Henry Miller once said "You can travel 50000 miles in America without once tasting a piece of good bread." While this may not have been the case when he said it, today his quote is true mostly because one can travel 50,000 miles without finding anywhere other than Subway at which to dine.
I've driven all over this nation many times in the past 10 years, and if I have one useful piece of advice to give road-trip planners it is to stay off of the Interstate Highways as much as humanly possible. Most readers can no doubt relate to the experience of taking long drives on I-80/90/5/Whatever and the crushing boredom that inevitably results. Like one of those cheap old Hanna-Barbera cartoons in which the animators re-use the same background endlessly, Interstate drives are a stultifying repetition of gray concrete peppered with a BP, McDonald's, Subway, and so on every 3 miles. It is possible to drive from Boston to Los Angeles on the Interstates without seeing a restaurant that is not in your home town.
If getting somewhere in a hurry is your objective, the Interstates are clearly your best bet. This is true. It is true for the same reason that the drives are so boring: the Interstate highway system is a massive military project, designed largely with the goal of moving large amounts of cumbersome, bulky military hardware (and personnel) over long distances in a short time. If one were to look up the legislation establishing the system in 1956, it bears the revealing name of the National Interstate and Defense Highways Act of 1956.
[sarcasm]Like all good things[/sarcasm], our military leaders got the idea from the Nazis. American commanders in Europe were stunned by the efficiency of the Third Reich's blitzkreig tactics, which relied heavily on the rapid deployment of highly mobile mechanized units. This was aided in no small part by the reichsautobahn which linked every major city and border point in the country. With the transport capabilities of aircraft still in their infancy, moving men and material relied on ground transport or rail (an infeasible option once aerial bombing advanced to the point that railroads could be easily disrupted).
Like our space and rocketry programs (thanks, Operation Paperclip!) the American plans for a highway system relied on German expertise and experience. The plan was simple: a standardized grid of four-lane (minimum) highways with a cleared, flattened median and shoulders and no curves sharper than 15 degrees. The soft curves (and maximized number of straightaways) allow today's drivers to use the highways without ever having to brake – there are no points at which, for example, one must drop to 30mph and take a sharp corner. The original purpose of this feature was to permit the transit of plodding, poorly-maneuverable equipment like tanks, artillery, and half-tracks. The practical effect today is that drivers can practically fall asleep or pay no attention to the road but still do 75.
The cleared median and shoulders, which today are the primary cause of the extreme boredom and featurelessness of Interstate driving, also have a military purpose. While it is an urban legend that the Interstates were designed as runways for bombers, they were in fact intended to allow the emergency landing of smaller fixed-wing aircraft as well as ground transport of large equipment that would overhang the sides of the road. One can hardly move a fighter jet down the highway if the median is full of trees.
In short, the highways were not in fact designed because the government was concerned about finding you a better way to get from Kalamazoo to Lubbock. While the Interstate system has functioned as a massive subsidy to American car culture (one that the gas taxes right-wingers bitch about don't even begin to cover), it was designed with the sole intent of getting Marines from South Carolina to California in a hurry and keeping open the flow of warheads from Pantex to SAC-Omaha once the nukes started flying. Over time, of course, the development of massive cargo aircraft like the C-5 and C-130 has substantially reduced the system's military usefulness. Nonetheless its military origins have left an imprint that is still felt strongly today, and it is this legacy you can thank every time you realize you have fallen asleep behind the wheel yet are somehow still on the road and doing 70.
As Charles Kuralt said, incorporating Mr. Miller's idea into his criticism of the highway system on aesthetic grounds, "The interstate highway system is a wonderful thing. It makes it possible to go from coast to coast without seeing anything or meeting anybody. If the United States interests you, stay off the interstates." Truer words never spoken.