How we decide when to grit the roads
Decisions are made daily based on local forecasts and on road temperatures rather than air temperatures. Salting is likely to take place whenever road temperatures are forecast to fall below +1ºC and ice is expected to form.
This decision is not taken lightly as each run uses approximately 65 tonnes of salt if we use a pre-wet mix, or 85 tonnes of rock salt if we lay the salt dry. Costs include the salt, labour and fuel.
The process for making the decision is as follows:
- Readings are taken from monitoring stations around the county. These stations monitor ground temperature as the decision to salt is based on the temperature of the roads rather than air.
- Daily weather forecasts are received including graphs showing the predicted ground temperature and dew point.
The dew point is the point where air has cooled to a degree that it can no longer hold moisture. If the ground temperature is forecast to drop to zero, ice would be expected to occur on untreated roads.
As well as the technical reports, experience and team work plays a big part in deciding when to send the gritters out.
Why roads may still be icy
No guarantee can be given that roads will always be completely clear of ice or snow. If we are responding to a late change in forecast, or if we have to wait for rain to clear before we salt, wet roads may freeze before they are salted. We can not salt during wet weather as the salt would be washed away.
In severe cold weather below -8ºC, even salt will not prevent ice from forming.
We usually send the gritters out at 7pm and 4am to ensure the roads are treated before the road temperature drops below 0 and before peak travel time in the morning. This also means the gritters should have a clear run of the roads as they can struggle to effectively salt a route if there is a lot of traffic.
If it is raining in the early evening, a decision may be taken to salt later so the salt doesn’t get washed away. If snow is forecast before 7pm the gritters may be sent out earlier, before the afternoon rush hour.
Two decisions are made over gritting in Buckinghamshire – one for the north of the county and one for the south, as weather conditions and road temperatures may vary.
Once a decision has been made, a Twitter alert is sent out. You can follow us on Twitter @tfbalerts for daily updates.
When we grit
We grit or salt a network of key roads around the county. These roads are known as Precautionary Routes and are salted routinely whenever a decision to salt is made.
These roads consist of all A roads and B roads and some C and unclassified roads.
At times if salt stocks are low these routes have to be reduced to what we refer to as Emergency Routes.
When all precautionary routes have been adequately treated a decision may be made in a cold spell to salt what we refer to as Secondary Routes.
These routes will generally be roads that will create links to roads on the precautionary routes improving the network. They are also roads that will provide improved access to key facilities.
In the course of a prolonged cold spell when all precautionary and secondary routes are clear of ice a decision may be made to salt other roads. This depends on salt stocks and does not include residential cul-de-sacs and private roads.
We decide which roads to grit based on a scoring system:
- Gradients 0 / 4 /10 / 20 points
- Bends (roads over 40mph) 0 / 4 /10 points
- Community link (200+houses). 5 points
- Traffic Flows 0 / 10 / 20 points
- Historically salted route 5 points
- Public and School Bus Routes 0 or 15 points
- Adjacent key facilities 10 points
- Route practicality and efficiency add/deduct points
A road will need to score on more than one issue to gain enough points to qualify for attention. A minimum of 28 points are required.
Trained and experienced inspectors carry out these assessments and the routes are decided before the winter season.
Why we prioritise
- Cost: Rock salt is supplied to the local authority at a cost and there is a limited budget available for winter maintenance.
- Suitability: Salt works most effectively on well used roads as the tyre motion plays a key role in the process. It is a better management of a limited resource to target such roads.
- Availability: Rock salt production is not sufficient in this country to salt every road. This was only too apparent in the winter of 2009–10 when a prolonged period of cold weather put pressure on the national salt supplies.
- Environment: Salting at the current level of 40% of the road network is not deemed to have a significant impact on the local environment. However damage to roadside plant life is evident at the end of the season. Though seepage into water courses is deemed to be at safe levels if we were to salt 100% of the network on a precautionary policy, we would see saline seepage into water courses at more harmful levels.