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How we decide when to salt

Hertfordshire County Council have eight winter duty officers who work a one week in five duty rota during the seven winter months.

The duty officer decides whether to salt or not, and that decision applies to the whole county including our agents where applicable, but not motorways and trunk roads.

The primary information used for that decision is the weather forecast. Hertfordshire have a contracted weather forecaster who provides specific winter road weather forecasts for the county.

The forecaster uses current and historical information from eight roadside weather stations installed by Hertfordshire which show air, road surface and ground temperatures, wind speed, dew point, and concentration of salt on the road. The decision is always made on the worst of these weather stations, which may often be the site located on a cold bridge.

The coming night's weather forecast is usually recieved at 1pm, along with a 2-5 day preliminary forecast. The duty officer at that time uses available information to decide whether further decisions will be required and whether to put the salting contractor on ‘standby’. Updated forecasts are received at 9pm, and 2am if necessary, and the duty officer can speak to the forecaster anytime 24hrs a day if he wishes.

The duty officers have direct access to information at each of these roadside weather stations 24hrs a day via a laptop computer and a telephone line, as well as from the weather forecaster.

The duty officers have had training as to what weather conditions are likely to produce frost or ice formations, and the forecaster will indicate to the duty officer the likelihood of these occurrences for the coming days and nights.

Just because the general weather forecast (what the public hear on local radio for example) says temperatures will fall below zero, it does not mean conditions will cause ice to form on roads. There can be enough heat in the ground to keep the roads above freezing. Conversely, temperatures may stay above zero, but conditions can allow ice to form.

Temperatures can be well below zero, but if there is no moisture present then ice will not form. Moisture can be in the form of rain, snow, dew, mist/fog or seepage from surrounding land.

Salt spread on the road will lower the freezing point of the moisture on the road surface. The higher the concentration of salt in the moisture, the lower the temperature has to fall before the moisture freezes.

If conditions are right for freezing, then the duty officer will also need to consider other factors before deciding whether to spread salt. The most important of these is whether there is already enough residual salt on the roads from other salt spreading.

Factors determining how much residual salt is already on the roads are;

• Local knowledge of the roads, can salt be seen on the roads, either white appearance on the road surface, or can the grains of salt be seen?
• Traffic volumes, speed and type
• Length of time since the last salt was spread, and how much was last spread
• Road conditions at the time the salt was spread, salt will stick better to a damp road with less ‘bounce off’ of the salt grains
• Whether rain or run-off might have washed some of the salt away
• Whether the salt has dried out and could have trafficked away or been blown away
• What indications are given by the roadside weather stations as to concentrations of salt on the road

In addition, the duty officers have access to salting decisions made by neighbouring highway authorities as well as the motorway and trunk road network running through Hertfordshire so that a consistent approach can be taken.

If all the indications are that ice may form on the highway anywhere on our salted network, then the decision will be made to spread salt at the appropriate spread rate at a suitable time (usually avoiding rush hour traffic) so that the whole of the salted network across Hertfordshire is treated before the time that ice is predicted to form.