[andrewflnr]

I don't understand why public transit doesn't have exactly the same induced demand problem as highways. If there's enough people to fill up new highways, they'll also fill up the public transit... Unless the plan is to make public transit miserable enough that only outside with no better options will use it, in which case it seems like it's all going according to plan already.

[asoneth]

Increasing public transit capacity does induce demand. In areas with excellent public transit people choose to live further away from work, take more discretionary trips by transit, etc. (That's one reason I would be wary of making transit free as some politicians have proposed.)

One reason many people are more concerned about inducing driving demand is that private vehicle travel generally emits more carbon, uses more valuable land (e.g. parking, highways), and results in more fatalities per person per mile than comparable forms of transportation.

Another problem with inducing driving demand is the degree to which those costs are subsidized by taxpayers (roads, highways), other shoppers (required/subsidized parking), or left as externalities (carbon, noise). Tolls, gas taxes, per-mile fees, and parking fees would have to be quite a bit higher in most places to cover those costs.

[azylman]

The way that public transit scales to meet higher demand is different than roads. Whereas roads require more lanes, public transit such as trains can scale by either adding more train cars to existing trains or adding more frequent service. More frequent service, in addition to improving throughput, also helps everyone else using the system by making it more convenient. And, if it "induces" people to move from roads to trains, that also reduces congestion on the roads. So induced demand for rail lines is a good thing.

If induced demand is high enough even that, too, may not be enough - but then building a new rail line is at least no harder than adding a new highway lane (in most cases), and can support substantially more throughput with equal or lower travel times.

[njarboe]

Before COVID, BART in the Bay Area was completely full during peak hours. You had to wait for multiple trains to finally pack into the car. That, or take the train the opposite direction for a few stops from downtown and then get back on in the other direction. They are starting to remove more and more seats to pack in more people, but eventually if use keeps increasing you will have to build a new subway and who knows how much that will cost or how long it will take. It is probably not really possible right now. See the failure of California's bullet train.

[azylman]

Yup, this is a big reason why I said "in most cases" and not "in all cases". When your trains have to go underground and underwater and your highways go over roads and over bridges, that dramatically changes the numbers. Of course, none of those are a requirement of rail systems - just how the BART is built. There are plenty of trains that go over roads (e.g. the L in Chicago) or over bridges over water.

By the way, even BART could increase throughput today without adding more lines. Not all trains are 10-car trains, because they don't have enough cars in the fleet. Adding more cars to their trains is a significantly cheaper prospect than adding a new lane to the Bay Bridge (which was also basically fully maxed out on throughput during peak traffic times, pre-COVID). And BART carries substantially more people across the Bay than the Bay Bridge does.

So, certainly the BART needs more capacity, both now and in the future - but so do the highways.

[dekhn]

Bay Bridge 260K ppl/day + San Mateo Bridge 93K ppl/day is within spitting distance of BART (411K ppl). If there was another whole bridge across the bay (well, maybe two or three) it would alleviate the Bay Bridge and the traffic around it.

[azylman]

Building an entirely new bridge isn't going to be cheaper than adding more cars to BART. Even assuming it was (which it wasn't), it's not going to help as much as you think it is. Not sure if you've ever commuted on the San Mateo bridge, but it's basically fully backed up from the exits onto 101, because 101 is also grid-locked. So in addition to building an entirely new bridge (which, again, more expensive than adding more cars to BART), you also need to add more lanes to 101.

[dekhn]

I live right by the 92/101 interchange. Adding a Southern Crossing would change the dynamics in ways that you probably wouldn't have to add anything to 101, it would just distribute the traffic that's on 101 into better locations. https://en.wikipedia.org/wiki/Southern_Crossing_(California). For example, for the set of people who are using 92W to get to SF, the southern crossing would move them north and alleviate traffic around san Mateo.

Note that I didn't make my point to say that we should build another bridge, just the pedantic detail that BART traffic isn't that much larger than Bay Bridge + San Mateo Bridge, which is a more apt comparison. I think actually you'd want to add in the Golden Gate Bridge, another 110K. So basically, there are more people commuting into SF driving over bridges than taking BART in.

