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Personal Rapid Transit System Safety
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| Concern: |
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The minimum headways required by PRT are not achievable, safe, or realistic |
| Reply: |
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| The minimum headways proposed for a PRT system are one-half second. While this is far too short for the human controlled automobile that we are used to using, this is achievable by a PRT system specifically engineered to operate in these conditions. Lets examine the differences between PRT and automobiles for some of the items that affect system safety: |
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- Reaction time
A person requires from .3 to .6 of a second to recognize a problem and begin applying the brakes to their vehicle, assuming they are not looking out the window, intoxicated, or otherwise distracted. A PRT system uses a zone control computer that continually monitors the vehicles in each section of guideway. The time for the computer to poll all the vehicles in a section and issue a stop command, plus the time for the vehicle computer to process that command and begin applying the brakes will be under .15 of a second.
- Braking system latency
Once the control system reacts to an event and begins braking, further time is needed for the brake system to begin braking fully. In an automobile, this is around .1 seconds - most of this is time taken up by the driver to fully depress the brake pedal. However, the linear induction motor on the PRT system has no moving parts. Its latency speed is limited by how fast an electromagnetic field can be reversed. This value is about 10 milliseconds or .01 seconds.
- Braking system effectiveness
The brakes on an automobile rely on friction between the pavement and the tires, and between the brake pads and wheel to stop the vehicle. Thus extra headway is needed to compensate for variations in the system. Tires wear down and become bald, some pavements are smoother than others, brake pads wear down or wear unevenly. Water, snow, and ice also reduce the amount of friction between tires and pavement, limiting braking effectiveness
In contrast, the PRT vehicle's linear induction motors never normally contact the running surface. They rely on the electromagnetic force between the running surface and the motor to slow the vehicle. This allows for more predictable performance since there are no friction surfaces to wear smooth. The PRT "brakes" are not affected by water or ice on the running surface as the electromagnetic forces between the motor and the guideway are not effected by water, ice, or oil on the guideway.
- Vehicle coordination
There is little coordination between automobiles traveling with short headways in a traffic lane. Each vehicle is being operated independently of the others by its driver. In contrast, PRT vehicles traveling close together can be commanded to slow, stop, or increase speed at virtually the same instant. If the zone controller notices a problem requiring an emergency stop, ALL the vehicles begin braking at the same time, and since the PRT vehicles are far more similar to each other than automobiles on a roadway, they can come to a coordinated stop with less risk of collisions between the vehicles.
| Perhaps the best evidence that these headways are achievable comes from research into improving the highway system. In 1997 the National Automated Highway System Consortium demonstrated a platoon of modified Buick LeSabres traveling at 60 MPH with headways of just over 4/10 of one second. The task of making these larger vehicles, with slower reacting brakes, and de-centralized control systems travel this closely is far more difficult than building a special purpose PRT vehicle to do the same. Yet hundreds of journalist and spectators safely rode in these vehicles during the Demo97 demonstrations. If we can do it on an open highway with 2000 lb vehicles that must also allow for human control, then we can do this with thousand pound PRT vehicles on a controlled guideway! |
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PRT Skeptic's Pages
Here we attempt to address issues brought to us by PRT skeptics in a question and answer format. If you have a concern that you think should be addressed, send us an email at Info@ACPRT.org or fill out the Skeptic's Question form and we will do our best to answer it.
Be sure to check back every once in a while as we will be adding items from time to time.
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