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Comment by illumin8 | Hacker News Reader

> There's been a lot of tax breaks/money that goes into these programs too, and that's the tragic thing, since that funding should be going to known solutions: public transportation (trains, trams and express buses).

Citation needed.

> Even if you had a full-autonomous only Interstate where each car could be fully loaded and ride bumper-bumper at over 100kph, it wouldn't even begin reach the carrying capacity of currently existing fully autonomous subways systems like those in Singapore.

Citation needed.

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From the comment guidelines:

> Please don't post shallow dismissals, especially of other people's work. A good critical comment teaches us something.

Handling 30k passengers per hour is manageable for most rail lines around the world.

Torontos Bloor line does 500,000 daily.

Similarly highways are constrained to their own maximums per lane, and counterintuitively raising the speed limit doesn't help. 1500-1700 vehicles per hour is a fair estimate.

http://buckholztraffic.com/UNF/Data%20Analysis/2%20Lane%20Hi...

He posted unsubstantiated claims about tax breaks (I'd like to learn where there are tax breaks for autonomous vehicles) and a claim about trains having more throughput than an interstate with cars fully loaded moving 100kph bumper to bumper.

Asking for citations might be a shallow dismissal, but stating hypotheses without facts to substantiate them is also shallow.

Did you click on the link? It goes the the article I wrote, which has a ton of sources at the bottom.

That's true, but taking the thread further in the wrong direction only makes it worse.

There are other ways to ask for more information, that open discussion up rather than stiffening into confrontation. Please post in that spirit instead.

A train or subway car can carry at least 10 people per meter vehicle length, while a car is usually some four to five meters long and usually carries only one or two persons. Of course, the comfort factors are decidedly different.

He said an interstate fully loaded. A fully loaded interstate has 10 lanes (5 on each side), and with 10x cars, each 3 meters long with 5 passengers, that's 16.6 people per meter.

Moving 100kph continuously bumper to bumper, not having to stop to let passengers on or off, my bet is that the interstate has more throughput than a subway train.

But how many rails can you fit in the same envelope as an interstate?

None, because trains can't handle the grades that interstate highways generally follow...

I've yet to hear of a subway where there is no space between consecutive trains, however. If there's only one meter of train per hundred meters of track...

That's true, but it doesn't resolve the disagreement. Consider the fact that trains aren't infinitely long, and so they have to slow down everyone to let some passengers off. There's an idea where a train "car" is boarded and accelerated separately then joins the rest of the train while it's moving, but this approach is rarely implemented for many reasons.

> and so they have to slow down everyone to let some passengers off.

So do roads, slow downs happen when cars get on or off of highways.

Going from a road with a high speed to one with a lower speed, if those roads are at capacity, will always result in a backup. Off ramps serve this purpose of course, they basically convert cars going at 60mph into traffic going 30mph through stop lights or some other sort of flow control, but if the roadway is at capacity (as many major cities are), that off ramp as a flow control mechanism falls apart and cars pile up onto the freeway.

The math is unfortunately independent of self-driving cars. If every car is self driving capacity does increase, cars can self coordinate and some of those stop lights become unnecessary, but if 100 cars want to get onto a road that has room for 20, someone is going to have to wait!

Subways naturally don't have this problem!

This argument seems to compare 1 interstate hwy to 1 rail. In fairness to the autonomous vehicles, there's many road routes to most places. Road infrastructure costs a tiny fraction of rail infrastructure.

Roads also allow for multiple lanes. This allows for somewhat degraded service when something goes wrong; a train stopped on the tracks blocks everything. With multiple lanes, roads can be maintained while still in service. Every subway struggles with maintenance windows.

I'm not saying one is always superior to the other - trains have their niche, but it's a niche that will never be as broad that of autonomous cars.

> Road infrastructure costs a tiny fraction of rail infrastructure.

Not necessarily.

Rail is cheaper to maintain, and there is a huge opportunity cost of paving over a cities most valuable real estate to make roads.

The math on how large the streets would need to be in downtown Seattle to alleviate traffic end up with a giant % of downtown Seattle being torn down and paved over.

The Urbanist has a good article (https://www.theurbanist.org/2016/10/12/brt-is-not-cheaper-th...) on why light rail is cheaper than a huge freeway.

Rail can also have multiple tracks, it costs more, but amortizes really well.

Not having to intermix pedestrian, bicycle, and cars is another huge win for, well constructed, rail projects. Autonomous cars still have to stop at crosswalks! In urban cores, stop lights seriously cut into average MPH for a trip, and start shifting things into favor of rail.

We definitely need at least four train tracks for mass transit to work. Two tracks each way: one express and one local.

What I don't understand is why do autonomous self driving cars and subways have to be either or. Why can't we do both? After all, we can't use the subway for ambulances and fire trucks so we still need to build some kind of roads even if we had full confidence in the subways...

Just thinking out loud... Unless we had helicopters for emergencies?

Can we have a town with no paved road? Only sidewalks? That's still paved, right? Can trash collection also take the subway?

With true self driving technology it's plausible the cars on the road will be notably different than what we have today, which might significantly increase the roadway carrying capacity.

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