> You are just joking, right? you can literally fit 10 bikes in the space of a small car, on the road

You are just joking, right? A Fiat 500 is 3.5 meters long and 1.6 meters wide. No way can you "literally" fit 10 bikes being ridden on the road in that space - or even 3. Parked, you might be able to fit 3 if they're staggered, but if you're going to tell me you can double-decker it, you won't also fit the stairs/ramp/elevator mechanism in that space as well.

> You think bike maintenance is the same effort as car maintenance?

You take it to a shop, drop it off, and leave it there, then get it back later? Yes, that sounds about the same effort. Except a car usually needs servicing and is still drivable, a bike is probably punctured or chain snapped and unusable, making it more hassle, and if you decide that means you have to do it yourself, more effort. Everyone remember how fun it is to take a bike wheel off and run it through the bathtub to identify the location of a puncture, yes? Never spent that much effort on taking my car to a garage.

> Bike maintenance is something most riders can do on their own, while car maintenance is something people take to a shop.

You've gone from "baffled by this claim" to "most people spend more effort on bike maintenance than car maintenance" in the space of a paragraph. No comment.

> What on earth are you referring to?

I'm referring to the things I said. If you didn't have paved roads made for cars, you would need to build them for bikes. You wouldn't need to build them for walking. Bikers never ever mention this, it's a kind of parasitism on the car infrastructure - a cost that bikers don't consider. If there were no cars, but we needed to upkeep hundreds of miles of tarmac roads, bikes would need to be taxed hugely. It's an externality in the sense that bikes need it, but aren't paying (directly) for it, and are offloading the cost onto car drivers (who currently need it more and do more damage to it).

> Are you including pollution in your list of externalities?

No I'm not including the cost of container shipping enough bikes for 500,000 people from China, or the cost of digging up the iron ore and making the steel and carbon fibre to build them, or the trash heaps where hundreds of thousands of bikes rot. Good point though, neither do bike enthusiasts.

> Are you including accidents and fatalities in your calculation?

The kind where a big heavy fast moving metal lump collides with a soft squidgy easily damaged slow-moving pedestrian? I'm not including those either, but I am against bikes being allowed anywhere pedestrians are, so let's add that in as well.

> I’m confused, I really can’t think of a single unmentioned externality where bikes don’t compare favorably to cars by a very wide margin.

Neither can I. That's a totally cherry-picked comparison because it's like saying "being stabbed compares favourably to being shot by a very wide margin". There's no unmentioned externality where walking doesn't compare favourably to bikes by a very wide margin - in a place which is designed and built for humans walking -- which all places should be because humans are more important than vehicles. Walking needs less tarmac, less machinery, less maintenance, less money, is more accessible to people of more abilities, takes less parking space, less infrastructure, causes fewer accidents, places fewer restrictions on clothing, has lower environmental cost, lower pollution, less waste, doesn't need helmets and high-visibility clothing and bike-locks, doesn't need insurance and breakdown recovery and loan-cars...

> So, it seems like the answer is a really clear and obvious no, there are other forms of transport with externalities so much larger than bikes that it makes the mere suggestion seem pretty absurd.

Cars pay for roads in terms of fuel taxes and vehicle taxes. Bikes don't pay for either. Cars often pay for car parks, bikes often use sidewalk, or car parks. Cars pay for accidents with mandatory insurance, bike riders are uninsured. That other things are worse was not my point, my point was that bikers gloss over needing roads and the cost of that, car drivers don't.