A Couple Things About Gas Mileage

(Reblogged from Nathen’s Miraculous Escape, from May 28, 2012.)

I heard a segment recently on NPR about gas prices and it reminded me of the link between driving speed and gas mileage. According to the story, driving over 60 miles per hour is equivalent to paying $.25 more per gallon for each five miles an hour faster. So if you paid$4.00 for a gallon of gas, but burn it at 65 miles an hour, it’s like having paid $4.25. Seventy miles an hour makes it $4.50, etc.

It seems funny that it’s very easy to imagine Americans complaining about the price of gas, but very difficult to imagine them driving the slightest bit slower in order to save that same money. In fact, I can easily imagine an American burning a 70-mile-per-hour gallon of gas to go out of their way to save $.25 cents per gallon at the pump. (It could be a good economic decision to do so, I suppose, depending on how many gallons of gas you need to buy, and how much your time is worth, but it still seems funny.)

If I remember my physics correctly, I think the loss of efficiency is actually not that linear, that a 75-mile-per-hour gallon probably costs significantly more than the $4.75 that NPR’s equation predicts. It has to do with the amount of force each particle of air hits the front of our car with–it’s the same principle we (if we are from the desert) use to remember to drive slowly or stop during a sandstorm, to save our windshields from getting sandblasted. Any physicists in the audience care to explain the mechanics of it?

In his excellent lecture “Climate Change Recalculated,” engineer Saul Griffith tells about how he gave an intern this incredibly boring job: Drive his wife’s Honda Insight in 100 mile stretches around a runway at constant speeds, twelve 5-mile-per-hour increments from 20 to 75 miles per hour. Seventy-five miles per hour was the worst, obviously, at about 40 miles per gallon, and the most efficient speed, at about 85 miles per gallon, was 30 miles per hour.

That’s pretty slow, but three times as fast as the average driving speed for large urban areas, he points out. I’ve been thinking about making my next trip to Portland at 30 miles an hour, to see how little gas I can use to get there. It’ll take 4 hours to get there, so I’d better bring some good company.

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On Transportation

(Reblogged from Nathen’s Miraculous Escape, from December 23, 2011.)

I’ve been visiting Vancouver for a few weeks and most days we end up commuting at least once from the west side of the city to the east side and back, mostly by car, sometimes by bus. (I’ve done it by bike, too, but not on this trip.)

It’s about six or seven miles each way and takes about 30 minutes. Google maps says 20 minutes by car, and I’ve heard rumors of 15-minute trips, but I’ve yet to experience one less than 30. Yesterday, our commute was 10 miles and it took 50 minutes (extra Christmas shopping traffic, I’m told). That’s five miles per hour in the middle of the day. It was worse on the way home, at 3:30 rush hour.

I found myself quite impatient with this situation. Five miles an hour does not seem a reasonable speed to travel. I think of Los Angeles as congested, but in non-rush-hour traffic I expect to be able to get to another city in 20 minutes–from the train station in Los Angeles to my brother’s house in Glendale, for example.

The thing is, I’d be on the I-5 most of that trip. There are freeways all over the place in LA. This is strikingly not the case in Vancouver. We are on surface streets wherever we go, hitting stoplight after stoplight, very often with no left-turn lanes so traffic piles up behind each turner. Suddenly I miss all of those ugly, loud LA freeways.

Reanna and her family argue that the fact that it sucks to drive in Vancouver is an accomplishment. The more it sucks to drive, the better, because more people will use public transportation or bicycle. We fought to keep freeways out of here, they say. I was reminded of how upset my grandfather gets when he talks about the freeways in LA. The house he built was one of the houses they demolished to put in a freeway (it might have even been the I-5 that went through his house). Freeways went through the middle of neighborhoods, loud and ugly, splitting them in two. It’s very hard to imagine that happening in Vancouver, if only because the real estate is too expensive.

I am pro-public transportation, so when I’m not stuck in Vancouver traffic I think it’s a shame that LA was designed for cars. Maybe it is the relative ease of car-travel that has kept LA’s public transportation from moving to the next level — though LA, at least according to this article, is quite low in miles of freeway per person compared to other major US cities.

This situation does not strike me as a straightforward win for Vancouver, though. People still drive a lot, and in cars constantly in their least efficient mode, stopping and starting all the time. The busses use the same congested, no-left-turn-lanes roads as the cars, so they lose efficiency and speed along with them. Maybe the answer is to have the government quadruple gas prices or insurance prices to make driving a rich-person-only thing, and leave the roads for public transit. I’d much rather see public transportation that wins because of how great it is, rather than because of how crappy driving has become, but I guess I would take what I could get. Not that I could get quadrupling the price of anything related to driving even here in the most progressive part of Canada. That might be less popular than putting in freeways.

In thinking about all this, I wanted to be able to compare the transportation systems in different cities and found it quite difficult to do. We need a single-number transportation index that takes into account the average speed of travel, average energy-expenditure per mile, and how far people travel on average to live their lives in their area. People-miles per gallon-minutes, maybe, or maybe people-kilometers per joule-minute. Any economics or urban planning students out there looking for a project?