Guest Post: Looking at PRT – Part 1

This the first of two guest posts is from commenter “Icebird” looking at PRT.  This post is solely the opinions of “Icebird” and not of the Auckland Transport Blog. 

The (eventual) future of public transport?

Nick R has made several persuasive posts on this blog for consideration of “automated rapid transit” or “light metro” as a solution to some of Auckland’s transit issues.

If you’re familiar with some of my comments on this blog and on the CBT forums, you might remember me for lobbying for a different horse in the “innovative transport solutions” derby:

“personal rapid transit” or “PRT” for short.

I wanted to take the opportunity of a guest post to more fully explain just why I’m still enthusiastic about PRT even in the face of widespread skepticism from many, and even outright hostility from some. Nick’s ART system certainly improves on traditional rail systems in many ways, and it’s a very attractive technology. But at the heart of it, ART is still basically a rail system, with both the pros and cons of any rail technology. But if PRT can prove itself in high-capacity operations, I think it could be a game changer, fundamentally altering the way we plan public transport systems and even how we design our cities.

In this post, I’m going tell just what PRT is (for those of you not familiar with the concept), try to explain why I’m so excited about the technology even in its current, mostly unproven form, give you a bit of a rundown on the history of PRT development (trying to answer the question “if its so great, why hasn’t anyone built a PRT system yet?”), outline some of the challenges that still have to be overcome, give my first-hand experiences of riding the “Heathrow Pod” PRT system, and float some ideas about where a potential Auckland PRT system could be built.

Designing the perfect public transport system

Almost every current form of public transport represents a compromise of some sort. There’s probably a bus stop within walking distance of you right now. But the bus that stops there might not take you directly to where you want to go, and its still subject to the vagaries of traffic, so its rarely going to be faster than driving a car without additional assistance.

Trains? Trains are much faster. They don’t have to share the rails with all those cars for one, and there are fewer stations, much further apart. The downside is that there probably *isn’t* a train station within walking distance of where you are right now. So you might have to drive or take a bus to get to the train.

Now imagine for a second that you were designing the perfect public transport system. What would that look like from the passenger point of view?

For a start, there would be spots to board a vehicle all over the place – think bus stop level coverage, rather than train station level of coverage.

Waiting for vehicles to show up at the stop? Forget about it! I want the vehicles to be waiting for me!

And when I board the vehicle, I want it to take me non-stop to my destination. No stops to pick up or drop off other passengers. No changing vehicles. No stop lights. And I want to be able to go from any stop on the network to any other stop on the network.

Oh yeah, and I also want the system to run 24 hours a day, 7 days a week.

Sounds like a pretty impossible combination right? Using public transit would be more like hailing a cab than getting on a bus.

Here’s the thing: PRT could actually potentially deliver that level of passenger service. I’d be a fan of the concept for that alone. If it could deliver that passenger experience – how many car drivers might decide to use the system, even if they weren’t compelled to by extortionate parking fees?

So what is PRT?

Start with a small passenger vehicle that can carry 3-6 people. Give it computer guidance so you don’t need a driver. Put the vehicles on their own track – at ground level if you have the land available, on an elevated guideway if you don’t. Put the steering gear in the vehicle so you don’t have to put switches in the track.

Then take the stations off-line from the main track (like a siding on a railway line). Vehicles stopping at stations don’t impede any of the vehicles behind. Your pod can bypass all the intermediate stations between your origin and destination.

Empty vehicles are automatically re-routed to restock empty stations. At off-peak times, a vehicle should always be waiting for you. At peak times, you should only have to wait a few minutes at most. Can you imagine a transit system that provides a higher level of service at 3 AM on a Tuesday morning than at 5 PM on a Wednesday afternoon?

Because the tracks don’t have complicated switching mechanisms, you’ve got much more flexibility in how you lay out your network. Forget long, thin corridors like traditional rail, and imagine instead a connected series of one-way loops, with new tracks constantly forking from, or merging with, the track you’re currently on.

