Even if you think what you would say is obvious, please add. This is genuinely something I think makes sense regarding local bus routes given the longevity of light rail and how infrequently routes change, but I also suffer from confirmation bias, so I'm hoping for reasons this would be a terrible idea but obviously would prefer reasons it would be an even more amazing idea than I thought.
Light rail/trams are better especially for avenues etc. But busses are more flexible, and you usually need a combination of both for best results
This.
I think of buses as the caterpillar to a tram's butterfly.
You can start with a comprehensive bus network, and as a particular route stabilizes and the bus starts struggling to meet throughput needs, that is an indicator that a tram may be worthwhile.
Starting w/ a tram line is a pretty big financial bet that it will be useful/needed, as once you build it, you're locked-in to that specific route.
Buses for longer journeys make sense. We have a bunch of buses in London that run from the city centre out towards the green belt. Buses for those especially long journeys makes sense.
Why not just build a train for long journies? Cheaper over time, more capacity, and reduces road dependency.
Buses are awful for long journeys. Trams for longer journeys make sense. You need the buses to get you to the tram stop.
The big benefit of light rail is you can make trains longer than buses, and fit more people. So if your system has outgrown buses, then you should move to rail.
But transit systems should always be trying to maximize frequency, because the more frequently a train or bus comes, the more convenient it is for riders. So if a bus fits 30 people and a train fits 90 and you’re trying to make a decision between providing a bus service every 10 minutes or a train service every 30 minutes, the bus service is the better option.
Different modes work best for different passenger demands, and you should use the right one for the number of passengers you’ve got. Overbuilding is expensive, and if you spend too much building out a network and the don’t have enough for operating expenses then you’ve got to reduce service levels.
There's pros and cons of buses vs trams.
The pros that I'd slate for trams do include a better ride, more throughput (carries more people), wholly electric, are more durable than busses, and very quiet in general. People in this thread have noted most of these already, but the one that I feel is very overlooked is that they're a commitment by the city along their routes. Many people will note that busses have advantages because their routes are easier to change. While true, I feel it's actually worth considering that this is also a negative from the perspective of anyone who wants to invest in property that relies upon the bus route. If you can't trust that the city won't just up and move the bus stop away from your shop or apartment complex, you'll be more reluctant to invest in the location. Trams are indeed much harder to change, but that's actually a good thing from the perspective of investors. If I'm going to invest millions of dollars in an apartment complex, would I rather do it next to a bus stop that might not be there next year, or a tram stop that's really hard to move away?
Another advantage is how well the tram integrates with pedestrians. Busses are only as accurate as the driver. As a pedestrian, I have to pay attention to every bus just as I would cars on the road. They're dangerous to be around. Trams are much more predictable (see: rails) so they can be used in/around public squares, markets, and along walkways with more safety for the people walking nearby. The rails themselves also show you where the transit is. Bus routes are invisible except for the stops and when you see the busses go by. When I'm walking in a city that has railed transit, I love seeing the rails because I know that I likely follow them to the next stop, and that by stops there will be shops, stores, and interesting places. They're a guide to the best places in the city even if I can't see the tram at that exact moment.
Trams are also usually larger inside. There's more room for wheelchairs, bicycles, and other mobility aids. They're a better conveyance for people who need the room. Those same people also need to pick where they live carefully so that their transit won't up and change on them. Having the bus stop move a block away could be a huge hurdle for their daily mobility needs.
Railed transit provides a permanency and a more equitable transit solution for a city. It's not the right solution in every instance, but as a city grows it needs to start investing in railed transit. Those rails provide the bones of where growth will centralize around giving the city focus and then identity as neighborhoods grow around tram/light rail stops. There's a power to railed transit that busses just don't provide in their stability, visibility, and statement of commitment to the longevity that a city should be investing in.
Also, look up grassy tram lines. That's peak urbanism!
I'd never heard of grassy tramlines, I love them! I've never seen any anywhere, and I've been all over Europe, they are either quite rare, or I haven't been paying attention.
We've even got them in Finland and we only have two cities with a tram network. Otherwise I've seen them a lot in Germany and newer systems tend to have them more often than older ones.
Light rail is infinitely more expensive to construct and it only takes one delay/accident and all subsequent trains after cause a log jam...vs a bus which can route around it.
A better solution uses corridors dedicated to buses that are electric powered.
