Dear Umbra,

Your recent column suggested that the questioners sell one of their two cars, but I can’t help wondering how much good that does for the environment, especially weighed against the annoyance cost of not having a second car when two people have to be going in opposite directions at the same time. I have a personal interest in this, as we have three cars: mine (a Prius, used for all errands and most weekend driving), his (for commuting), and ours (a four-wheel-drive minivan, which we use when we have too many people or too much stuff for the Prius, when our road is bad in winter, or when our teenage daughter has my car and I have to get somewhere).

I wish we lived somewhere with public transportation or where the roads were safe for bicycling, but while I work on that, my three-car approach seems smarter than going back to just the minivan and his car. So what’s the environmental cost of merely owning a car, if there’s no extra mileage involved?

Ivy Main

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Dearest Ivy,

I generally encourage readers to get rid of extraneous vehicles, and I stand by that. But let me answer your direct question as best I can.

And behind door No. 3?

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Photo: iStockphoto

As you can imagine, a car’s environmental impacts are many and varied, and they occur all along the road of life: from manufacturing the vehicle to extracting and using fuel to keeping the ol’ jalopy clean. As a starting point, let’s assume this assessment of the environmental impacts of a car’s lifecycle is correct. (It’s a few years old, but things haven’t changed all that much.) It tells us that approximately 10 percent of a vehicle’s total energy impact comes from its manufacture. The other 90 percent or so comes through use. This is good news, in a sense, because use is something we have control over.

If, in the car-ownership equation you present, you were literally treating two cars — the minivan and Prius — as if they were one, that wouldn’t be so bad. Using a rough calculation (mathematicians, avert your eyes), let’s say the two cars combine to make a mythical mega-car — call it the Privan. The manufacture of the Privan would have totaled 20 percent of its energy impacts, while its use would still total 90 percent (split between the two cars, and assuming you never drive them at the same time). So the energy impact of your Privan would be roughly 110 percent that of an average car. Not all that dramatic, right?

Not so fast. We know that sometimes all three of you are driving: your daughter in the Prius — and good for you for being an eco-role model — you in the minivan, and your husband in his car. So all my (admittedly fuzzy) math just went out the window. And like it or not, you are a three-car family. That, dear Ivy, is too many cars.

The best reason to get rid of a car is simple: owning vehicles encourages their use. Whether you have one car or three, you are contributing emissions that cause planetary problems. If you had one less vehicle, you and your family might be inclined to arrange a ride-share, or combine trips, or go to a store that’s closer to home. You might lobby your local politicians to make the roads safer for bicycles. You might use some of the money you’d normally put into car insurance, gas, and upkeep to buy a folding bike or electric bike and brave the roads. You might join a car-sharing program. Could your husband carpool? Could you and your daughter work out a car-sharing schedule?

It’s lamentable that your area doesn’t have good public transportation, but I see that they’re working on it. With that prospect looming and this challenge from me on the table, consider dumping one or two of your cars (not the Prius). Yes, someone else might end up driving your cast-off, but it could actually improve that household’s efficiency.

Check out these handy EPA rankings for more details. And remember: transportation innovation is fun.