Sand is thrown on tracks or roads covered with snow. Explain why?

When the roads (dr tracks) are covered with snow then there is considerable reduction of frictional force between the tyres of the vehicle and the road (or between the track and the wheels of the train) which leads to the skidding of the vehicles. Thus, driving is not safe. When sand is thrown on the snow covered tracks or roads then the force of friction increases, so safe driving is possible