[jltsiren]

Public transit depends on the assumption that some trips are more common than others. If any given person is equally likely to go to any direction, public transit becomes too expensive to build. And it becomes impossible to make the city dense without turning the traffic into a nightmare.

A typical direct bus line starts from somewhere, goes through a number of neighborhoods, reaches a major street, and follows it to a central location. The number of directions that need a bus line is typically much higher than the number of streets reaching the central location. (For example, you need ~10-degree intervals at 10 km from the center to guarantee a reasonable walking distance to the nearest bus stop.) Hence the bus lines eventually converge.

Once you have enough bus traffic that a dedicated lane makes sense, transforming an ordinary lane into a bus lane will make the traffic faster for the average person. It's not a Pareto improvement, as the traffic will become worse for those who drive on that route. But it's not a huge loss for them either. If you already have 20+ buses/hour making frequent stops during the rush hour, the throughput for that lane will already be much lower than for the other lanes.

[AnthonyMouse]

> Public transit depends on the assumption that some trips are more common than others.

Public transit depends on the assumption that there is enough density along a given route to justify its existence. Take a look at the NYC subway map. In the highest density boroughs (Manhattan and Brooklyn) the routes basically go from everywhere to everywhere. Even more so for the Manhattan bus map. That's what you want in a large dense city.

In smaller cities, the "build a high density core surrounded by lower density areas" model is the thing that causes more congestion, because then the core ends up as a bottleneck but people don't want to take transit to get there because it doesn't have frequent service to the areas outside the core at one of the traveler's endpoints. For those cities it's better to have medium density everywhere than try to make transit work in a city where a large proportion of the population is coming from an area with density too low to make it viable.

And if the whole "city" is low density, i.e. it's a rural small town, then it's not likely you're going to make public transit work there whatsoever. The best option there is to use mixed zoning so people so inclined can live within walking distance of shops.

> It's not a Pareto improvement, as the traffic will become worse for those who drive on that route. But it's not a huge loss for them either.

It is though? The premise to begin with is that road is already too congested and is slowing down the buses. Removing a third to half of its capacity is going to make it dramatically worse. That's what many of the proponents of bus lanes are after -- they want to force people onto the bus by snarling the cars.

They refer to this as "induced demand" by inverting the sign when what they really mean is to suppress demand for driving by making it more miserable, but don't want to call it that because it would be unpopular.

[jltsiren]

High density over a large area is a rare exception. Public transit is mostly used in regions that are locally dense but have low-to-moderate population density over the entire region.

Consider a low-density urban area with 1500 people / square km (~4000 people / square mile). You could achieve that with a uniform sprawl of single-family homes on half-acre lots, or with a network of towns / villages / neighborhoods surrounding the city center. The former generates more car traffic, while the latter makes public transit useful for a large fraction of trips. And the latter also makes local services viable, as there will be enough population within a walking distance.

And if you have a 2+2 lane street with enough bus traffic to justify bus lanes, most of the capacity is in the inner lanes not used by the buses. Urban buses stop frequently, making the traffic flow much worse than in lanes without buses.