If you want the nearer I-5 sign in focus, you'll have to use 1/500 second (or faster) for exposure time for highway speeds. You can get away with something a little slower at speeds under 35mph, but I wouldn't go with too much slower unless it's a more distant object. A sign bridge that's 0.2 mile away doesn't need as fast a shutter speed as a closer-up reassurance sign at 300-500 feet away.
If you want the defroster vent lines to disappear, you'll have to use a circular polarizer on your lens. From a quick search, the Nikon D3500 kit lenses use either the 52-55-58mm diameters. Best $40-50 you'll ever spend on your camera (other than another battery). Usually the lens cap has a number stamped with an Ø on the inside of it.
My solution for that is a set of $3 dollar black pillow sheets from Walmart. Just throw those in your travel bag, iron at a hotel and they’ll easily eliminate 90-95% of windshield glare in a rental car.
I tried using black felt when I was really into road photography and it was no use; I just ended up with the texture of the fabric glaring off the windshield instead. Plus it was a huge pain to keep the cloth in place on the dash.
A polarizer seems like an intriguing solution to the problem.
What I’ve found is that I have to get a nice sweet flat spot on the dashboard. Once I hit it getting a good photo with the vent glare gone is easy. My Impreza has a somewhat ornate instrumentation cluster which makes this challenging something, the Challenger and Forester are much easier due their flatness.
Worth noting, I use a iPhone 11. Too much zoom makes for blurry signage photos, up to 1.3 magnification seems to the furthest I can take things without problems. I go up to 1.5-1.7 if I’m not taking photos of signage.
The really tricky thing with the black cloth in the Impreza is getting the placement right. If I get too much vent covered the cloth will flap all over with the defroster on. Too close to the wheel and the cloth will potentially slide back on uphill grades. Tape doesn’t do me any good given it tends to bunch up the smooth surface I ironed.
One thing that is worth mentioning is that no matter how good you are at taking road photos they likely will still not come out completely perfect. I always post edit my photo stock to minimize/crop out any artifacts from the image. I also usually two shot bursts for non-signage photos and three shots for photos with signage. I’ll go through after and pick the best images out of the bursts I like, the rest get deleted.
As of late I’ve been using natural shade cover when the terrain provides it to me. Objects like the side of a hill and a row of trees tend to yield the best lighting results.
