Who invented the interstate highway, and where was the first one?

If you mean the first in the world, I think that would be Hitler and the Autobahn.
http://www.german-way.com/german/autobahn.html

well, in terms of long distance highway development, Hitler had engineers develop the Autobahn before World War two as both a way of providing employment and preparing for war-machine movement in the impending war.

(Almost) anything you want to know about the U.S. Interstate Highway System:

Dwight D. Eisenhower National System of Interstate and Defense Highways

This link mentions: "The oldest Interstate segments actually predate the establishment of the Interstate system. Early examples include a portion of the Grand Central Parkway in Queens, New York, which was opened to traffic in July 1936 and later was incorporated into the Interstate System as I-278."

Three States Claim First Interstate Highway

Omnes viae Romam ducunt.* - Old Roman Saying


"The road system of the Ancient Romans was one of the greatest engineering accomplishments of its time, with over 50,000 miles of paved road radiating from their center at the miliarius aurem in the Forum in the city of Rome. Although the Roman road system was originally built to facilitate the movement of troops throughout the empire, it was inevitably used for other purposes by civilians then and now."
" The Romans were the first ancient civilization to build paved roads, which did not prevent travel during or after inclement weather. Indeed, mud or gravel would hinder, if not completely halt many vehicles pulled by animals or other people, not to mention discourage travelers on foot. Roman engineers, however, did not stop with just paving Roman roads. Roads were crowned—that is, they were higher in the middle than on the sides to allow water to run off—and they often had gutters for drainage along the shoulders. Probably the mostincredible engineering feat concerning the Roman road system, though, is how well the roads were built. Many are still major thoroughfares for cars today. Indeed, their road-building methods were unsurpassed until the invention of the macadam in the 19th century." -
library.thinkquest.org

"Expansion of the Roman Empire was both facilitated by and an impetus to the development of an efficient system of roads. They were usually built by a consul or other important magistrate, starting in the early Republic. (The Roman Republic dates from circa 509 BCE - DG) The roads were important militarily, as they bound together the steadily growing Roman Empire. A good road would shed water during the rainy season and permit travel at a rapid pace during all kinds of weather. they were surfaced with stone paving blocks, had a drainage ditch on each side and were crowned to shed water. The major Roman roads were built upon a foundation of carefully laid rock which was constructed from a large ditch dug into the underlying earth. In this way, if the ground became waterlogged, a tight foundation layer helped prevent anyone traveling on the road from sinking out of sight in the mud. City streete were paved with large fitted stones lain upon a foundation of rock also." - Discovery Channel

* All roads lead to Rome.

Roman roads underly many old roads here (England) , or did until the introduction of tarmac roads; these simply followed the main routes of the Romans. Just a mile away from here there is one that is still used as a cart track on the neighbouring farm.It runs across fields and also serves as a public path; one of the older ' public rights of way' in this area (not the oldest; these follow Iron Age routes). This road, so far as anyone from Cambridge University can tell, has been unaltered and untouched by significant repair since it was constructed as a connector road for the Roman military. So the ancient joke that the difference between modern roads and the Roman ones is that theirs have lasted until today, has some truth in it

Incidentally, it was a Roman road that proved that they drove on the left; naturally being so classically educated the Britons continued their practice. One leading to a quarry had much deeper grooves worn on the side where the wheels of laden carts came from the quarry to those on the approach side where the carts were empty. The 'empty', approach side,is the one on the left as you approach and the 'laden', exit side on the left as you leave.