PODCARS – A Personal Rapid Transit primer

Seattle Transportation Examiner

United States

By David Gow

This year has seen an increasing stream of news, Examiner.com included, about the mass transit alternative concept Personal Rapid Transit (PRT) — also known as “podcars.”

Stories on podcars are usually followed by discussions among readers speculating about what PRT is, how it would work, or why is it needed. Answers tend not to resolve their questions to any great satisfaction.

I have some insights into the subject, having observed PRT development for nearly twenty years. Wider public understanding is needed about PRT, because there are two PRT projects that are to begin operating soon — short initial phases of what could become larger PRT-based transit networks.  In the next few years, your community could start thinking about adding PRT to existing transit services, and your thumbs-up or thumbs-down needs to be an informed one.

How PRT would work

Taxi 2000

The PRT concept is pretty straightforward: imagine splitting trains into small segments — 4 to 6 seat pods. Each segment can run around separately on an elevated guideway, driverless, under computer control.  The guideway connects stops (stations) distributed across a service area, forming a network. Each stop is located on a siding off the main guideway, so that loading or unloading passengers at one stop doesn’t block pods going elsewhere. Designers of PRT systems believe small light weight vehicles have economic advantages — they can use smaller profile guideways, and therefore could have lower per-mile capital cost.  It is hoped that PRT can build more miles of transit, reaching more places and expanding the base of transit users.

Pods would operate on-demand instead of according to schedules. During off-peak periods the pods would wait at stops until needed.  A traveler would go to a PRT stop, request a ride by selecting a destination stop from an ATM-like machine, provide payment, then board a pod that would take her on the most direct route (balancing distance and time) through the network to her destination, bypassing intermediate stops. When the ride is over, the pod is available for another user.

You might be surprised to learn that on paper PRT is more energy efficient than trains and buses. Due to our experience with automobiles, small vehicles are assumed to be more wasteful. But a big part of energy use in any type of transportation correlates to the amount of vehicle weight that must be moved with the passengers.  A 50 ton light rail car with 70 seats is moving over 1,400 pounds per seat, a 23 ton articulated hybrid bus with 58 seats has 769 pounds per seat, and a 7,000 pound six-seater Escalade has 1,166 pounds per seat.  In contrast, a six-person PRT pod might weigh only 900 pounds, or 150 pounds per seat.

Other sources of energy waste in transit are frequent starting and stopping (addressed when vehicles have regenerative braking) and low occupancy.  Because the average occupancy on transit (the occupied percentage of all available service) is on the order of 15%, government statistics on transportation energy show transit as sometimes less efficient than automobiles (see Fig. 2.12 and 2.2 at this Center for Transportation Analysis page).  However, the automobile’s dominance has a cumulative effect that more than overcomes any small statistical differences — and for CO2 emissions as well as energy.

Capacity in a PRT system is mostly a function of the number of pods, and short headways between them. Congestion is avoided by having a set number of pods, in contrast to the continual increase in new automobiles being put on the roads.  Capacity is the number of trips each pod makes, times the number of seats per pod, times the number of pods in the system. Just as an example, in a fleet of 1,000 four-seat pods each making five trips per hour, the capacity is 20,000 passengers per hour. Therefore on-demand service is the chief difference between PRT and light rail — light rail is good at moving large groups in trains many minutes apart, along corridors; PRT serves the same number in smaller groups, with pods sometimes separated only by seconds, around a grid-like network. In addition to bus and rail schedules, there is another feature of typical transit that isn’t part of PRT: each trip is an express ride to the selected destination. Rider groups are determined at the start of the trip.  The odds of people going from the same point A to the same point B at exactly the same time is quite low, so travelers share a pod when they plan to travel together, or several strangers going to the same place can negotiate ridesharing.

And because pods are usually ready and waiting, crowds aren’t expected to accumulate inside PRT stops, so most stops can be comparable in size to an elevator lobby. Crowds at train platforms and bus stops are partly caused by having to wait for scheduled departures — that’s not a judgment, it’s just how scheduled transit works.

PRT seeks to address the need for convenient transit access by having relatively short distances between stops.  Because stops are on sidings, they don’t slow down PRT traffic the way average speeds of trains and buses are reduced by frequent stops.  The ideal is that, within a PRT service area, people should never be more than a quarter-mile from a PRT stop — they are more likely to walk to PRT and not drive.  Thus the ideal distance between stops is about a half mile.  These small ridersheds also benefit the PRT network’s performance — rider demand and pod traffic is more dispersed than if there were fewer stations. This also helps keep the size of stops small.

PRT is network-based and on-demand, and therefore can’t be evaluated in the same way as corridor-based, scheduled conventional rail. Forgetting this difference has been a major source of misunderstanding over the years, and continues to this day.

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One Comment on “PODCARS – A Personal Rapid Transit primer”

  1. metoo Says:

    sure wish TransLink here in BC had the brains to adopt the latest in technology rather than using the 18th locomotives

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