I’m sure you know how bike-sharing and carpooling works. Split the costs enough ways, and suddenly a big lumpy cost like purchasing a bike or car becomes cheaper than a cup of coffee. It is simple algebra and doesn’t require a rocket scientist to calculate. You ever hear of the airline metric “cost per available seat-mile” (CASM)? It’s the same thing – simply the airline’s total costs spread out over the capacity put out onto the market. It sure sounds fancy but it all comes down to simple math.

As a recreational pilot, I tend to prefer to fly medium haul (300-500 miles) distances than driving, especially for weekend trips. For one thing, it keeps me current and proficient. The other is that I save a bunch of time traveling. So while I pay a premium to rent an aircraft from the local flight school, along with fuel and landing fees depending on the trip, I find the time saved to be well worth it. For example, I recently flew from Ann Arbor to Indianapolis for a weekend with three other friends. The total roundtrip cost was $481, and the total travel time over the weekend was about 3.5 hours. We all paid $120 each. Compare that to 9 hours of roundtrip driving time over a three day period, and I think it’s actually a pretty good deal, despite the higher cost.

But where is the crossover point? At what point does it make more sense to fly an airline or just drive yourself? What are the factors and variables that determine this decision? This can definitely be quantified. That’s right, ladies and gents. There’s a model coming your way.

As I said before, this is not rocket science. Building a model just means being aware of the important factors and writing some formulas. So I created an excel model that calculates the total roundtrip cost of trips in 50 mile increments, up to 2000 miles, for driving, airline flights, and flying yourself. The “fly yourself” case is for a rental aircraft only, and does not apply to owned aircraft which would require additional factors. The outputs are two metrics: total roundtrip cost per person and the percentage of total trip time depending on the mode of travel. However, to get these two outputs, there are 15 variables, such as travel time, fuel cost, aircraft rental rate, and others. You can download the model here, and there are notes in some of the cells that describe the variables. Also, the second “Summary” tab includes live versions of the charts I show below.

So what does the model tell us? Let’s assume that I’m flying the plane I typically rent, a Piper Arrow, and that I have three friends to come along with me to fill the seats (this is the default case in the download link above). For a weekend trip (54 hours), how does the cost of flying myself, driving, and purchasing an airline ticket vary with distance?

As you can see, driving will always be cheaper than either mode of flying. No argument there. Furthermore, when comparing just between flying yourself and taking an airline, flying yourself is cheaper up to just about 1,600 miles roundtrip. That’s a roundtrip from Ann Arbor, MI to Daytona Beach, FL! But still, why fly when you could drive and saves hundreds of dollars?

Well, time is money. You might save hundreds driving to the sunshine state from Michigan, but you’ll dedicate a lot more time travelling as a percentage of the total trip. Flying is obviously the faster method of travel, and flying yourself actually saves more time overall up to about 750 miles roundtrip. This crossover point is much lower than the 1600 miles in the first chart, so there is a gray decision area between 750 miles and 1600 miles. What premium are you willing to pay for an airline to save some time? Is the saved time worth it?

Also, the time argument begins to break down for longer trips. The example above is for a weekend trip, which I have at 54 hours (48+6 hours on Friday night). For longer, say week-long trips, your travel time as a percentage of the total trip gets smaller and smaller, so maybe you would be willing to take that 17 hour drive to save money instead of flying. If the time savings are negligible, I’m willing to bet that people would make a decision based principally on price.

Again, the math here is nothing groundbreaking. I’ve just always been curious about the quantitative decision points between these different modes of transport. However, this model can be used as an effective decision-making tool. Trying to convince your friends that flying yourselves is more cost and time effective than driving for a weekend trip? Do you need to evaluate if you should begin renting an aircraft for business travel rather than driving or purchasing expensive airfares? This is the tool for you. It currently does not model aircraft ownership costs, but that may be a new feature in the future.