And what ISO were you shooting with f/16 at 1/100? Somehow I doubt it was ISO 100, with a polariser and ETR.
To give an example, a photo I've taken from an almost cloud free day at Yosemite overlooking the Nevada Falls is 1/25 at f/8.0 for ISO 100 (zoom of 25mm). In the interest of seeking to maximise my chances of the photo being sharp I use a tripod even if it is borderline ok for handheld with IS.
No, I was a bit above ISO 100. I wasn't using a polarizer. If I had used one, then yes, I can see why you would want a tripod. However, on a clear sunny day in fall, unless you are shooting extremely bright clouds, or again...water falls...I don't see a reason to use a polarizer or ND filter. Some of those people I saw, were indeed shooting water...but I was referring to the ones that were just shooting landscape with a lot of bright sky, mountains, etc.
The weather on the day I was referring to, was cool, with clear blue sky, few clouds. There was haze compared to Arizona I suppose, but you could only see it from mountain tops, toward the horizon.
Here are a couple shots I did, cropped to web size. (No the Corvette isn't mine...I was in a Bugatti Veyron...not!)
Many of the landscape shots didn't turn out all that well, compositionally (I've not spent much time going through them yet). We were very rushed to cover a lot of ground in a short time. However, quite a few did turn out ok that I shot through the car's windshield (I was passenger). On their own, sure they're compromised and mundane, but not if you consider they could be made into a photobook that would be a tour of that part of the parkway.