On the average roadway there is much unused space, mostly for safety reasons due to the relative ineptitude of human drivers. Computers could do the job with fractions of inches tolerance instead of several or more feet. That means that your average interstate highway going in one direction with two lanes of travel plus shoulders, could have eight lanes of traffic, four in each direction. Like todays highways, there would be slower lanes and faster ones, with top speeds exceding 200 mph. The first lane would be for merging and freight, the second for freight and very short trips, the third for longer trips and off-hours light freight, and the fourth for the longest trips and will go the highest speed. The heaviest and largest freight may be unmanageable by the system but anything smaller than about one-and-a-half times the size a standard freight cargo container and weighing less than several hundred thousand pounds ought to be doable, even if it takes up three lanes of traffic and must be shipped during low use hours. Vehicles would come in a few different sizes with two seat and four seat vehicles being what is referred to when talking about personal vehicles and all other vehicles referred to as freight or light freight even if passenger transport is their main purpose. An average freight vehicle would occupy two lanes, could carry a large shipment container, be refrigerated or climate controlled, be enclosed or not, and would have top speeds near 100 mph (smaller vehicles such as those for more than four passengers or light freight could go faster). An onboard computer recieves a controlling signal from a central computer telling it what to do to seamlessly navigate the system. The same computer controls vehicle environment, monitors operating parameters, and provide feedback to the central computer, which is itself part of a system of regional central processors which co-ordinate with neighboring regions to keep traffic moving in an orderly and expeditious fashion. These computers should be among the most reliable in the world and run on a dedicated system not linked to any other in order to prevent hacking and other malintent. Hookups for internet, cellphones, or other applications will be through wireless portals inside a vehicle that will also provide many of the amenities found in cars today. Users will also be able to do emergency bathroom (or other) stops within a few minutes of asking or change destination en route (try that on a city bus).
The area that "The Scientific American" claims is required to provide half of Americas energy use in the year 2050 with solar power is about the same as the area the U.S. Department of Transportation says is roads in this country. While it is true that S. A. estimated for southwest desert and roads meander everywhere, it shows that the area already being used for roads has potential to be used to power all of the vehicles on those roads (transportation being only a little over one quarter not half of our energy use). The use of windmills to supplement solar should ensure the capacity to produce energy beyond what the system uses most of the time far into the forseeable future. A nuclear advocate might want to use reactors to power the system and it would be a relatively proper use of the technology, there are two serious drawbacks, in my opinion. Firstly, what does one do with the most poisonous and radioactive substances ever accumulated by humans and which will remain that way for many times longer than his civilization has even been existance? Secondly, what can be done about nuclear proliferation when America is the main proliferator? I am well aware that reactor fuel cant be directly made into a bomb but the same type of technology used for refining reactor fuel is used to refine bomb making materials and should be known and understood by as few people as possible, i.e. sanctioned researchers only. Once all of the current nukes (plants and bombs) are out of operation much of the technology used will be nearly impossible for anyone other than a state actor to obtain and use. I will get off my soapbox now and finish describing the system. All the electricity generated has to provide power to something. As of this writing, the systems being examined include electric rail, mag-lev, and conventional tire on pavement. All have benefits and drawbacks and will also affect the final cost analysis which is in itself in the earliest stages. Electronic eyes for a GPS-like system will keep traffic in line and efficiently going where it needs go.