I can think of three major reasons why the correct salt-sand mixture on highways in the winter time would be important. First, using too much sandcan actually become a hindrance to drivers as it is thrown into the air by truck tires and if there is any depth to it, sand can actually adversely affect traction depending on the speed of the vehicles. Secondly, salt in quantity can eat away at cement and asphalt and shorten the lifespan of the road.
Lastly, saltmixes with snow and rain and runsoff into creeks,streams and the water supply. Too much salt can actually change the salinity of the groundwater, as well as the soil surrounding the roadway, and while this would likely be a small amount, habitats are quite sensitive to salt.