My car is nearly 20 years old, with 300,000+ miles on it. Thus, minimal depreciation. The largest remaining expense is oil and oil changes, and the occasional repair. My insurance is also minimal. Moreover, there's no marginal cost to it; I'm paying my insurance regardless of whether the car sits or I drive it constantly.
Let's say that all adds up to another $4 on each trip, on average.
If I'm cheap, I drive down I-95 through Providence, and avoid all the Pike tolls.
So it costs me about $40 to do the round-trip by car. And it's usually under 3 hours.
Amtrak, Boston to New Haven, is $43, purchased in advance, on one of the non-Acela services. Add another $1.70 for the T and another $2.50 for Shore Line East. So that's nearly $50, each way, or $100 round-trip.
Greyhound to New Haven is $31 with a seven-day advance purchase. So that makes the bus alternative around $35 each way, or $70 round-trip.
I could, were I insane, take the Chinatown bus to Manhattan, then take Metro-North back from Manhattan to New Haven, then Shore Line East to my parents. That's probably the cheapest possible way. Occasionally, between Bolt and the Chinatown buses, you can get to NYC for a buck. But Metro-North is still $14, even off-peak, and there's still the $2.50 for Shore Line East, and $2 on the subway to get from Chinatown to Grand Central, and the $1.70 to take the T. Add it all up and it's more expensive than my drive and takes more than twice as long, because I'm travelling about double the distance, with multiple changes and 75 miles of backtracking. Sorry, that's nuts. My time is worth something, after all.
(I'll add here that Shore Line East leaves me a couple of miles walk from my parents', and the sidewalks end about a half mile before their place. I've walked it. The car dodging is merely annoying, not suicidal.)
In order for me to break even on the trip, taking Greyhound to New Haven, my fuel costs would have to rise to around $60. That's around $6/gallon in my car. And for my taking Amtrak to make economic sense, it would need to go to above $9/gallon. I mean, it might happen, but it hasn't yet.
The point is that it costs me money to take public transit on the long-distance trip I do most often. I want to do it. I'm a transit buff, so I even have non-economic incentives to take transit. But given the price difference, of course I'm going to save the money and drive; on average I do this trip weekly. And I need a car once I get to my parents'. It's suburban Connecticut, after all.
This is all quite broken if we want to encourage people to get out of their cars.
Is this an exceptional case? Sure. I'm going from an urban area to a suburban one, but I'm doing it in the Northeast Corridor. That's a part of the U. S. that actually has some usable public transportation. In a lot of the rest of the country, the comparison's even worse. (San Fernando Valley to Las Vegas springs to mind, from recent experience.)
But it's the case that makes the most difference to me.