When comparing costs of various modes of transit, units measured “per passenger-mile” are very common. It makes sense intuitively – people take trips of varying length, and longer trips are more expensive than shorter trips, so the desire to standardize and compare makes us want to simply divide the trips by their length and call it even. Both supporters and opponents of light rail use per passenger-mile costs and subsidies to justify their positions, the government keeps tabs on them, and Randal O’Toole at the Cato Institute has even used carbon emissions per passenger-mile to claim that cars are more environmentally friendly than rail.
The problem with measure at the rate of distance traveled is that the purpose of transit is not to travel long distances – these are not pleasure travelers trying to get as far from home as possible, but rather commuters trying to get to wherever their jobs and schools are located. But the distance to this “somewhere” is not a variable to be held constant – it actually varies with population and job density, which is highly correlated with mode of transit. Places with train lines generally have and allow for denser development and thus less distance between your house and your workplace or school – the difference in average commute distance between urban and exurban areas could be as much as an order of magnitude.
Measuring costs in terms of total costs per person and not per passenger-mile makes intuitive sense when you think about it in terms of personal finances. If you lived in Brooklyn and your office moved from Manhattan to somewhere out in suburban Long Island and your transit expenses rose from $89/month for a Metrocard to $300/month for the cost of the car plus insurance, gas, and upkeep, it would be hard to trick yourself into thinking you’re saving money. Your cost of commuting per-mile might have fallen, but you have to travel more miles to do the same task. This would obviously apply to your carbon footprint as well – emitting two pounds of CO2 to travel four miles by car is still more polluting than emitting one pound to travel one mile by bus.
Walking is also possible in high- to medium-density, transit-accessible areas, which reduces some numbers in the denominator of the cost per passenger-mile equation to 0, further heightening the disparity between total cost and cost per mile. And while walking is rarely an option for high-income earners commuting to work in cities, it is often an option for both quick shopping trips and low-wage local employment. The short car rides to the store to buy some milk and short car commutes to the local fast food restaurant to work that take place in the suburbs are within easy walking distance for most urbanites, the use of wheeled transportation being eliminated entirely from some trips while still retaining the same basic utility.
Of course commuting by car and by transit are not perfect substitutes. Cars can be more comfortable, walking gives you more exercise, and riding a train or bus allows you to devote more of your attention to other things. Some people have scenic drives to work, but some people enjoy the vibrancy and people-watching of walking to and then taking the train to work. People enjoy these activities in different proportions, but financial costs are a huge component of most people’s decision-making calculus, and if individuals aren’t going to be making decisions with their own money, then the government and commentators should at least be accounting for these subsidies and environmental costs properly.