The Beltway at 8:30 a.m. on Monday, Jan. 4, 2100, isn't going to be much less crowded than it will be tomorrow, despite futurologists' predictions about telecommuting. The kinds of cars, how they are powered and the experience of being on a major divided highway all will be different, but the numbers will be remarkably similar.

Let's start with the simplest prediction--naming the energy source for transportation systems in 2100. It won't be petroleum, given that our reserves are already dwindling. My own guess is that cars in 2100 will be powered by new kinds of batteries, and that the electricity to recharge them will come from an environmentally benign mix of solar collectors in the Arizona desert and nuclear reactors.

Being in the car will be different, too, because you won't be driving. By then we will have worked out the technical and legal problems associated with smart highways, and on-board computers will be steering you to your destination. Cruising the Beltway will be something like being in a private train car--you will be free to get on with your business while someone else gets you where you want to go. The car of the future will look more like a mobile office than a means of transportation.

You probably won't be going to the office every day, because you'll be able to do a lot of your work at home. This would mean that the percentage of the work force commuting on any given day will be lower than it is now. We can be sure, however, that the number of people living in urban areas will have risen considerably, which means that there will be more cars around. The result: roughly the same number of cars on the road then as now.

The same will hold true for longer-range travel. The number of business trips per person will decline as teleconferencing gets more sophisticated, but the number of people needing to travel will increase. The way we will travel, though, will change. High-speed rail links (most likely involving magnetically levitated trains) will connect major urban areas. Washington travelers in 2100 will no more think of taking airplanes to Boston than you would think of flying to Baltimore today. With trains capable of moving you from one downtown to another at speeds of 300 mph, who wants to deal with airports?

Real long-distance travel, though, will still be the domain of airline companies. There are a number of designs on the drawing board that could reduce the time for intercontinental flights--beating even SSTs. My favorite candidate in the breakfast-in-Washington, lunch-in-London sweepstakes is the suborbital airliner, which follows an ICBM-like trajectory above the atmosphere.

And, of course, if you really want to let your imagination go, picture adventurous travelers spending New Year's Eve 2100 in a resort at an orbiting hotel or even on the Moon. I think, though, I'll leave that one for science fiction writers.

James Trefil is the Clarence J. Robinson professor of physics at George Mason University.