The dark path from the door to the sofa is the most common carpet complaint there is, and it is not one problem. It is three separate things happening in the same place, and they respond very differently to cleaning.
Knowing which of the three dominates in your case tells you whether a clean will fix it, improve it or barely touch it.
Cause one: soil in the pile
The obvious component. Grit, dust and oily residue accumulate along the route people walk, and the concentration is far higher than in the rest of the room because every visitor takes the same line.
This part responds fully to cleaning. Remove the soil and the colour returns, which is why a well cleaned traffic lane on an otherwise sound carpet can look almost new again.
Cause two: crushed pile
Carpet fibres are cut ends standing upright, and they reflect light from those ends. Walk on them repeatedly and they bend over, so light reflects from the sides of the fibres instead. The area appears darker even when it is perfectly clean.
This is why a lane sometimes looks better in the morning and worse in the evening light, and why the difference shifts as you walk around the room. Cleaning helps considerably here, because the process of agitating and grooming the pile lifts the fibres back upright.
- Crushing shows as a change in sheen rather than a change in colour.
- The appearance changes depending on where you stand.
- Grooming and pile lifting during cleaning improves it substantially.
- It returns with continued traffic, so it is managed rather than cured.
Cause three: abrasion
This is the permanent one. Grit trodden into the pile acts like sandpaper, scratching and fracturing the surface of each fibre. Damaged fibre no longer reflects light cleanly, so it looks permanently dull and slightly grey.
Nothing restores abraded fibre. Cleaning removes the soil sitting on it, and the area will improve, but it will never match the untrodden carpet beside it. This is wear, and the only real answer is preventing more of it.
Working out which one you have
A simple sequence tells you most of what you need. Vacuum the lane thoroughly, then look at it from several angles and compare it with a protected area such as under a sofa.
If it looks much better after vacuuming, soil dominates. If the darkness shifts as you move, crushing dominates. If it looks the same from every angle and the fibre tips look frayed or shiny under a bright light, abrasion has set in.
- Improves greatly after vacuuming: mostly soil, cleaning will fix it.
- Appearance changes with viewing angle: mostly crushing, cleaning helps a lot.
- Uniformly dull from every angle: abrasion, cleaning improves but cannot restore.
- Compare against carpet that has been under furniture for years.
Preventing new lanes
Since abrasion is caused by grit, everything that keeps grit out of the carpet extends its life. Barrier matting at the door does more than any other single measure, and it needs to be long enough for several steps rather than a token mat.
Spreading the traffic helps too. A hallway runner takes the wear instead of the carpet, and it can be replaced for a fraction of the cost of the floor beneath it.
- Fit a barrier mat long enough for three or four steps at each entrance.
- Vacuum entrance areas and traffic routes several times a week.
- Use runners on the heaviest routes and replace them rather than the carpet.
- Rearrange furniture occasionally so the walking line changes.
- Consider carpet protector after cleaning on the busiest areas.
Carpet protector on traffic areas
Protector coats each fibre so soil sits on the surface rather than bonding to it, which means vacuuming removes more and spills have longer before they penetrate. It does not make carpet dirt proof, and it wears off with traffic, which is why it is reapplied after each deep clean.
It makes most sense applied selectively to the areas that actually need it: hallways, the route through a lounge, and the stairs. Treating a spare bedroom is money spent where nothing was going to happen anyway.