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If the average bus is 8 years old, that means that buses are replaced approximately after 16 years. According to this source, the average bus in New Zealand is more like 16 years old, so they’re actually running for 32 years 😱

For a reasonably sized transport association it will be outright impossible to electrify an entire fleet at once:

1. Where do you get all the buses? For Hamburg alone, you’d need to buy 2600 new vehicles, which would be half of all buses sold in all of Germany in a year.
2. Where do you park those 2600 new vehicles until you’ve sold the old ones? For some transition period you’d have twice the number of vehicles while at the same time…
3. You’d need to overhaul all your bus depots at once, which adds to the parking problems. In Hamburg we have more than a dozen bus depots, where do you find all the construction workers to upgrade them all at the same time? Where do you get all the architects, planners etc?
4. You’d need to get a lot of electricity to your depots in a short period of time. Your local power company might be able to build a new substation for one depot a year, but 10? Probably not.
5. Where do you educate hundreds of mechanics on the new technology? And who’s maintaining your buses while all your mechanics are at the training?

You don’t need too much infrastructure to start transitioning: You can add charging infrastructure to on one or two terminal stops, upgrade one bus depot, educate 10% of your mechanics and start by upgrading all the lines going to these terminal stop. In the next year you upgrade the next terminals, the next depot and train another 10% of your mechanics. After a decade you’re fully electric without a big hassle.

English is such a weird language. 💋 EDIT: According to Meriam-Webster and OED both spellings are allowed (at least in US-English)

Busses are replaced regularly anyway. An average bus in Germany for example is 8 years old, so 6-7% of all busses are replaced every year. Just buy electric busses when replacing the old ones instead of throwing out perfectly new combustion engine models. That’s also more environmentally friendly, as a large part of its lifecycle pollution happens during the construction of a vehicle.

The other question is obviously highly dependent on the city (size, density, geology etc.) and the type of transit you’re building (underground vs overground, separate rails vs. tram on streets etc). As a current example Hamburg is building a new subway line that’ll go through the entire city (25km, 24 Stations, almost completely underground) is estimated to cost 15 billion €. So, depending on how mucch your city needs it could be anywhere between 10 and 100bn for a subway net. However, the national accounting will benefit 1.28€ for every 1.00€ that’s spent, due to savings in travel times, fuel, cost for accidents and road maintenance, freed up real estate in the city etc. according to the calculations.