Friday, July 30, 2010
What is PRT?
In very few words, PRT (Personal Rapid Transit), is a system that provides riders with on-demand personalized transportation from origin to destination without any stops and without being slowed down by traffic. In most cases, the vehicles wait for passengers rather than the other way around. I like the name Smart Transit much better than PRT, because it represents better the fact that the system provides personalized transportation controlled by computer programs that are programmed to maximize efficiency. Watch this video about the Echo Control of Taxi 2000
There are a few companies that designed PRT systems and all of them opted for the overground solution. In my next post, I will talk about the reasons PRT was designed the way it was as well as the results of optimizations that were exercised to achieve the most efficient and cost effective system. There are many ways to do it wrong and some of them were already tried. The basic design consists of a track detached from the ground and small vehicles big enough for a traveling party but also small enough to make it cheap to construct and operate. Some systems designed it for four passengers and others for three. The main principle is that vehicles that don't need to stop at a station don't need to stop. The stations are off track and only vehicles with passengers that wish to stop at a particular station go off track into a bay to stop and therefore do not obstruct other vehicles that wish to continue to a different station. This is a simple and powerful idea. This concept is shown on the picture above. The tracks are designed as a a grid and cover an area. The intention is that there will as many stations as possible. More stations, as opposed to other methods, do not slow the system. They only increase the number of possible origin-destination pairs and therefore the usability of the system.
There is a control center that commands the vehicles and "senses" where demand is needed and regulates traffic to that location. This control was not easy to develop but also here a few companies have already done it. The operation from the passenger's point of view is extremely simple. If you stumble upon a station one can enter it, swipe a bar code card, press a button with the desired destination, enter a vehicle, sit down and the vehicle will get you there at the shortest possible time. You don't have to worry about which station to go to, or schedules, or transfers because any station will take you the your destination, on your schedule, without any stops.
Since the operation is based on on-demand trips, tickets can be personalized. One can have an account with the control center with some credit in it and when the bar coded ticket is scanned, the system charges his or her account. Buttons can be personalized - for instance the number 2 can mean "home" and number 3 could mean "work". A family could have different tickets that belong to the same account and certain tickets could be programmed to allow access to only predetermined destinations, such as "school", "home", or "friend A".
Vehicles tend to wait for passengers in every station. Vehicles are being stored at stations when there is low demand. In a computer simulation a PRT system designed for Cincinnati, the programmers simulated a very challenging situation. The end of a baseball game where many passengers want to use the system at the same time in a confined area near the stadium. The result was surprising - 98% of passengers had to wait 2 minutes or less for a vehicle. This is an amazing performance. We need to understand that stations are not at all similar to subway or train stations. Since the wait is so low, people don't stay at the station and therefore stations could be much smaller. In fact, stations can be a post in an open area, like a bus. The only restriction is that tracks should be off limit for pedestrians, so some safety issues have to be addressed. Those have been addressed by PRT designers and there are some with very elegant solutions.
Another feature that allows such a performance is the headway of 0.5 seconds between vehicles. It was demonstrated both mathematically and with live vehicles that this distance (which varies with speed) is safe and feasible. This allows a guideway to have a capacity of 3 highway lanes (the headway between vehicles in highways is about 2 seconds which translate to a capacity of 30 vehicles per minute, as opposed to 120 vehicles per minute in a PRT system which is controlled by computers). This spacing between vehicles was demonstrated 13 years ago. To view a video of this, click here.
As I mentioned, there are many ways to design a PRT system. My next post will be about the factors that determine the design. How you would you optimize the system economically and socially.