Why do motorcades upset me?

On my way back to work from a midday walk, I found cop cars blocking each cross street of 16th Street, which is a north-south boulevard leading to the front of the White House. Auto and pedestrian traffic was halted in all directions. Sixteenth Street was eerily barren, with people tapping their feet on the sidelines. The president, you see, was returning to his house. The parade, in order of appearance:

9 motorcycles in a row, spaced 100 feet apart
3 black SUVs
1 armored conversion van
2 black limousines (one presumably carrying POTUS or his deputy)
1 ambulance
1 helicopter
3 black SUVs
3 police cars

This list does not include several police cars that zoomed down side streets, as if to flank the president in case of an attack from the side.

Whenever I see a motorcade, I'm left with the same thoughts: Is this really necessary? What does this accomplish, other than making a spectacle? If the president really wanted to be protected, wouldn't he travel in a single, quiet, nondescript, armored car rather than in a blaring Macy's Day parade? The president might as well be waving from the limo's sunroof. And shutting down a major urban thoroughfare for 10 minutes seems like an intensely irreponsible thing to do. It's very Triumph of the Will-ish, except the citizens lining the motorcade are grumpy, not ecstatic.