In many instances removing or repurposing lanes (road diet) decreases congestion much more effectively than just adding capacity.

Congestion occurs naturally regardless of capacity due to fluctuations in flow (jitter) more often than being over capacity (this is civil engineering 101). By repurposing lanes for dedicated turn lanes, bus lanes, bike lanes, and/or pedestrian walkways, you remove many of the triggers for jitter.

Synchronizing traffic (connected autodrive?) would do much more for removing congestion than just adding high over head capacity.

https://highways.dot.gov/safety/other/road-diets/road-diet-c...

Induced demand is a real, observed phenomenon for both roads and public transport. However, because of density differences, it is generally easier to increase service frequency for public transport than to add lanes for low-density road traffic.

Empirically, this pattern is observed repeatedly.

> If you believe that adding lanes doesn't reduce congestion, then you must also believe that adding transit doesn't reduce congestion.

The problem with road congestion is the number of personal vehicles inefficiently carrying 1-2 people while taking up a lot of space on the road.

In any situation where the roads would be clogged with personal vehicles an effective transit network will be carrying dozens or even hundreds of people per vehicle in vehicles that may not even be sharing the same lanes and thus can't contribute to road congestion.

1. Like others said - induced demand. You add one highway lane - more people will jump into cars to use it. You add tram/metro line instead - same people would like to use that instead of sitting in traffic jam, taking away traffic from the highway.

2. Continuing from first point - see linked graph in parent comment. Light rail has an order of magnitude capacity per lane compared to cars. For 20 lane highway you'll need 2 lines of light rail or 1 lane of "heavy rail" (as per graph). Insane difference!

3. There's a reason "one more lane" is a meme: https://knowyourmeme.com/memes/one-more-lane-bro-one-more-la...