To me, one of the most annoying features of the details of transportation policy is the persistent use of passenger-miles as a denominator in various calculations. As Alon Levy argues this is borderline incoherent and senselessly biased on favor of auto-oriented road projects:
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.
The use of passenger-miles as a unit of measures embeds the assumption that the goal of a regional intra-urban transportation system is to have people traveling as far as possible. Now you could imagine a city in which individuals, firms, structures, natural resources, etc. are just strewn about at random. If that was the case, then you probably would want to organize transportation to maximize distance traveled. People would have arbitrary transportation needs, might need to get very far away, etc. But when you’re talking about a real growing city, a focus on passenger-miles just implies a focus on spreading your urban area out as widely as possible. That’s great if you’re in the business of manufacturing and selling automobiles. And because the US spent decades crafting an industrial policy oriented around the automobile manufacturing and oil extraction industries, we’ve embedded these kind of ideas in a lot of our bureaucratic processes.