates are conservative because the optimal efficiency as shown in the graph above was for cars in 1997. Since this time, average U.S. automobile fuel efficiency has actually been dropping. This is mostly due to the increased use of gas-guzzling SUVs like the cheesy Hummer. Such SUVs and light trucks have a drag coefficient about twice that of more aerodynamic cars, they have more than double the frontal area of small cars, and can weigh two to three times what more reasonable cars weigh. In general, smaller, lighter, more aerodynamic cars will get their best mileage at higher speeds than SUVs. Bigger, heavier, more cumbersome and less aerodynamic vehicles will get their best mileage, which is still sad, at lower speeds. Thus strategically driving the speed limit provides an opportunity to do as Jesus commanded: “Love your enemies”. By helping SUV owners drive closer to their optimal operating speed you will be helping them even MORE than you will be helping normal people. In fact many an SUV driver, having realized how much I saved him (by performing the necessary mental calculations), kindly and vigorously gestured that I was number one in his book. It is, of course, better to have good deeds go unrewarded. Luckily, only the first driver or two in your caravan will ever know what a good citizen you are. Of course, there can only be a limited number of negative CO2 drivers (if everyone was a responsible driver it would not work) but there is a lot of room for improvement. Strategically obeying the speed limit is more than fun and games. Each gallon of gas burned releases 19.56 pounds of carbon dioxide into the atmosphere.1 If fuel economy were improved by 5 mpg, which can be accomplished by pushing the average velocity on the highway down to about the speed limit, American consumers would save 1.5 million barrels of oil per day. This would equate to an annual reduction of approximately 55 million metric tons of CO2 emissions per day. That is a lot of savings and undoubtedly a boon for the environment. However, it is also apparent what a large problem we face- what of that other 95%? We have a lot of work to do – so get in your car and strategically drive with a clear conscience! For other ways to increase fuel efficiency see http://www.fueleconomy.gov/ The graph is based on information published in the Transportation Energy Data Book: Edition 23, Table 4.24. Available: http://www-cta.ornl.gov/data/chapter4.html