A lot of people criticised the tunnel extension into the city. After all, it only saved ~5 minutes of travel time. I don't think most people in Adelaide fully appreciate public transport yet.
Adelaide is often called the 20 minute city because you can just drive anywhere in under that. But that's changing as our population grows. I really hope there are more projects like this that get funded.
Adelaide used to have _the most extensive tram network in Australia_. It was pulled up around the same time as Sydney pulled theirs but Melbourne didn't (because they're the original hipster). Sydney had the money to reinstall theirs but not Adelaide. There's a lot of politics around fixing our trams - we almost dropped 40 million on a building a right hand turn.
The road going past Paradise Interchange is always lined with cars for hundreds of metres on both sides during working hours. Modbury Interchange now has an inner-city style multi-storey car park to feed the demand.
The O-Bahn is almost a victim of it's own popularity and efficiency, but it took 20-odd years to reach that point, and it's still the best way to get into and out of the city during peak hour (with the possible exception of the linear park bike track, which is a beautiful way to see Adelaide 'behind the curtain').
Adelaide also is well past it's moniker of the 20-minute city. It's nowhere near Sydney / Melbourne levels of traffic, but the population is also much smaller.
Edit: I just read about its speed limit being limited to 85 km/h due to wear and tear and unevenness on the track. It seems the track is reaching the end of its life.
Things wear out...
I found this paper from the university of Washington which has a bit more information (DOC warning) https://faculty.washington.edu/jbs/itrans/adelaide-o-bahn-pa...
The 12 km busway cost $97 million in 1999 AUD, which is ~$119 million USD 2018. I have a feeling that a similar project would end up costing something like $75-80 million per mile, just based on transit projects in the US.
Chicago spent $41 million building 8 bus stations and painting a mile of street red.  It was billed as BRT, but plans for multi-door boarding, and off-bus fare collection were scrapped.
Projects like this aren't happening in the US because there is no political will. If you are going to have bus lanes you need to have enforcement. Buses should have cameras ticketing other vehicles in the bus lanes. Turn lanes should not merge into bus lanes, drivers should be forced to wait for a green arrow to make a turn across the bus lane.
There is definitely a need for service like this in the US, but we seem to think that the best option is to have people like Elon Musk build private tunnels that wealthy individuals can bring their private cars in. No matter how you design that system there is no way it can provide the same capacity as BRT. It's just another road with low-occupancy vehicles.
BRT has notable disadvantages; buses can move from side to side unlike rail, so the tunnels and bridges have to be bigger or use proprietary guided technology like this one. In general road surfaces wear away much faster than rails. And rail vehicles perform much better once you factor in labor costs. BRT is useful in some cases, like where there is no dominant trunk line and buses can leave the roadway to serve different destinations; but eventually the common trunk gains enough critical mass and you convert to light rail anyways, as Seattle and Ottawa have discovered, and LA is considering for the Orange Line BRT.
Chicago owned all of the land used for the Loop link and it still cost $41 million, and has little of the benefits of actual BRT.
People here get incredibly upset when any road diet is discussed. Our politicians rarely ride public transit, and laws and engineering guidelines prevent projects which would reduce the volume of traffic carried. It's absurd.
In fact, the fact that it was on an existing street would make it very expensive to modify; look at a picture of the O-Bahn and there's plenty of space for construction vehicles and staging to set up, no one to get in the way of, etc.
I can probably speak to a bit of this as I am from Adelaide... and have also driven in the USA before.
The driver mentality in Adelaide, compared to USA (and even the eastern cities of Australia like sydney / melbourne), is a lot more laid back. Adelaide feels more like a really large country town rather than a city. Speeding isn't that common (still happens but its not every single person) & cutting people off doesnt happen as much in america.
The O-bahn, is mostly designed for only specific public transport busses. About once or twice a year a car or mini bus (usually driven by tourists) gets stuck on it and has to be lifted off. They recently did an extension to connect it into the city via a dedicated bus lane that turns into a tunnel. I drive along this stretch of road every day, and I think i see about 1 or 2 people a month driving in the dedicated bus lane, usually because they didn't know what was going on rather than doing it on purpose.
The O-bahn services about 26% of the workers in Adelaide City.
Maybe we don't cut off people as much, but we sure can't do a zip merge without someone getting upset.
It makes a ton of sense for medium/large cities with sprawl (versus super-large cities like New York). That traffic segregated lane can also be used by emergency services, and buses can leave it if they have to for parts of their route.
Everything on the left of the dotted line can go without rails/guidance and everything on the right has to stay on rails.
1) regular buses
2) buses on specialized roads (busways)
3) guided buses which have extra wheels or electronic guidance to follow the busway without the driver steering: https://en.wikipedia.org/wiki/Guided_bus
4) bus/tram hybrids like the Bombardier Guided Light Transit, which usually operates with a guide rail but has the ability to go without any rails: https://en.wikipedia.org/wiki/Bombardier_Guided_Light_Transi...
5) rubber tyred light rail systems (e.g. automated guideway transit systems, often seen at airports)
6) rubber tyred heavy rail (e.g. Montreal Metro)
7) heavy rail
The lines between buses and trains will only get more blurred in the future with autonomous bus platoons and the such.
With adult eyes it seems over-engineered, but I still want to ride it again just because I want to support whoever thought that was a good idea and managed to convince enough people to build it.
It surprised me to see they didn't simply use retractable railcar wheels like the railroad maintenance trucks, which is apparently a fairly standard "road-rail vehicle":
Alignment could be easily automated today, or a low-tech mechanical guide mechanism could be added for the bus.
I'd expect the maximum comfortable speed to be substantially higher than the guide wheel approach. That guide wheel design seems better suited for aligning the rail wheels than continuous operation.
I travelled along the O-bahn for most of my childhood/uni years and I think it's a brilliantly simple system.
Stick a couple of guide wheels on a bus, and you've now got a safe, fast, relatively low cost transport system, that still has the flexibility (once it exits the track) to go where ever it needs to go.
I'm sad to read that they limited it to 85kmh. In my time, they definitely were going 100kmh, and every nuts and bolts rumbled on these old buses... Mostly the fault of the cement track, not the bus themselves. :)
The buses look like normal every day buses but if you look closely you can see the little wheels that protrude out the sides to keep the special buses on the bus way. The bus otherwise drives around like a normal bus until it drives onto the cement track and the little side wheels are then used just to keep the bus from scraping the sides and just rolling along easily.
The track is also more compact than an equivalent two-lane dedicated bus roadway - especially given the safety margins roadways need when you have high-speed vehicles travelling on them.
Being more compact meant the O-Bahn route could follow the Torrens creek which gave it an unobstructed route from the outer suburbs right to the CBD without expensive land purchases etc.
Brisbane is also in the process of upgrading their busways to something like rubber-tyred trams (bi-articulated busses) with the Brisbane Metro project .
It seems the Boring Company expects much higher top-end speeds, however. Perhaps smaller vehicles and autonomous driving will help with that.
So a normal road wasn't possible.
It's just an excuse to justify it.