Gas cost more than $4 a gallon until recently, which encouraged people to drive less, buy bicycles, sell their SUVs, and consider carpooling. But is carpooling a good option? Sheldon Jacobson and graduate student Doug King tackle this question in a recently published paper in Transportation Research Part D (Transport and Environment), which made a bit of a splash.
They show that although carpooling saves gas, it guzzles time. For carpooling to become attractive, we would have to value our time at almost nothing ($4.24 for car drivers and $4.68/hour for light truck drivers). Car pooling makes sense if the carpoolers live close to one another (so they don’t waste a lot of time picking everyone up) and if they can save a lot on tolls and parking.
My husband carpools about three days a week, because he otherwise has a long commute (his work offers additional incentives for carpooling). The hardest thing about car pooling for him is the lack of flexibility in work times (he leaves early in the morning to return home at a decent time). If he isn’t driving, he can nap during the long drive to work. So if you’re a napper, car pooling can purchase two naps a day.
On a related note, a couple of years ago, Sheldon Jacobson and and I predicted that if lighter people from the early 1960’s drove today’s cars, we would save 938M gallons of gas a year due to the extra weight (both men and women gained an average of ~24 pounds between ~1960 and ~2002). Sheldon Jacobson and grad student Doug King updated this research, since (1) Americans weigh even more now, (2) Americans drive even more now, and (3) Americans have grown taller. Baesd on these factors, we could now save 1.21B gallons of gas per year if people from the early 1960’s drive our cars today (an increase of 21% over my estimate).
I found a couple of things to be fascinating when working on this research:
- Although men and women take the same amount of trips per day (4.1), men drive much more than women (37.6 miles/day compared to 21.2 miles/day). This is particularly accentuated for adults aged 25-54 years old, who do the vast majority of the driving. Say what you about how much we drive in this country, but most of the driving is done by adults going to and from work. Unemployment obviously saves a lot of gas, but I can’t exactly see this as an upside to the rising unemployment rate.
- The extra weight in my car is due to car seats. Nearly every state has rigid car seat laws that require parents to lug around heavy car seats in their cars for years and years. Buying a lightweight car seat could save a lot in gas over its lifetime. One of my car seats is very heavy, and I feel guilty about this, but heavy seats often do well on the safety tests, and I am not willing to trade safety for gas.
What heavy items do you carry around in your cars that you can’t do without? Do you carpool, and if so, why?
Punk Rock Operations Research 
February 19th, 2009 at 10:34 am
It strikes me that this is a classic conflict between what is optimal for part of the system and optimal for the whole system. The people who are carpooling are looking for what is optimal for them. But they are part of the larger system (the world) and one objective for the objective for that larger system is to minimise pollution.
Incidentally, has anyone measured the difference in travel times because carpooling on crowded highways reduces congestion?
February 21st, 2009 at 10:16 pm
I have a 30-mile commute. I carpool with someone who lives about six blocks from me. I don’t think anyone carpools for financial advantage. I carpool because I want to nap on days that I’m not driving. Also, I’m an occasionally drowsy driver and so carpooling feels safer than driving alone, even if I’m driving.
Carpooling also allows me more flexibility. For instance, both myself and my carpool partner have been able to get to work on days when our own cars were in the shop or on loan to friends.