Friday, August 8, 2008

Efficiency of PRT and LRT

Which is more efficient, a PRT (personal rapid transport) or LRT (light rail transport)? What is the difference?

A PRT operates like a taxi. It offers point to point transport. It is used exclusively by the initial passengers and does not stop at intermediate points to pick other passengers. The journey time is shorter. All passengers have a seat.

A LRT operates like a bus. It stops at all intermediate stops to allow passengers to embark and disembark. The journey time is longer, as the train has to stop at every stop. Some passengers have a seat; others have to stand.

The train (i.e. LRT) is less efficient as all the passengers have to make many stops, which takes up energy. If the train is replaced by 100 pads (say) and each pod is able to travel non-stop on its journey to its destination, it should save energy.

Studies have shown that the energy used to transport a passenger on the PRT is lower than the LRT or MRT.

1 comment:

  1. A Transportation EnthusiastAugust 9, 2008 at 9:25 AM

    If trains ran only during peak periods when they are full, and didn't run at any other time, they would probably exceed the efficiency of PRT. But they can't stop running when there's lesser demand, and that's why overall they are significantly less efficient than a well-design PRT system.

    I've often seen misleading quotes about rail energy efficiency - misleading because they quote energy usage per seat rather than per passenger. A train may have 600 seats, but on average maybe only 75 are in use at any given time. If they quote energy usage per seat, that's 8 times larger than the per-passenger usage, and what really matters is per-passenger usage.

    The problem with trains is that they don't adapt well to variations in traffic levels. Too few vehicles and there's not enough capacity to support the rush; too many vehicles and off-peak energy usage (and costs) become unwieldy. It's an unavoidable tradeoff.

    Some trains can run with fewer vehicles during non-peak periods, and this can increase overall efficiency. But most systems just reduce schedules to conserve energy and cost, and this results in a less convenient system for passengers.

    PRT runs only when needed, and therefore can scale both up and down - it consumes the same amount of energy per passenger regardless of whether 1 or 10,000 people are riding, and does so while providing 24x7 service. In that sense, it scales down as well as it scales up.

    Trains are still efficient in some places where density warrants it - but in most places PRT is better in terms of both efficiency and passenger service.

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