Keeping a container of salt in the car sounds strange, but the idea comes from two old winter-driving tricks. The first is moisture control. Salt can absorb some moisture from the surrounding air, so people sometimes leave an open container inside the vehicle hoping it will reduce condensation and help prevent windows from fogging or icing on the inside. It may help slightly in a very damp car, but ordinary table salt is nowhere near as effective as a proper moisture absorber or silica-based product.

The second reason is more practical during freezing weather. Salt lowers the freezing point of water, so a small amount can help melt thin patches of ice around tires or on a frozen surface. Some drivers historically kept salt, sand, or cat litter in the trunk so they had something available if the tires became stuck on an icy driveway or parking spot. Sand or non-clumping litter usually provides better immediate traction, while salt works mainly by helping ice melt.

There are drawbacks, however. Salt is highly corrosive to metal. Spilling it inside the vehicle or leaving salty residue around the bodywork can contribute to rust, especially when moisture is present. Keeping an uncovered glass of salt loose inside a moving car also isn’t particularly smart—it can spill or become a projectile during sudden braking.

So there is some truth behind the old trick, but a sealed container of traction material and a proper car dehumidifier are better modern alternatives. The salt idea isn’t magic, and it certainly isn’t a secret police technique. It’s simply an old-fashioned winter-driving habit that survived because it can occasionally be useful in the right situation.