Why Roads Are Gritted as Temperatures Drop Below Freezing

With temperatures expected to fall as low as minus three degrees tonight and gritting operations planned across Cork's main routes, here's why road crews apply salt to prevent ice forming on our roads.

Gritting works by lowering the freezing point of water. When spread on cold road surfaces ahead of a frost, the salt dissolves into moisture already present and forms a brine. This mixture requires a much colder temperature before it can freeze, which means black ice is far less likely to form.

Pre-treatment is essential because once a road is already frozen, salt becomes much less effective. This is why crews apply grit in the evening whilst temperatures are still above freezing, giving it time to work before the coldest hours arrive.

The salt used in road gritting is the same chemical compound as table salt, sodium chloride, though it comes in much coarser grains. When it comes into contact with moisture on the road surface, it creates a saline solution that remains liquid at temperatures well below zero degrees. Pure water freezes at zero degrees, but a strong salt solution can stay liquid down to around minus six degrees or lower.

Timing is crucial. Grit needs traffic to help break the salt crystals and spread them evenly across the road surface. The first application typically happens in the early evening whilst there is still traffic movement, then again in the early hours when temperatures reach their lowest point. Continuous monitoring overnight allows maintenance teams to respond quickly if temperatures fall faster than forecast.

Crews target specific routes that carry the highest volumes of traffic and those most exposed to risk. Bridges and flyovers are particularly vulnerable because these structures lose heat faster than ground-level sections. With no insulating ground beneath them, they can freeze earlier than the surrounding road network, sometimes catching drivers by surprise when the air temperature is still marginally above zero.

However, gritting has its limitations. It becomes less effective when temperatures drop below minus six or seven degrees, as even the salt solution can begin to freeze at that point. Heavy rainfall can also wash away grit before it has a chance to work, whilst very dry conditions mean there's less moisture for the salt to dissolve into.

Drivers should still take care, as gritting does not make roads completely ice-proof. Shaded areas, exposed routes and untreated local roads may still freeze. Speeds should be reduced in cold, dry and clear conditions, which are the most common setup for black ice.

The science behind gritting is simple but effective. By lowering the freezing point of water on road surfaces, salt gives drivers crucial protection during the coldest nights of the year.