The 1905 picture above shows the Riverside Drive as it transitions from being of ordinary street construction to an elevated viaduct that is a third of a mile long. The street was not part of the original grid plan, but rather a reaction to the topography of the Upper West Side: what is now Riverside Park was, pre development, a steep hill from the UWS plateau down to the Hudson River, and laying out the originally-planned grid would have been a nightmare. Rather than build unusably steep streets, the edge of the grid was modified, with Twelfth Avenue eliminated above 59th Street (a rail freight yard belonging to the Hudson Division of the New York Central blocked it there anyway) in favor of the Drive, laid out, about half a block to the east, on top of the hill. The land between the Drive and the river was called Riverside Park long before it was developed into anything that actually looked like a park; the Drive became a prime location for high-end housing because of the uninterrupted views across the river.
The Upper West Side more or less seamlessly transitions into Morningside Heights, but then there’s a geological interruption: there’s a steep valley running diagonally northwest across the grid (today, it defines the path of 125th Street, which deviates from the regular grid at its west end). That valley is what the viaduct spans over. (It should be noted that the valley is regularly described, sometimes in breathless terms, as an earthquake fault line. It is, but it’s actually a small piece of a line the extends both east and west of the visible valley, and its dramatic drop is primarily the result of erosion, not seismic activity.) Since there are reasonably large plateaus to the south (Morningside Heights/UWS) and to the north (Hamilton Heights) it made sense to simply continue the Drive across at a constant elevation. Similarly, the IRT subway under Broadway, a long block to the east, changes from a tunnel, to a bridge over the valley, and then back to a tunnel, simply by running at a constant elevation.
The best view of the viaduct is from underneath, where the regular street grid from the east comes in below the high structure. You can see the 1900-era engineering that holds the Drive up, in all its trussed and arched glory, creating one of the most overtly Gotham-City-like settings in New York. The south end:
Past 125th Street:
Near the north end, running past the granite-faced retaining wall for the Hamilton Heights plateau:
And the north end, where the Drive curves to head onto the plateau: