What are the advantages of PRT when compared to tram (LRT)

It is common practise to compare PRT to trams as both require large investments before traffic can start. But this is almost the only thing that these systems have in common. The big difference is of course that a trolley or LRT line has the same inherent disadvantages as a bus: Waiting times, low mean speed, change overs and lack of seats in rush hour traffic.

Building a tram line in a denser area in a city is a much more complex operation than building a PRT line along the same route, as you have to prepare the ground and install rails for heavy vehicles (for instance 37,5 tons for Tvärbanan, Stockholm). For PRT you only need to dig for poles every 25 m (80 ft) or similar. The much tighter cornering and steep slope ability also simplify installation. As the ground works is the dominant part of tram installation cost the cost per installed mile for PRT is less than half of the tram’s in the typical case. This despite the fact that the PRT guideway system as such is more expensive than the tram track hardware.

The higher travel speed and greater comfort of PRT allows for a much higher ridership than a tram line can ever hope to attract. This makes the cost advantage of the PRT system even more obvious.

Trams are delivered in small quantities per model, a few dozen is a typical quantity. This leads to extremely high NRE (non-recurrent engineering) cost which in turn lead to a very low replacement rate of the rolling stock. Typically tram cars are used for at least forty years before being replaced. As the level of comfort expected by passengers rises at the rate of the improvement of cars the trams are hopelessly outdated before they are replaced. PRT vehicles will be delivered in thousands per year which allows the NRE cost to be divided by much larger numbers. A PRT system will typically upgrade its fleet of vehicles continuously which allows the operator and passengers to take advantage of the latest developments within the comfort, travel time and energy consumption areas as they become available.

What are the advantages of PRT compared to the car?

PRT has many similarities to the car. You don’t have to wait for the bus or tram and get a guaranteed seat in a separate vehicle which takes you directly to where you want to go. But when you leave the PRT vehicle you don’t need to find a parking spot. Also you don’t have to drive the vehicle yourself and computers make sure that accidents and congestion is avoided.

Today the capacity of large parts of the road network is fully utilized in the rush hour so to keep or improve the possibilities of uncongested travel we face major investments whichever way we select to improve the situation. Out of the available options PRT offers by far the largest capacity for each invested dollar. Granted, roads are cheap to build when availability of land is not a problem, but to increase capacity of congested highways today you often have to tear down entire blocks of housing or dig the roads down into tunnels. Both of these options are extremely expensive.

What are the advantages of Double direction lines?

A common way of thinking among PRT advocates has been to draw networks of single direction lines. The rationale of this is that you can provide a tighter grid of single direction lines at a certain investment level. This gives a shorter mean walk-in distance to stations. Intersections are also simpler with single direction lines than in a double direction system.

When compared to a double direction system each trip on the single direciont network is on the other hand longer by 1,5 times the grid size on average. This is as you have to travel in the wrong direction at each end of the trip, just like a city with only one-way streets. If the grid size is 500 m this means an average of 3 km extra on each trip. Riding the vehicle for 3 km takes 3 minutes at 60 km/h. This corresponds to an extra 300 m of walking. For the 500 m grid size the walk in distance is 250 m, so to offer the same trip time including walking a double direction system can get by with a 800 m grid size. With a 800 m grid size the amount of guideway required is reduced by a factor of 1.6 which means that a double direction system with the same total trip time is probably cheaper to build than a single direction system.

As the double direction system replaces 3 km of PRT travel with 300 m of walking we get a whole array of additional advantages:

  • Better capacity: While we are building 1.6 times less guideway, the total beam length is in fact 25% longer than for the single direction system. At the same time each trip is shorter by 3 km. As the mean trip length is on the order of 5 km in a typical system there is a 60% difference in trip length. In total the capacity of the double direction system is thus twice that of the single direction system of the same cost.
  • In a typical, non-square city there are many places where it is unnecessary or impossible to have guideways at a fixed 500 m spacing. A double direction system is much more adaptible to different shapes of the city as you don’t have to plan for a returning guideway at a suitable distance from each outgoing guideway.
  • As more of the trip is made on foot the electricity consumption of the system is down by almost 40% for the double direction system.
  • The health improvement of the 400 m walk compared to the 250 m walk of the single direction system is not to be neglected.
  • The visual intrusion of two slender Beamways beams is about the same as for one ULTra or Vectus beam. But as we are building 1.6 times less guideway miles the total visual intrusion is much lower.

A report which compares three different networks connecting the same set of stations is available here.