Trackless trams are, essentially, bendy electric buses that run along a set route. They don't need rail tracks, but rely on other technologies to hold them in place.
Adelaide's O-Bahn has wheels that run against a kerb on each side. Other versions include magnetic strips on the road and cameras on the buses that combine with radar and GPS to guide them very precisely along lines painted on the road.
New Zealand has already had a "trackless tram": That was the name they used in 1924 for a Wellington service connecting Thorndon and Kaiwharawhara.
But the route wasn't busy enough and it was decommissioned in 1932. That "trackless tram" was what we now call a trolley bus.
And that goes to an important point about this debate. They're buses. But they have some of the advantages of trains.
In particular, you can get on and off quickly and easily, especially compared with double-decker buses, and they provide a smooth ride. That's great for the safety of everyone not sitting down.
It's quite possible that if we don't build light rail, parts of our bus fleet will evolve into articulated buses anyway.
The contemporary version was invented just a few years ago in China and can now be found in many cities there and in Europe. Judith Collins raised the possibility of their use in Auckland and Wellington more than a year ago, and National prefers them to light rail.
It's easy to see why: They're cheaper to buy and it's easier and faster to set them up on the road. There's enormous merit in those things.
Also, they're flexible: They can be redirected when there's a problem on the route, and they can easily be introduced to new routes.
But there are some real problems the backers of trackless trams don't seem keen on mentioning.
As Matt Lowry at Greater Auckland reports, light rail can carry 400 or so people at a time and will benefit from "smart" traffic signalling, meaning the lights will turn green as they approach.
Trackless trams would carry about half that number, so we'd need twice as many. That could make priority signalling impossible, because it wouldn't allow much time for cross traffic.
Building light rail will be extremely disruptive, because the roads need to be dug up and strengthened. Service pipes may need to be moved so the trains don't run directly over them.
But that disruption will be invaluable in time, because it will allow all the underground services to be renewed and expanded. Water, sewage, power and other utilities all made good enough to cope with population expansion for 100 more years. We need that anyway.
Trackless trams can be installed without the road being dug up. But because they follow a very precisely calibrated route, the wheels of every unit, every time, run over exactly the same strip of road. And they're much heavier than ordinary buses.
The result: Rutted roads, constantly being degraded and requiring more maintenance. That's a heavy ongoing cost, whereas light-rail maintenance needs are light.
Transport engineer Richard Young, who has studied trackless trams for several years, suggests the only viable solution for the bendy buses is to lay heavy concrete pads for them to run on. In other words, they'll have to dig up the roads anyway.
This issue has already led some cities, including Eindhoven in the Netherlands, to remove their trackless trams.
One of the great advantages of light rail is that it can run on a green swale: Grass, usually, that carries away rainwater and helps filter pollutants. Trackless trams can't do that.
They're less green for another reason, too: They use specialised rubber tyres that shed carbon.
"If we want to get serious about water pollution and microplastics," says Ed Clayton, an environmental scientist who presented on this to Auckland Council recently, "we will build green light-rail corridors."
He says about 30 per cent of all primary microplastics in the oceans come from tyre wear. "Contaminants such as zinc and other heavy metals are also released in the same process."
When it comes to the "good choice for green" test that all new infrastructure should have to pass, trackless trams don't score as well as light rail.
Another issue: Trackless trams are proprietary systems. You have to keep buying from the same supplier, even if your needs change, the price rises or the quality of the units is unsatisfactory. Right now, the leading company is the same Chinese firm that sold asbestos-laden trains to KiwiRail.
The thing is, there is no good, cheap, easy option for public transport. Whatever we choose will have problems that have to be managed.
Light rail is disruptive and more expensive to build. But is it right to make the decision on short-term criteria?
Don't we want a good long-term outcome, coupled with a robust method of protecting the local businesses that will be harmed during construction? That's the part of the equation that urgently needs adding.
Meanwhile, the Auckland Light Rail (ALR) "establishment" team, set up by the Government with council involvement, is holding "community events" around town. It wants to explain the project and hear what people think.
But it's not explaining very much. It hasn't released any of the extensive work done over the past four years on light-rail options for the city. It's saying very little about the route options and nothing at all about where the stations might go.
ALR is looking at two modes: "Light rail", which would run on the roads with frequent stops; and "light metro", which would be faster, have fewer stops, be more expensive and include both underground and elevated sections. But it won't say what it thinks of these options.
As it happens, ALR is also considering a range of other options, including trackless trams, but it won't talk about this either.
This is preposterous. Handing out leaflets at community markets but refusing to divulge what's going undermines the very efforts to build the confidence and trust they say they are so keen on.
ALR will report to Government later this year.
So what is the best option for Dominion Rd? A quick bendy bus fix, knowing it will create problems down the line? Or a more disruptive light-rail solution with a better chance of meeting future needs?
Or is there another option? Well, yes there is.
It's cheaper and it's better at meeting all the goals of climate, congestion, community building and public health. For a tiny proportion of the construction disruption of light rail or trackless trams, we could transform that suicide zone of a street into a safe, quick, easy and fun place to travel on.
Yes, with bike lanes. Nothing should happen on Dominion Rd without bike lanes getting priority.
The best thing about the DRBA's public statement this week is that its own visualisation for the street includes bendy buses, just two single lanes for cars, and dedicated bike lanes.
They definitely need to be made safer than is shown, but the point is made. If the business association is happy with bike lanes, let's build them, right now, while we continue to fret about the rest.
Come on Auckland Transport, you're not keeping up.