Cars are regressive taxation
Okay, technically "cars" are't regressive taxes. Our focus on them is. That we prioritize and promote facilities overwhelmingly for private automobile use is a cost we impose on everyone, but for those who are poor they are a regressive tax.
Let's start with the cost of a private car. Even a beater that doesn't fall apart immediately is going to cost $2,000. You'll be lucky if it lasts even a few years (and it certainly won't without a lot of maintenance). But let's give it five years, or $400 a year. Maybe you'll only spend half the car's cost into it in repairs over the same period. These repairs you always have to prioritize over other household costs because you need the car to get to work. So that's another $200 a year. This is completely unrealistically low, but I'm trying to be conservative.
My family's insurance on a six year old car is nearly $1400 a year. I doubt you can get legit insurance that doesn't leave you open to massive liability for less than a $1000 a year in my area, so let's go with that nice round number. A cheap beater is, fortunately, going to have a pretty cheap yearly license and registration fee, maybe even less than $50 despite Sound Transit 3 taxes in there.
Gas is obviously an ongoing cost. An old beater is unlikely to have good gas mileage (it certainly won't be electric and non-carbon-emitting, nor are you going to find a running electric used car for anything less than $15,000). So let's guess 25 miles to the gallon as a compromise between cheap, small car that's naturally a bit more efficient and old and prone to inefficiency due to repair issues. Going along with that, let's assume a small 10 gallon gas tank. That's 250 miles a tank which gives you 8 miles a day a month if you stick to one tank of gas. The cheapest gas around here is at Costco – typically $3 a gallon – and let's just go with that because I like nice easy to multiply numbers (conveniently located gas stations that don't require an annual membership fee run closer to $3.30 a gallon or higher). A tank is thus $30 and we are filling up only once a month so $360 a year.
Total costs per year: $1950.
Savvy readers will immediately note this is already less more than the full (unsubsidized) cost of a monthly transit pass.
If you work a $15/hour job, 40 hours a week, 50 weeks a year (you're not getting paid for two weeks of vacation and unlikely to get paid to work every week), that's $30,000 a year before taxes. The car you need to get to work costs you 6.5% of your gross income. As your income goes up, this percent would go down. Of course, in practice people buy more expensive cars and drive more, but at least arguably they could chose cheaper options and as you go into higher and higher incomes even a pricier car is less of a proportional cost.
But why did you need a car? You needed a car because other options – trains, bus, biking and walking – were inconvenient, unsafe, too infrequent or too unreliable. The less expensive public options didn't go where and when you needed to go for you to get to work. So you have to get a car. Instead of paying money to a local or state organization that maintains and improves access for all, the money you pay for a car goes to the car industry and the oil industry. The latter of which of course just spent $31 million to block their ability to pollute and harm you and your children.
In practice, many poor people (see note) make do without cars. Instead they "pay" in the form of time and inconvenience. They arrange their lives and jobs so that long, unreliable transit trips don't get them fired (or deal with upset managers and changing jobs). They spend hours walking or on a bus that they could be working or spending time with their families. They risk biking on unsafe streets next to cars going four times as fast because it's faster and more reliable than the bus. While people in private cars zip by on over-large streets, they wait for 120 second light cycles in the rain to get to the bus stop. With rents in Seattle so high, many poor people choose to maintain their car over even paying their rent because you can at least sleep in your car.
But it doesn't have to be that way. We built it this way. We can build it other ways. We have sixty foot wide roads where we say there's no room for accessible sidewalks, transit priority lanes or safe bike facilities, often to preserve free parking and underused travel lanes. We spend hundreds of millions on spot projects to "improve" a small interchange or section of road, but leave out sidewalks and bus lanes. We are spending billions on a tunnel highway in Seattle that only people in private cars can use.
Over the many decades since cars became ubiquitous we have built our places to require a car if you want to fully participate in society. We see the amount we spend on it as normal and a given because we "always" have. It's not a given. We can change.
Note: I talk about "poor people" in the abstract. I was poor growing up and for much of college and a bit after. This is not abstract to me. There is probably not a day that I do not think about how different my life would have been if I hadn't been lucky enough to get a tuition waiver to a state university (something the state of Arizona did at the time), a degree in computer science and eventually a job programming computers.