Within those basic parameters, there are various ways of executing the design. The Ultra system uses vehicles with fast-recharging batteries and a sophisticated laser guidance system. The pods recharge while docked, and the guideway infrastructure is largely passive, keeping the construction costs low. Other systems have proposed using linear induction motors, or more fancifully, “mag lev” propulsion.

Here’s the amazing thing about PRT for transport planners: it should be cheap. Elevated guideways sound expensive, but they only have to support small light pods, rather than large, heavy train carriages. So the guideways can be quite small with, relatively speaking, an unobtrusive footprint. A light narrow guideway is easier to integrate into the existing urban environment (by running it above sidewalks or carparks). It can accommodate tight turns and steep climbs. You could attach guideways to existing bridges (I’m looking at you, Auckland Harbour Bridge!).

I attended an Engineers for Social Responsibility evening where the New Zealand holder of the Ultra licence talked about the technology. The cost number he quoted for the system? $13 million per kilometre – total system cost, including guideway, vehicles and supporting infrastructure. That’s pretty competitive with any competing rapid transit technology. As Nick as noted in his Light Metro posts, any driverless system leads to massive saving in ongoing operating costs.

Furthermore building a PRT system doesn’t require an all-or-nothing commitment. You can start small, then expand the system. The nature of the system means that vehicles have the freedom to pursue alternative paths between to their destination. That means that part of the system can be shut down without necessarily affecting the rest of the network. Thus you can continuously expand the network in an incremental fashion, rather than the current model where the rapid transit network tends to expand in large, very expensive chunks, with little activity in between.

Go on, what’s the downside?

This sounds a bit too good to be true, so your skepticism levels might be on high alert. The debate between those who passionately believe in the technology, and those who believe it cannot and will not work is vigorous.

The first major problem of PRT is that its largely unproven. The first “modern” systems have only just rolled out, and only in low-capacity operations that don’t really test how the technology will perform in a busy urban environment. Many of the benefits of PRT will only be realised if it can be succeed in high-capacity operations.

The conveniently brings us to another issue frequently raised in opposition to PRT: “It doesn’t have enough capacity”.

Now a high-frequency rail system will have a higher absolute capacity than PRT. I’ll happily concede that point. But PRT might very well have enough capacity.

The Ultra system is currently licensed to travel at 40 km/hr with 3 seconds gaps between vehicle’s by Her Majesty’s Rail Inspectorate in the UK. This is very conservative licensing. On a motorway, the recommended gap between vehicles is 2 seconds travelling at 100 km/hr. So if the gap in the Ultra system was lowered to 2 seconds, the guideway would have the same capacity as a lane of motorway traffic. (Ultra is aiming to increase the speed allowed by the license to 80 km/hr).

The real benefits of PRT start to kick in however when you lower the gaps between vehicles even further. A commonly cited “target” by PRT developers is half second intervals between pods. Even factoring in capacity lost due to empty vehicles being redistributed throughout the network, PRT’s capacity begins to match about three lanes of motorway traffic – at a fraction of the land allocation.

High-capacity PRT and high-frequency rail aren’t necessarily in competition with each other. The existence of motorways doesn’t negate the need for a local road network. There are opportunities for PRT to act as a feeder to the rail network, bringing rapid transit closer to a much large section of the population than now. Think of PRT as a bus replacement technology, rather than a replacement for the rail network, at least to start with.

The other major obstacle I see to PRT’s adoption is visual intrusion or the “NIMBY” factor. I don’t think there’s any getting around the factor that an elevated system, even with all the mitigation in the world, will always cause some visual intrusion. If you presented the plans to the good citizens of Auckland today, I’m sure most would say “not on my street”. If you built a system somewhere non-controversial (like Wynyard Quarter perhaps) and they could try it, and ride it, and it lived up completely to all the expectations I’ve laid upon it… I think they would still say “not on my street” but might demand that it be built one street over so they had a station within walking distance.

In the next post Icebird talks about the history of PRT, his personal experience and potential applications in Auckland.

Share this