Well sure you can. We know how much it costs, the budgets are public. Completely zeroing out fares would be a single-digit percentage of the government budget. Meanwhile it would save the public money on net, because collecting the cost as taxes has lower overhead than operating a parallel fare collections infrastructure. And it benefits drivers by giving them exactly what they've always wanted -- an incentive for other people to use mass transit:

https://theonion.com/report-98-percent-of-u-s-commuters-favo...

> Instead, we should be honest and price it at the full 100% recovery rate, with 100% capital cost return. People will then start to think: "Should I continue paying that $20 per trip on a light rail, or should I get a car?"

Which is exactly the problem with your plan. If you build a rail line and set the price at $20/trip then people don't use it, so the amortized cost of the rail line becomes $30/trip because you have to pay for all the same fixed costs with fewer riders. But a $30 fare reduces ridership even more and soon there is no mass transit which in turn makes it suck to drive because there are too many people in cars and your commute is 20 miles in two hours.

Whereas if you set the price to zero, the actual cost per trip which is now being covered by taxes comes out to $4/trip, because at lower cost you get higher ridership and more usage to spread the fixed costs over. Which in turn means less traffic congestion on the roads for the people in cars.

> I'm all for sponsoring car purchases for poor people and/or giving them money to buy transit passes at full cost.

You're all for subsidies as long as they're paying the full cost? Subsidies are the thing where they're not paying the full cost.

Moreover, you want the same incentive for everyone -- if a free fare would get someone at the 70th percentile income to take the subway instead of a car, give it to them so they do that.

The converse where you use means testing is not only bringing in high administrative costs, it creates a poverty trap where making a little more money causes you to lose the subsidy and thereby removes your incentive to do it. Means testing is effectively a scheme to impose high marginal tax rates on the poor.