adjacent gravel shoulder. Sometimes, the paved and gravel shoulders are combined as the "recovery zone" beyond the rumble strip. However, if the gravel is loose, soft, non-level, eroded, or there is an "edge-drop" from the pavement to the gravel, then the gravel shoulder portion will be ineffective for recovery, especially at highway speeds. When a vehicle's tires sink into a soft shoulder, thus compromising vehicle handling, it is known as "vehicle tripping". Virtual rumble strips require an adequate recovery zone as well. Deterioration Climate Climate is another factor that affects the success of a rumble strips installation. If they are installed in a northern climate, they may be filled or partially filled with a deicing salt and traction sand mixture. They may also be filled with ice. This is a particular concern in regions with freeze-thaw cycles requiring frequent deicing. Furthermore, strips filled with water, snow, slush, and ice may cause or aggravate occasional accidents. Generally, air turbulence and vibration from passing large trucks keep rumble strips clear of debris and ice, but this process may take several days. Moist traction sand tends to cake together or freezes, and is not easily dislodged by truck traffic. This is problematic on low-volume highways with frequent deicing, and can significantly reduce the effectiveness of rumble strips in winter months. When rumble strips are installed on a very narrow paved shoulder, sometimes sand and gravel can fill the rumble strip which is usually a problem in the winter and early spring. If the snow-cover is substantial, then the shoulder (including the rumble strip) is usually partially snow-covered as the snowplow's wing-blade doesn't clear the entire shoulder. Vehicles going off the road usually collide with the shoulder snow bank or go into a snow-filled ditch which reduces the possibility of serious damage and injury. In these situations, the rumble strip effectiveness can be negated but the crash implications are mitigated by the snow bank. Pavement deterioration Generally, deterioration of the shoulder asphalt pavement due to rumble strip installation is not a problem. However, if the sub-grade under the CSRS is poorly compacted or has poor drainage characteristics; or the gravel shoulder has eroded, crack(s) may form in the CSRS. Sand tends to fall into these cracks resulting in "jacking" of the cracks. Water percolates vertically down through the soil, but it also creeps horizontally under the paved shoulder. This may be a particular problem