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Comment by laurencerowe | Hacker News Reader

Most of the city buses in my UK home town are now battery electric double deckers. These carry up to 85-90 people (with 25 standing), weigh 11 tonnes, and is 10.5m long.

A typical jaguar Waymo usually carries up to 4 people (usually just one), weighs 2.1 tonnes and is 5.2m long.

So the double decker bus should be more environmentally efficient with maybe 5 passengers making independent journeys, though operational costs for paying a driver probably make the Waymo more financially efficient in that situation.

I suspect robotaxis will become very important in suburban areas with distributed jobs where buses don’t work well. In busy cities there simply isn’t the roadspace to replace buses with robotaxis (or any other form of car) for commuting.

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The real question is what's the optimal size for a bus if you don't have to pay a human to drive it? Once the technology is proven, there's no reason you can't put the Waymo or other self-driving technology in a bus. A bus designed for a dozen people instead of 30, and a fleet of them that did point to point rides instead of a fixed route, just all booked via the app so you share rides with other people. I dare say that would be the best of both worlds.

> The real question is what's the optimal size for a bus if you don't have to pay a human to drive it?

It will depend on the area. Where I am in San Francisco we have articulated buses designed for 100+ people running every few minutes above BART. (We probably need more BART stations.) There’s simply not enough road space for smaller buses (let alone cars) to carry commuter traffic downtown.

(I’ve never worked downtown though so always drove to work in desolate Silicon Valley office parks.)

Even in my smaller UK home city (250k people) you need double decker 70+ capacity buses running every 15 minutes on two routes from city centre to university to handle demand because the jobs are not so distributed.

Road damage scales hypercubically with axle weight, so an 11 ton 2-axle bus does ~1400 times more damage to the road than a 1.8 ton Tesla.

https://en.wikipedia.org/wiki/4th_power_law

This is the right way to think about it. Larger buses make sense when there are a lot of people going to and from the same places at the same time, but I think that is much more rare in my city after remote work became widespread.

> A typical jaguar Waymo usually carries up to 4 people (usually just one), weighs 2.1 tonnes and is 5.2m long.

A more reasonable taxi for single riders is going to weigh around 1.5 tons (e.g. the prototype Tesla Robocab).

You also incorrectly assume that the efficiency depends linearly on weight. It doesn't, it depends more on aerodynamics, speed, and the number of stops. London buses (11 tons) average around 2.2 kWh/mi, Tesla Model 3 is at 0.21 kWh/mi while weighing 1.8 tons.

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