This applies if you're well-off, but the less wealthy don't get a choice in mobility. If you don't have money, a secure job, housing, car, etc, mobility is quite limited, costly, and time-consuming (if it exists at all). Others' increased mobility can even hurt the less well off (see: white flight). That hurt then becomes a burden on everyone, as it manifests as poverty, crime, unemployment, over-reliance on public services, and so on.

But increased accessibility reduces those problems, because it becomes less difficult to do things people need to do, regardless of socioeconomic status. You have the option of being mobile, but you don't have to be.

Now, getting more specific to the OP: traffic congestion is due to inefficiency. Adding lanes/roads doesn't address the source of the issue: cars are inefficient mass transportation mechanisms. Traffic jams only occur when large masses of people are transporting themselves. You can either A) add more mass transit, or B) bring the services closer to the people, so they travel less.

The latter helps the traffic jams, but it also helps people who don't even have an option to be in the traffic jam. Mobility helps the well-off, accessibility helps everybody.

(downvote away, you know i'm right)