Something like this was done in Colombia with these routes being connected by ground hubs, similar to subway stations.
That’s like saying a ship is more expensive than a car. It depends.
A tram is not „infinitely“ (what absurd statement is that anyway) more expensive than a bus.
Construction cost is not everything, and they’re not even that much higher, you also need to consider service life (much longer with trains), energy cost per passenger mile (much lower with trains thanks to the lower resistance), etc.
What is best is always depending on the specific circumstances.
The biggest limitation of buses is capacity, and a highly used tram is cheaper per passenger mile than a bus. Try replacing the S-Bahn in Berlin with BRT, see how far that gets you. You’d probably need to bulldoze a new highway… speaking of which:
Germany is actually hellbent on building a highway right through its capital Berlin, which currently clocks in at 700 milion € for 3.2 km. I expect the whole thing to end at ~2 bn € for ~7 km.
So I think the costs of public transport are really not the issue people should be focusing on.
Electric busses are actually a lot more complex logistically than electric trains. With a train, you just need a bunch of big-ass transformers and overhead wires. Expensive to install, but very reliable and relatively low maintenance over many years.
Batteries on the other hand are heavy, relatively fragile, degrade quickly, and very expensive. With a 100KWh EV, about 1/3 of the total cost is the battery, so it would likewise increase the cost of a bus.
Charging is another problem, instead of the whole system using energy real-time, you now need a distribution system that can take hundreds of busses at night and charge them all back up, requiring a massive amount of power in a somewhat short time. While it’s nice that energy is generally cheaper at night, you still need the infrastructure that can take that load.
So, it’s not to say that there’s no place for them, just that our main focus needs to be on rail in most places. There are lots of low-density places with cheap power and temperate weather that absolutely need BEV busses, but a lot more with challenging weather, older grids, and medium density that are a better fit for rail.
IMO electric busses needs to have a trolley bus infrastructure on some route so the bus is recharged during the day. Won’t cover 100% of the energy needs, but will spread out the charging time.
Here in NYC, we switched to hybrid electric buses many years ago and are currently transitioning to all electric buses. I’m not sure about other cities. 
Yes, we certainly can route around it, but having lived in London for most of my life, I can tell you that we seldom route around it. However given the capacity that light railway how. If we keep the vehicles moving on the main arteries, we can move more people alleviating the frustration.
Probably biassed as I'm a bus driver but the city I'm in has a tram and it's fantastic until one gets blocked or broken. Benefit of busses is they can detour if needed, and if one breaks it doesn't (always) block the entire route
edit: extra annoying when they break down and I have to carry a tram load of passengers on one double decker bus
Biggest drawback for anything on rails really, it works either really well or not at all. I think it is still worth it, but I am also incredibly biased towards trains.
It's also very easy to reconfigure bus routes, just slap some new paint on the road.
I usually prefer light railway too, because it usually is less impacted by traffic than buses (depends on the road structure, of course)
It’s less impacted by traffic only because it’s given priority.
Sometimes light railway have their own lanes as well, physically split from the rest of traffic, which also helps
Light rail, imo, has its own right of way. If it doesn't it's not light rail, it's a tram/streetcar.
We tried in Denmark (Aarhus). Quite expensive, and too many issues. Electrical busses (with dedicated lanes) seems like the better solution, ~~bus~~ but this is also not cheap.
Edit: Spelling
But electrical buses still create an outrageous amount of rubber waste
Aarhus is not really a good example of replacing a bus, as it is a rather regional light rail system with a short inner city section. The difficulties they have seen are probably mainly caused by the technical and budgetary choices made during planning.
For Denmark, Odense looks like a better example, that should be successful if they manage to solve the initial challenges, e.g. with noise/vibrations.
As for the longevity that OP is mentioning, the systems in Bergen (Norway) and Tampere (Finland) show how important this is, with huge private investments being made along the lines. A bus line can be gone next year, but rails will stay for at least decades.
With the caveat that this only applies to my city, San Francisco... I prefer buses. SF horribly mismanages its "trams"* where they run at ground level through the streets. They must follow all stop signs and traffic rules. They don't even get signal priority. So it's a quite jarring experience to get into a train underground, exit the tunnel to the street, and begin stopping every block and waiting at red lights.
