IIRC the trend is observed over longer periods, where it feeds into people’s decisions about whether to have a car or second car, and how far to commute. And even then, congestion and travel time are two different things - an extra lane probably makes no difference, but a new route or higher speed limit will get you there faster regardless.
This is definitely not always true. They added a lane in my city a few years ago and my commute went from 45 minutes to 30 minutes.
IIRC the trend is observed over longer periods, where it feeds into people’s decisions about whether to have a car or second car, and how far to commute. And even then, congestion and travel time are two different things - an extra lane probably makes no difference, but a new route or higher speed limit will get you there faster regardless.