One other aspect is the design of the vehicle itself, more specifically how a uni-body is attached to modern vehicle frame, how body panels are made or shaped, and their location on the vehicle itself. Folds in sheet metal lips on wheel well arches or lower door panels just seem (no pun intended!) to retain winter salt brine and road debris.
I cannot stress as well the need to keep drip water drains in sheet metal panels clean to allow them to work as designed. There SO many places for these drains that car owners forget about, giving evidence to the adage "Out of sight, out of mind." If you park your vehicle outside in a wooded area or drive on dirt roads or off-road on a regular basis (like here in "the sticks" of Northeast Wisconsin), these drains tend to get clogged rather easily. Drains are located in:
1) All side door, rear hatch, or trunk panels, usually on the front and/or back lower corners
2) Gas filler neck near the bottom of the gas cap
3) Front cowl by the lower windshield ends on both sides
4) Rear hatch or trunk sill/gutter on both side, usually by the tail-light housings
5) Front fender near the the bottom where it meets the front door on both sides. Might even be an extension from the cowl area mentioned above.
I take a garden hose with a spray nozzle set to a jet stream and flush out that area on vehicle washes at least twice a year (early spring/late fall). Yes, it splashes water everywhere if the water-source pressure is higher, but you`d be surprised at how much gunk/crud/debris washes out of that area.
I use tweezers to pull out old leaf stems, small twigs, and grass clippings that get embedded or stuck in those drains.
Vans with passenger sliding doors gather a lot of sand, silt, and gravel detritus (small rocks or stones) in the bottom body sill area where the door support and slide mechanism is housed. I take a larger long-handled artist`s brush and try to brush areas where it gathers as I vacuum with a skinny nozzle to remove that sand. I do this before the washing to keep it from turning to mud.
Another area that get a lot of dirt trapped is door seals or weatherstripping itself next to the body. I take an old microfiber or shop paper paper (thick heavy-duty blue stuff) soaked in APC (Megs D101 or Optimum Power Clean) and force it between the seal and body to try to get out dirt that gets embedded in that area. if I cannot reach far enough in, I use a cotton swab (Q-Tip) soaked in APC to remove the debris. Works well on the seal itself if it has a deep track or recess. All part of being an Obsessive-Compulsive Detailer.