It's convenient to think about only the out-of-pocket cost (fuel) since the other things will only show up over several months to a year. It's true, though, that driving costs more than just fuel. AAA costs are probably a little high since their fleet still includes a lot of high-cost vehicles, but a likely range is from around 35c/mile (my small, paid-off car) to 80-90c/mile (a fully-mortgaged Hummer, say). If you figure 2 gallons of gas, that's a 30-50 mile trip depending on your gas mileage. In my region, that's an hour (AM peak)/1 1/2 hours (PM peak) on the road (assuming nobody has an unscheduled meeting on the road) or 1 1/2 hours with light rail (assuming drive to train, 20-25 miles on it, and short walk downtown) - minimal difference in the afternoon. If you can score a carpool to use the HOV lane save maybe 5-10 minutes - roughly what a limited-stop train would save if they ran one. Some corridors have commute buses that use the HOV lane and run in about standard driving time even with stops at the home and downtown ends - no-brainer except for the higher fare.

So using transit really doesn't burn a lot more time than driving except in the morning, when you can use the extra time to nap or read something (usually not too many trolls on the train at that time of day). It does save a lot of vehicle expense - your driving trip is cut at least in half which helps on gas even though in-town mpg will be worse than even congested freeway. If you're close enough to walk or bike to the light rail or bus (say, 1/4-1/2 mile walk or 3-5 mile bike) you can leave the car home several days a week and get a head start on the gym. On days I drive to the local station, I look at it (rose-colored glasses, I know) as Really Cheap Parking with a free shuttle since I get a substantial employer subsidy on the monthly pass.

The real problem is that transit (especially rail) doesn't go everywhere, but cars do. If you live and work in the transit corridor and your work times fit the schedule reasonably well, you have no business driving all the way. If that condition doesn't apply, you drive, and the condition does NOT apply for many many people. Even one transit transfer (bus/bus or bus/train) will add at least 1/2 hour to the trip probably making it uncompetitive until gas prices top $6.