MPG nerd trivia: What happens when you add a mile of highway?

I popped in on Sightline’s open house last night and took their Cascadia sustainability quiz. I did OK, but came up pretty stumped on a couple of questions. One of them asked “How much extra CO2 gets added to the atmosphere per year when you add one lane-mile of new highway?” The choices ranged from 0 to 100,000 tons of CO2, but I don’t think in terms of tons of anything except when measuring my devotion to Marisa Tomei.

So this morning I set out to do a little Googlage and get myself knowledgable about weighing CO2.

• Step one: How many miles per year for the average US vehicle? About 12,000.
• Step two: What’s the average MPG for a US vehicle? About 24.
• Math! Eeek: 12,000 miles/gal / 24 gal = 500 gallons (Nerd tip: Always include the units.)
• Step three: Fun chemistry trivia from the US government! One gallon of gas weighs about 6.3 pounds, but produces 20.3 pounds of CO2. Chemistry is cool. So our 500 gallons of gas = 10,175 pounds of CO2, or 5 tons per vehicle per year.
• Step four: 100,000 tons / 5 = 20,000 vehicle’s worth of CO2 per year per mile of new lane.

I’ve left the 5 tons/vehicle/year in bold above. That’s the thing I think I’ll try to remember when thinking about climate change. (My output is less, but even 1.5 tons is a lot.)

Footnote: The question might have been about roads, or highways specifically, I can’t remember for sure. Also, I’m sure there are tons (ha, ha) of qualifications I should make to the numbers above, but I just wanted to get a quick estimate. ZOMG, the EPA has a more elaborate paper on this topic, and they come up with pretty similar numbers: 4.8 to 5.5 tons per year, depending on which fuel economy estimates you use.

Marisa Tomei footnote: I resisted the urge to add a photo to this post.

HOOOLY CRAP! Update. I just found the question from last night, and it goes like this: “For every lane-mile of new highway built, how much carbon dioxide will be released over the next 50 years?” Jeez, that’s a hell of a different question, and it shows how out of touch I am with carbon numbers.  20,000 vehicle-years divided by 50 years is 400 cars…. that doesn’t seem like such a big deal to me. I’m not a pro at thinking about traffic issues (surprise!), but the CO2 argument doesn’t seem particularly scary when thinking about new roads. Of course, there are plenty of other awesome reasons to avoid new roads in favor of mass transit, densification, etc., I just wouldn’t put this one at the top of the list.