Where I live (in San Francisco), there’s a definite shortage of parking. If people intend to just stop for a minute (to carry something up to their apartment, or to get a friend, or whatever), they will generally double-park, leaving their car out in the street, or pull into a driveway. The first option is clearly superior to the second, and I’m mystified that people continue to park in driveways.
Though multiple factors go into the decision of where to park, the most important one should be how likely you are to get a ticket or get towed. Convenience is also important, as is courtesy. On all three, double-parking wins.
When you double-park, you block a lane of traffic. This is generally not a problem, however, and if you’re pulled over to the side, people can easily get by. Even if you aren’t pulled over very far, people in SF are still used to it and will deal easily. Sometimes a tourist will get confused and just stop behind you, unsure of what to do. Don’t worry about them.
You also block the car you’re parked adjacent to. You may block two cars, but if you’re careful, you’ll only block one (and, if you are really careful, and park next to a driveway, the person you’re blocking will be able to get out with some effort). Given SF parking and driving patterns, it’s unlikely that the single car will be moved while you’re inside. That is, because parking is so difficult, most people move their cars as infrequently as they can, and the likelihood that the one person you’re blocking needs to move is relatively low.
If you park in a driveway, everything is inverted.
You’re out of traffic. So you have that going for you. But, as I addressed above, blocking traffic is a relatively small issue, especially compared with the prospect of blocking multiple people who move their cars more frequently.
With garage parking, there’s no penalty for using your car whenever you want, so a single person with a space is more likely to enter or exit the garage at any point (I’d say that I’m at least five times more likely to drive somewhere than I would be if I had to deal with street parking — and this might be conservative).
Multiply this by two for entering and exiting (on the street, it only matters if someone wants to leave, not come) and then multiply by the number of cars in the garage, and you can estimate how much more likely you are to be blocking someone.
We have six cars in our garage, so when someone parks in our driveway versus parking next to someone, they are 60 times more likely to block someone.
Finally, add to this the fact that people are more probably more likely to have you towed for blocking their driveway than for blocking their street-parked car, and I’m utterly mystified why anyone would park in a driveway.