“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Saturday, July 01, 2006

Economic choices

The other day I stopped at the bank to get some cash. The particular branch I chose to patronize had a drive through window, and I watched the action for a few minutes. I saw a Hummer (H2) use the window and it set me to thinking about the motivation.

In an earlier post I gave some fairly detailed estimates of the consequences of drive through windows, estimating that if they all went away we could save about 350,000 barrels of oil per year. Clearly, this is not a huge amount. What about the Hummer pilot?

The engine is a 6.0 liter V8. My Grand Cherokee has a 4.8 liter V8 and uses about 0.38 gallons per hour at idle. It seems very reasonable to assume that fuel consumption at idle is directly proportional to engine displacement so I'll estimate that the H2 uses (6.0/4.8)*0.38=0.475 gallons per hour at idle.

I don't know what the driver's bank transaction was but let's assume that it was something she (I saw that it was a woman - no implications intended) could have done at the walk-up ATM. This matters since it is clearly worth more to avoid going inside the bank than to avoid the walk-up ATM.

I think the estimates I made for times at a window in my earlier post would be applicable here, so I'll say that she burned a net three minutes worth of fuel that could have been saved had she parked and walked up. That three minutes of pistons going up and down in cylinders used about .024 gallons of fuel which cost about $0.074. So her use of the drive through meant that it was worth at least about seven cents to avoid going to the trouble of parking and walking to the ATM.

I'll guess that she might use some sort of drive through window four times per week, so she might burn about five gallons of fuel per year while driving through, worth about $15.00. If she were to be presented with these numbers, do you suppose she'd immediately cease use of the drive through? I'm thinking that this is one of the few answers of which I can be sure. Actually, she'd have two answers: "no" and "HELL NO!!"

I wonder where her "point of indifference" would be? Surely she wouldn't use the drive through if it cost, say, $100.00 per use versus the walk-up ATM. What about $10.00? $1.00? I'm going to hypothesize that an average H2 driver would drive through for a dollar and walk up if the price differential were any higher. That would imply that a gas price of (100/7.4)*3.12=$42.16 per gallon would be the threshold of removing her from her car. There's a lot of room for gas prices to go up before certain lifestyle choices would change. I don't think drive through windows will be shuttered anytime soon.


Bill Anderson said...

Hmm on newer vehicles such as the H2 you can't assume idle consumption based on displacement. At idle, the EMS will drop the fuel input down significantly in an attempt to produce a better MPG-city rating. This is very similar to the process used during deceleration (coasting).

That said, the H2 driver can still use drive through windows and achieve a reduction of fuel consumption by literally shutting off the engine while stopped for more than ten seconds. Ten seconds is the break-even point between starting the engine and idling. To put that in English, if you idle for more than ten seconds you've used more fuel than if you had shut the engine off and started it back up again.

So it would be interesting to consider the amount of time spend in a drive-through that involved sitting still versus moving. Maybe I'll do some tests on this and post them. It'd make a nice compliment to what you've done here I think.

Finally, on the point of indifference regarding walk up ATM versus drive through, given that many ATMs charge 75 cents to a buck fifty we may have some reason to believe that it woudl largely depend on the ATM. If it is one owned by her banking institution the fee would be low and she would tolerate a higher fuel cost. We've seen the outcry of 1.50/use ATM fees, so I suspect in that range we should find a PoI.

If it were known that she could cut that cost by shutting the engine off while waiting That point would shift toward the higher end I suspect. After all, turning the key is less effort than getting out and walking. And we live in a society where people will pay $50+ to run/walk on treadmills and Stair Masters but insist in parking as close to the store doors as they can.

King of the Road said...

Ten seconds is the break-even point between starting the engine and idling.

I've done a fair amount of looking around and seen figures of 10 seconds, 30 seconds and one minute cited for this number. None were published in a way that gave me confidence in their accuracy. I'm sure it varies from vehicle to vehicle and from era to era but I have yet to read definitive information by an automotive engineer. If you are or can site an autoritative source for this figure, please elaborate.