When we analyzed driving versus flying from San Francisco to Los Angeles, we used these calculators and sources.
Using the AAA TripTik travel planner , we calculated driving time from San Francisco to Los Angeles to be five hours and 35 minutes, which we rounded up to six hours.
To find the cost of the drive, we started with the total trip distance from the AAA TripTik, which is 380.2 miles. This we multiplied by $0.54 a mile, the AAA estimate of the cost of maintaining an average car driven 15,000 miles a year, including fuel, in 2009. This came to $205.20, which we divided in half, assuming that the car would carry one driver and one passenger, to get $102.60 per person. For costs specific to your type of car and driving habits, see the report AAA Your Driving Costs 2009 .
DRIVING: GREENHOUSE GAS EMISSIONS
To find the trip's greenhouse gas emissions, we first calculated the number of gallons of gas it would consume. We multiplied the trip distance of 380.2 miles by the average cost of fuel reported in AAA Your Driving Costs 2009, $0.10 a mile, to get the total cost of gas for the trip, $38.02. This we divided by $2.30, the average price per gallon in late 2008 when the AAA report was written. The result was that the average car would consume 16.53 gallons of gas during the journey. Driving 380.2 miles on 16.53 gallons, a car gets 23 miles per gallon. Using the Environmental Protection Agency's Personal Emissions Calculator ,we found that the car would produce 337 pounds of greenhouse gas emissions (represented in carbon dioxide equivalents , since burning fuel produces a variety of gases with varying impact on global warming). As with the cost, we divided the emissions in two to get 168.5 pounds of carbon dioxide equivalent produced per person in the car.
To find the risk death during the trip, we looked to the Federal Highway Administration  for the number of vehicle miles traveled in California in 2007, a whopping 328,312,000,000. Then we divided this number by 2,708, the number of passenger and driver deaths, plus the small number of fatalities whose relation to the crashes that killed them was unknown in the state in the same year. Fatality data came from the National Highway Traffic Safety Administration . The result was the chance of death per mile traveled in a car in California, 1 in 121,237,814. To determine the risk over 380 miles we used a Poisson function calculator  and found a 1 in 319,047 chance of one or more fatalities on the trip.
FLYING: TIME AND COST
We calculated the flying time and cost by choosing the cheapest one-way flight we could find, one month in advance of its departure, on the AAA Travel Planner . To account for wait and travel time before and after the flight we added two extra hours.
FLYING: GREENHOUSE GAS EMISSIONS
To find the flight's contribution to greenhouse gas emissions we used the TerraPass Carbon Footprint Calculator  and averaged the pounds of carbon dioxide equivalent  (CO2e) produced for one passenger flying a one-way trip, economy class, on the airlines traveling the route. At the time of our survey, those airlines were Alaska Airlines (154 pounds CO2e), American Airlines (198 pounds CO2e), Southwest Airlines (202 pounds CO2e), United Airlines (172 pounds CO2e), and Virgin America (177 pounds CO2e). The per-passenger figure was 181 pounds.
To find the risk of death while flying, we divided the number of commercial flights in the United States by the number of fatal crashes, since it is very rare for anyone onboard to survive a plane crash. We started with 94,467,463, the number of flights for the years 1999 to 2008 as listed at the Research and Innovative Technology Administration's Bureau of Transportation Statistics . This we divided by 12, the number of fatal accidents on such flights during the same years, found on the Web site of the National Transportation Safety Board . Our result was a risk of 1 in 7,872,289.
Photography courtesy PDPhoto.org 
This article was first published in March 2010. Some facts may have aged gracelessly. Please call ahead to verify information.