Machine Design

Letters - 06/05/08

I don’t think Mr. Rapp’s suggestion, the Public Personal Mover (Letters, April 10), should be rejected out of hand. With slight modifications, his plan could answer your objections.

Power to the people mover

Suppose the PPM is a purely electric vehicle with enough battery capacity to get from home to a rail terminus and back. This would most likely be relatively slow driving over fairly short distances.

The PPM would also have the electrical and mechanical connections to the rail system. So you drive to the terminus where your PPM hooks up to the rail. Then the rail controller takes over and directs the PPM to the rail terminus nearest your destination and the driver is now just a passenger.

While en route, the PPM recharges its battery. While on the rail portion, speed and traffic density could be higher than current highways, thanks to computer controls. Once at the destination, the fully charged PPM disconnects and the driver takes control.

This system would alleviate most design issues standing in the way of pure electric vehicles, give some measure of personal choice in style, and provide many of the benefits of public mass transit without giving up door-to-door service. The state (or whatever organization owns the rail) has reduced costs because many of the moving parts would be in privately owned and maintained PPMs.

The concept could be expanded to include those who choose not to buy PPMs by having the rail operator invest in some rolling stock to carry them from depot to depot. This mix of publicly and privately owned PPMs would also be beneficial during start-up: more public PPMs running depot to depot initially, then more and more privately owned PPMs as individuals see the benefits.

Kim L. Ground

Mr. Rapp’s letter does a fairly accurate job of describing the Personal Rapid Transit concept developed by Prof. (Emeritus) J. Edward Anderson, formerly of the Mechanical Engineering Dept. of the University of Minnesota. His work started in the late 1960s when he saw that existing rapid transit was a money pit, just as Mr. Rapp described it.

Prof. Anderson and his team of graduate students (and later his company Taxi2000 and now PRT International) investigated rapid transit with the goal of devising one that made economic sense and that people would want to use. They ended up with a system of small three-passenger cars moving people in urban areas than any ground transportation and for about the same cost as a taxicab. It would not require ongoing taxpayer subsidies to stay afloat, unlike light rail, buses, and subways.

PRT would run at about the second floor level on guideways supported by reinforced poles, so there would be little harm down to the urban environment and practically no use of street-level area. The system could deliver people and reasonably light packages, and most urban streets could be converted to parks over time. The system uses electricity so urban transportation would need little, if any, fossil fuel if we can produce electricity cheaply and with minimum environmental impact (say with nuclear-power plants).

There would still need to be other vehicles and many could be electrically powered, but some would undoubtedly end up needing an internal-combustion engine. However, PRTs in urban areas running on electricity from nuclear power could reduce America’s CO2 generation by around 40% and cause little harm to our economy or standard of living. And air quality within our urban areas would improve.

More information is in white papers from and

William J. Brooks

Your answer to Mr. Rapp’s letter about his idea for a People Mover was typical of the responses many ideas get. Your first sentence is about what you don’t like about the idea, and then you tell Mr. Rapp he would be better off to forget the idea. Looks to me like your “editor” status and ego got in the way. In my opinion, Mr. Rapp was trying to describe his out-of-the-mainstream idea and stimulate some positive discussion.

When are we ever going to get away from the urge to put down new ideas rather than looking at it with a critical eye, selecting what’s good about it, and then going on to improve it? That’s what engineering is. After analyzing a new idea, we should try to keep its good features and improve on the design. The initial idea might not be perfect, but a subsequent development of that idea may reach the goal.

David F. Carlson

Keeping track of CFLs
I’d like to add my two cents on compact fluorescent bulbs. In summer and fall of ’98, I converted all 35 of the 100-W mercury-vapor sidewalk and alley lamps of my condo complex to 27-W (now 23-W) CFLs. This is in southern California where it seldom gets to 32°F or below. These are all outside, on photocells, and run all night. I’m something of a skeptic, so I kept all receipts and made careful records. If a lamp didn’t last at least a year, it went back for a refund. (Home Depot was most gracious.)

Maybe colder weather is a factor in the shorter lifetime results in other parts of the country. I am more inclined to suspect power surges, perhaps caused by lightning. Mercury-vapor lamps are almost indestructible. In my experience, they last well in excess of 10 years, and the electronics in the CFLs have got to be relatively puny.

Bill Kelly

My understanding is that you Californians are lucky in that your cost of CFLs is somewhat subsidized by state and local government.

At any rate, my GE source told me that a lot of the premature CFL failures they found were due to the bulbs being used in recessed lighting fixtures where operating temperatures exceeded what the bulbs were designed for. Apparently some CFLs are designed for recessed lighting, but that’s a special kind. And this is not entirely clear from what’s printed on the bulb packaging. — Leland Teschler

Many of the “super cheap” CFLs I have bought lately have been subsidized by Southern California Edison. The latest, a box of four 23-W CFLs with the brand name “Greenlite,” has a sticker saying “The price includes a discount courtesy of Southern California Edison. The program is funded by California utility customers and administered by SCE under the auspices of the California Public Utilities Commission.”

I have to assume the discount is substantial because the price of the four-bulb box at a discount store was $1.49. However, we are getting to pay for it through our electric bills (a relatively local loop) and not through Federal taxes (which I suspect would be much more costly).

I have also seen essentially the same statement on at least one other brand of bulb. If there are other types of subsidies, I am unaware of them.

I can’t argue with the possibility of overheating the bulbs. Certainly, incandescent and mercury bulbs are required to run comfortably at high temperatures. However, most applications would involve putting a CFL into a fixture built for a much higher wattage lamp, such as a 23-W CFL in a fixture rated for a 60 or 100-W incandescent. Overall it would run much cooler, but if that is still be too hot is the question. In most cases I doubt heat would be a problem.

Bill Kelly

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