………….because trams have very short boarding times compared to buses and usually have Green Wave Traffic Light Pre-emption applied. which speeds them through traffic and which cannot generally be applied to buses.
Note the stream of cars following the rapidly moving tram which has nothing in front of it. See this video the Brussels traml above with a stream of cars behind the fast moving tram this is due to https://bathtrams.uk/solving-baths-traffic/trams-give-journey-times-cars-buses-even-whilst-sharing-congested-roads/
Rapid Boarding of Trams mean they stop only briefly in the road
Trams stop only briefly in the road forcing cars to wait for only a short period so when the tram moves off there is no traffic in front of it. This makes the whole journey much faster and hugely attractive to car drivers so many will switch to trams. Buses on the other hand with longer boarding times have to pull over, during which time traffic fills up the space in front.
This is because cheap and cheerful buses ( they have to be cheap due to the higher cost of bus drivers* fuel and maintenance) have only a single front boarding door which everyone has to go through one by one, and can’t have large multiple door openings, because they are of too flimsy construction.(This also makes them noisy, vibrate and so disliked). Passengers also often have to climb stairs.
First Bus did an experiment – it over 10 minutes using cash to fully board a bus – even with everyone using the ticketing app it took 2.30 secs – not everyone will have the app and not everyone will have change ready so in reality it will take longer than this.
Trams are stretched out ( see below) and due to the different engineering and structural strength of a rail vehicle have large multiple large doors and lots of space meaning tram boarding time can be down to 40 secs meaning they can pause in the road, and not pull over like buses.
Since the world wide experience is that car drivers will switch to trams, each tram can take 130 cars of the road, meaning the tram can sweep in, and the car drivers who must use their cars get a much quicker journey in because there are less other cars – even taking into account the occasional short delay caused by tram boarding. Everyone wins.
This tram is like a double decker but stretched out with lots of large doors at ground level ( only possible due to it being a rail vehicle) and no stairs to slow things down
This is like a tram but with one car on top of the other with smaller doors due to the automotive construction and stairs which slow boarding and deboarding
54 cars could be replaced by one bus and driver, but a Heidelberg tram could take 3 times as many, note the numerous large doors
Because a bus drivers costs, the largest operational costs, have to be shared over fewer passengers these means it makes sense for the bus operator to have a longer service interval to get enough passengers to pay for the driver. ( this also means they are crammed into bench seats). Whilst trams are expensive in first cost, this cost can be shared over unborn future users so is in fact less per passenger than a bus. Also the fact the driver is shared over more passengers coupled with the rapidity of the service means they can afford to operated with much shorter service intervals.
There are long bendy buses that try to emulate this, but they cannot get round the bends in Bath, and are much more expensive than the cheap buses we have which still struggle to pay their way.
Also car drivers simply do not like rubber tyred vehicles, which also emit toxic particles from the tyres, brakes, and engines.