Most of us travel the Interstate Highway system often, either for our daily commute or errands or longer trips. The expressway or freeway (or possibly a toll road) in your area usually provides the easiest way to get from here to there. I thought you might enjoy knowing what went into this system of highways.
I spent some years of my working life preparing environmental impact statements for some Interstate highway connections proposed in the late 1970s, so very much appreciate the engineering aspects of these roadways and have one word of advice: when you drive a section where the speed limit has been reduced to something low, like 50 or 40, PAY ATTENTION!!! There are reasons for that restriction and your safety is involved.
The U.S. Interstate Highway system had its origins during FDR’s Presidency, when the feasibility of designing three north-south and three east-west routes were conducted. Dwight Eisenhower, returning home from his WWII tour in Europe, appreciated the carrying capacity of a wide high speed highway built for long distance commerce. The original plan to build 26,700 miles in 1941 was then increased to 41,000 in 1956 when Congress initiated finding. There were over 46,700 miles constructed by 1972.
The design standards at that time mandated fully controlled controlled access (which is why we have highway on and off ramps and not just cross streets), at least two lanes in each direction (with several exceptions, including the “Super 2 through Franconia Notch in New Hampshire), 12-foot lane widths, 14-foot overhead clearance in urban areas and 16-foot overhead clearance in rural areas (to permit transportation of military apparatus on top of flatbed trucks) , 10-foot right paved and 4-foot left paved shoulders, and the speed capacity of 50 to 70 miles per hour. Initially, the design had to meet traffic level expectations for 1975, but, as we well know, that standard has been modified over time with more lane construction where it was economically possible.
Routes with odd numbers run north and south, while even numbered run east and west. For north-south routes, the lowest numbers begin in the west, while the lowest numbered east-west routes are in the south. By this method, Interstate Route 5 (I-5) runs north-south along the west coast, while I-10 lies east-west along the southern border.
Interstate exit numbering systems have mostly changed to the Milepost numbering system — All Interstate routes are mileposted beginning at the most westerly or southerly point. The beginning point is milepost ‘0’. If the first interchange on the route is located between milepost 4.0 and 5.0, it is numbered as Interchange #4. The next interchange, if located at milepost 8.7, would be numbered as Interchange #8, etc. With this system the motorist can easily determine the location and distance to a desired interchange.
It is not true that one mile of every five miles of Interstate highway must be straight enough to allow planes to land on it. This is an urban legend.
Other facts found on AA Roads Interstate Guide http://www.interstate-guide.com/interstate.html