The advent of smart cars can solve many problems, including an increase in traffic density for faster commutes, the promise of increased safety (due to better roadway decision making), and driver convenience. Electric and hydrogen cars promise cleaner energy, but rubber tire to road is still a relatively high-friction and inefficient means of transportation, requiring high energies to overcome rolling resistance. What, you may ask, is the most efficient and energy saving form of transportation in terms of passenger miles per btu? (the standard for measuring transportation energy efficiency). The answers may surprise you!
Some of the answers lie in the table below, in energy measured by the number of btus per passenger mile (a btu is an international measure of energy…don’t worry, you’ll see by the relative values which mode is more efficient) and wherever relevant, we show the equivalent “miles per gallon” of the mode (this is tricky because “people walking” uses caloric joules and obviously not gasoline, plus efficiencies of the transportation device come into play so that joules/m and mpg are sort of apples and oranges).
You can see that “what transportation device has the highest energy efficiency” is a sort of trick question, because it depends upon how close to capacity that device carries (for instance, a bus is very efficient when it carries a full load, not so much when it doesn’t)
As expected, walking/cycling/motorcycling is very efficient, if impractical for moving large groups of people. When it comes to larger conveyances, rail is very efficient if filled to capacity, even more so than buses since there is less friction and rolling resistance to overcome. Somewhat surprisingly, air travel can be very efficient, especially if filled to capacity or if planes are to have larger capacities, and this is where it gets interesting.
Very large capacity planes such as the Airbus are capable of carrying as many as 800 passengers, which could be very efficient if planes were filled to capacity. In addition, if air hybrid technologies were used, (jets shut off at cruising altitudes while engines run off batteries or fuel cells, much the same way an auto hybrid works) they could be even more efficient, and thus pollute less than other competing modes. Since air travel doesn’t need the land acquisitions that trains and highways typically need (they only need the land for airports) they are also more adaptable and cheaper to implement. Efficiencies are lower for shorter trips, as could be expected, since the time reaching cruising altitudes are short and the relatively longer times to “get to and from your plane”, which favors intercity trains for trips of 300 miles or less.
Similarly, the hassles of train travel make it less viable for trips of less than 30 miles, in which case buses or efficient cars may do the job more easily and efficiently. Of course if you are just going to the corner grocery, walk or ride a bike…most of us could probably stand to burn that 335 btus/mile!