Fact of the matter is that, if you're going to be treated like a car, it's better to be more maneuverable as a bus. Buses can avoid double parked cars, and have a fighting chance of squeezing through a gridlocked intersection. With a bus lane, they can use it but they don't have to, where's trams are trapped in a traffic lane (frequently the centermost lane) while idiots make (frequently illegal) left turns.
* Muni light rail - K, J, L, M, N, T, F
Busses have their uses. Lots of commentor have mentioned the flexibility in setting up / changing routes. But there's also the flexibility in sizes. You can start a line with a large van or small mini bus and your only overhead is the driver. From there you can scale that up according to demand up to frequently run articulated busses. Meanwhile your minimum investment for tram includes at the very least a not inexpensive track installation.
Don't get me wrong. If you have the passenger volume that investment definitely pays off. But I don't like this unnecessary competition between two modes of transport that can be very complimentary to each other and are both better than individual cars.
When did we stop calling them trams?
Yes but it makes economic sense only on the routes with top demand that pass through the center or CBDs or other high traffic areas
Tramways and Light Rails are much more silent and comfortable. Rails are smooth, no pothole etc. They are also much much more durable than asphalt.
Rails take less space and can be covered with grass/greeneries. Looks prettier and absorb heat.
It's more reliable, usually runs on electric rather than buses, can run more frequent without causing congestion.
Only real con is that you need some time, money and maybe more space to add it
We have trams in the city where I work. Two problems have been: 1 cyclists having accidents when wheels get trapped in the tracks 2. Reliability problems because if a tram beaks down the whole line is interrupted.
Trams deserve to be separate from car and bike traffic: it allows trams to be faster than cars and it protects cyclists from falls. We need to stop thinking that cars should be everywhere by default.
The busiest core routes should be served with light rail, allowing an efficient high-frequency service for the most common journeys, and most parts of a city should ideally have some kind of connection to that rail system within a kilometre or two. But you can't just put rails and stations literally everywhere, so buses (or trolleybuses with batteries if you're so inclined) remain useful for less common routes, gaps between stations, the neighbouring areas of rail routes or last-mile connections from light rail to within a short walk of a person's final destination.
Buses are also necessary as a fallback during maintenance or unforeseen closures on the rail network. Even if it's just a temporary station closure, that one station will likely be the only one in walking distance for quite a few people (especially if we're talking about an interurban network where a small, outlying town or village might only have one station connecting it to the rest of its metro area), whereas that same area could have several bus stops, giving pretty much everyone there a way to continue getting around, perhaps even to get a bus to neighbouring stations.
And bus routes don't change that infrequently. Certainly, not infrequently enough that you'd want to tie them to placing or removing fixed infrastructure like tracks or wires. Diversions also happen sometimes. All of this isn't to argue against light rail, but to argue for a comprehensive multi-modal vision of public transport. Let passengers use the right combination of services for their particular journey's needs.
Shouldn't we be switching buses with light railway?
Yes
Trams are the cosiest things to sit in. I enjoy being half asleep in the morning and just look at all the people being busy. Wish my town had some more grassy lines, but they don't lack on where you can go.
(edit: I want to add that I am also happy with the buses here, don't think there is a reason to be either or and rather focus on reducing cars in town and in its suburbs. Obviously easier to do for smaller towns).
Good luck climbing hills. Not saying it wasn't done before (I use a "tram" which does it daily), but it's one of the more expensive problems to solve.
San Fran managed.
And Lisbon too, as a side bonus tourists love to see old trams going up the hill (in both cities).
Not really. Unless you mean cable cars. But that's not really an alternate to a bus. Pretty sure they're way way way more expensive to maintain
But why put them on rails? As a kid I remember busses running on electricity from cables that were located above them. Isn't that the best of all versions?
Tyres wear down and produce nasty pollutants, and metal-on-metal is more energy efficient.
Trolley buses are good. But light railway lasts longer and does less damage to the road for vehicles that actually need to drive. Also you can go autonomous with light railway which is far easier on tracks than without.
It depends on the type of light rail.
Here in my city the trams share some of the roads with regular traffic, which not only means they can get caught in traffic (though they have priority where possible), but it also means the rails become a real tripping hazard for cyclists (over 800 injuries since 2015 at the last count). There's been an active campaign to introduce more safety measures but the council has been reluctant to do anything about it.
The tramlines are such a well-known hazard to locals that they actually put people off from cycling, which is surely counter-productive.
Light rail transit has its own right of way. Sharing the road means it's a tram/streetcar.
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