[asdff]

Increasing service is very difficult to do. For example, before the pandemic I would take the red line in LA which would be packed to the brim by the time it rolled into downtown LA, not enough room to even turn around while standing. During my commuting I would do a lot of reading about transit and about the redline in particular.

To increase the capacity of the red line would take a lot of work that would not be cheap. For instance, the length of the trains could be increased, but to do that you have to construct new stations, since the train is already the length of the entire platform, at least the ones used for rush hour. LA has actually lengthened platforms that are on the surface before to accomodate longer light rail trains, but underground this is so much more difficult.

You could lower headways from the 10 minutes they are currently at, but this becomes a physics problem fast. One issue is a lack of turnback stations at the ends of the line so the train has to come to a stop then 'reverse' at the end. Another issue is a subway with a train works like a pneumatic tube, there is a volume of air moving that needs sufficient ventilation, which is why you see these big ventilation grates on sidewalks where subways run below, and to run more frequent trains would require significant upgrades to the ventilation systems along the entire line.

[azylman]

That may be the case for LA, I'm not familiar enough with their situation to speak to it, but it isn't the case generally. There are lots of rail lines in the US and globally that have not come anywhere close to maxing track capacity. One such example is BART in SF, which could just add more cars to existing trains and see 60% increase in capacity [1]. The tracks and stations support trains up to 10 cars long, but very few are actually that long enough because they don't have enough cars in the fleet.

[1] https://www.bart.gov/about/projects/cars/faq

[mrsuprawsm]

This is a case of under-investment and poor design. The London Underground, for example, the oldest underground railway in the world, manages 2-3 minute headways (29 trains per hour) on many central lines, both sub-surface and deep level.

Of course, you need to invest in the infrastructure to make it work, but it's very possible.

A quick Google suggests that in Tokyo they even hit 50 trains per hour, so one every 1m20s, which is insanely impressive.

[andrewflnr]

I don't buy the lower travel time thing, not unless your trains or whatever are running every five minutes. In practice public transit has always at least doubled my travel time. Waiting at stops (a) introduces substantial latency that (b) is unpredictable, forcing me to pad my travel times further. I mainly use it to avoid parking at the destination.

Mostly DC metro rail and a bit of LA buses, FWIW.

[azylman]

DC and LA are both infamous for having bad public transit systems, so don't base all your decisions on your experience there. Within the USA, Chicago is really good - taking the L is way faster than cars because you get to skip road traffic and the trains come very frequently. I've heard mixed things about NYC (some very positive, some very negative, likely depending on where you're traveling from/to), but have no experience there myself. Outside of the USA, there are lots of countries with fantastic train systems (e.g. Hong Kong) that are way better than driving.

[asoneth]

Besides NYC that's been my experience as well -- US cities are designed for driving as the primary transportation method. Even mass transit in the US is often pitched as something to make life easier for drivers by lowering traffic.

Lots of places talk about the importance of walkability and mass transit but if you actually look at most transportation budgets the vast majority goes to automobile infrastructure. Even what passes for "pedestrian infrastructure" in many places is there for the benefit of drivers[1]. I mean, it makes sense because everyone in the US drives everywhere, but it's a chicken-and-egg problem.

[1] https://www.strongtowns.org/journal/2021/1/29/pedestrian-inf...

[tshaddox]

I think that technically the same problem exists with public transit. It's just that the constant factor people-moving density difference of multiple orders of magnitude gives you a lot of headroom.

Of course, there are also other potential advantages of public transit unrelated to induced demand, like pollution, safety, cost, impact on the design of living spaces, etc.

[meheleventyone]

Improving public transport absolutely does induce demand and that’s part of the point of making it better. In particular because it’s much more space efficient it also reduces congestion as people switch to it. Same thing with improving cycling facilities.

It’s just inducing demand on an already overused system like private cars doesn’t fix the system being overused unless you can get beyond the desired capacity.