One of the pushbacks I got about my post on road boondoggles is that I didn’t control for passenger-miles of travel, and the number for car subsidies is much lower when one divides it by the appropriate number of trillions. This is not the first time I hear people talk about passenger-miles as a measure of inherent worth, but it doesn’t make it any better.
Passenger-miles don’t vote. They’re not a unit of deservedness of subsidy. They’re one unit of transportation consumption. They’re like tons of staple as a unit of food production, or calories as a unit of consumption. We don’t subsidize food based on cents per calorie.
Even as a unit of consumption, there are flaws in passenger-miles as a concept, when it comes to intermodal comparisons. The reason: at equal de facto mobility, transit riders travel shorter distances than drivers. It’s very obvious when comparing total passenger-miles in transit cities and car cities (see e.g. page 36 here). Transit is slower than driving on uncongested roads, but has higher capacity than any road. In addition, transit is at its best at high frequency, which requires high intensity of uses, whereas cars are the opposite. The result is that transit cities are denser than car cities – in other words, need less passenger-miles.
What passenger-miles are more useful for is measuring intercity transportation. At intercity distance, mode choice has less influence over travel distance (though, even then, HSR and driving are shorter-range than flying, and thus passenger-miles can overstate the importance of flying over ground transportation). It is also a proxy for revenue, whereas on urban transit the fare is either flat or weakly dependent on distance. As a result, intercity railroads usually cite passenger-miles or passenger-km, and urban transit operators usually cite passengers.
But when it comes to local transportation, it doesn’t work very well. A country’s mode share expressed in passenger-miles is lower than that expressed in passengers, and this is going to make transit and especially walking look much less significant than they actually are.