Often, when we talk about all door boarding, we talk about the real and potential time savings. Muni, in San Francisco, started experimenting with all door boarding, and it turned out it worked really well, and they went system-wide, and it has saved passengers time. According to their final report, it saves 1.5 seconds per passenger boarding or alighting, and speeds overall vehicle speed by 2%. 1.5 seconds does not seem like a large number, but it begins to get a lot bigger when aggregated over a large number of passengers.
SF Muni and the MBTA have a similar number of surface passengers: about 500,000. (The T has about 400,000 bus passengers and another 100,000 or so surface boardings of light rail; looking at only surface lines, the T and Muni are actually quite similar in terms of size.) So, if we can save 1.5 seconds per person—we'll look only at boardings, since many trips either end at a terminal station where all doors are used or are surface Green Line boardings that end in a tunnel—we wind up with 750,000 seconds saved per day. This is, rounded down a bit, 200 hours saved. The cost of operating an MBTA bus is about $163 per hour, and for a light rail vehicle $250 per hour. Let's assume that half of that is direct operating cost: operator wages and such. Assuming the lower bound, it would save $16,000 per day. Even if there were no savings on non-weekdays, in 250 weekdays it would equate to operational savings of $4 million.
Savings add up, quick.
Let's look at it a different way. A full, two-car Green Line train in the morning carries approximately 300 passengers. On the B or the D lines, the surface portion of the route takes 32 to 34 minutes to run at peak rush hour (according to the T's scheduled time). Saving 1.5 seconds on each of these boardings, would equate to 450 seconds, or 7.5 minutes: more than a 20% savings for the above ground route. With the addition of signal priority on the B line, you could be looking at speeding the route by 30% or more—a game changer for one of the slowest—and most heavily-used—surface lines.
The transit planners will say "well, these savings will probably just be added in to headway recovery time." First of all, if you actually do realize a 7 minute saving, you're talking about an entire rush hour headway, so I doubt it will all disappear, unless you are going to be lining up multiple vehicles at rush hour. But second of all, if these wind up making headway recovery times much more even, that's great. That means you'll have the same capacity without having to dispatch a train as soon as it arrives, but rather on even headways. This is likely to reduce the number of vehicles that wind up bunching, overloading and slowing down.
But let's run with the $4 million figure. There are, on buses and the Green Line, probably about 1500 doors that would need car readers. If a pole-mounted reader costs $4000, the system could be paid for in a year and a half. Or if the system were assumed to last five years, you'd have $20 million to put towards the cost of the readers ($6 million) and additional enforcement ($12 million, or $2.4 million per year).
Oh, and customers? They'd get a faster ride. It's a win-win, for everyone. Except the few curmudgeons who are less concerned with how the vehicles run, and more about the anecdote about the person they once saw jump a fare gate.