Aron Levy makes a case against using passenger-miles travelled as a measure of a transportation projects’ values:
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.
This is true, using passenger-miles to compare transportation projects will make transit and walking projects compare less favourably to driving. It’s not true just that passenger-miles should not be the goal of a transportation project, something I’ll call but that passenger-miles are actually undesirable!
Of course freedom of mobility is good and allowing people to live where they want and work where they want is great, but most people would rather not spend large amounts of time travelling. And with motorized transportation, most other people sharing the mode with you do not want everyone else travelling as long a distance as possible. That’s how congestion comes about, or how you get to the video above.
Now obviously, the goal of a transportation system shouldn’t be to simply reduce travel, but a policy that included transportation and land use that worked toward a goal of people spending less time in vehicles would be a good thing.