Dear Tom and Ray:

In January, I visited the California Academy of Sciences in San Francisco. I parked on a slightly sloping street in Golden Gate Park, locked the car and went off to the museum. When I returned to my car, I noticed a chunk of a tree under my front tire and a note on my windshield that said my car had started rolling down the hill. Apparently, I didn't turn my wheels to the curb, didn't put my standard transmission in first gear and didn't set my parking brake. What are the chances of some people noticing my car moving, stopping it, having the piece of wood close enough to put under the tire and (this is the amazing part) be willing to do all of that?

-- Barbara

RAY: Well, you don't say whether you left the car in ANY gear. Obviously, if it was in neutral, it would have rolled quite easily. But even if it was in a higher gear, the force of gravity could have overcome the resistance of the compression in the cylinders and allowed your car to move.

TOM: It's also likely that it moved only a few feet at a time. As gravity begins to overcome the resistance of the compression, the crankshaft turns, and the wheels turn with it. But after one revolution or so, it might have stopped again, and held the car for a while. Cars left in gear often will begin to roll this way, until they build up momentum and then have to be stopped by another car. Or a redwood tree.

RAY: If someone saw your car make that first move of a few feet, it's entirely possible for him or her to have grabbed a large nearby branch and stuck it under your wheel.

TOM: Another possibility is that your car rolled into the car parked in front of it. And the person who owned that car had to figure out how to get out without then feeling responsible for letting your car roll into the Lexus LS 460 a little farther down the hill.

RAY: In either case, you've learned an important lesson, Barbara. When you park on a hill, taking just one measure to secure your car is not enough.

TOM: Right. It's not enough just to put the car in gear. It's not enough just to use the parking brake. It's not enough just to turn the wheels toward the curb.

RAY: And it's certainly not enough to do none of them! You need to have redundant systems in place, because sometimes one of them fails. A parking-brake cable can break. A clutch can slip. A curb can be low enough to allow a car to roll over it.

TOM: So make a habit of doing all three. And thanks to the good Samaritans out there who helped Barbara, and the nearby Lexuses and redwood trees that day.