Visibility on roads is typically poor during and after an ashfall, and total darkness may result during a heavy ashfall. During such conditions vehicle headlights and brake lights are often ineffective and barely visible to other drivers, and driving may become difficult or impossible. After an ashfall, fast-moving vehicles will stir up ash along roads and create billowing ash clouds a few tens of meters tall.
For example, following an eruption of Mt. St. Helens in Washington on 25 May 1980 that deposited 10-40 mm (0.4-1.6 in) of fine ash within about 120 km (75 mi) of the volcano, hundreds of vehicle accidents were likely caused by such stirred-up ash. This description appeared in the newspaper Seattle-Post Intelligencer on 28 May: "Brake lights could be faintly seen through the dust which rose 30 feet (9 m) above the freeway. Often, though, lights couldn't be seen, and all that could be heard was the tinkle of broken glass and the crunch of crumpled metal as cars from the rear headed into the ash clouds, and rammed vehicles already hidden in the clouds" (Blong, 1984, p. 291-292).Slippery surface
Ash deposits will absorb a considerable amount of water before being eroded and washed away. When ash on roads becomes wet, the mud-like mixture can cause vehicles to lose traction and drivers to lose control of steering. During such conditions, the braking ability of vehicles may be significantly reduced. Dry ash also causes roads to be slippery.Road markings covered
Ash deposits thicker than about 1 mm will obscure or completely cover markings on roads that identify lanes, road shoulders, direction of travel, and instructions to drivers (for example, stop or slow). When such road markings are not visible, drivers may become confused and disoriented.
Before ash is removed from roads, several actions can help reduce vehicle accidents and lower the amount of ash that is constantly being stirred up by moving vehicles. The following actions were taken in communities and for major highways in eastern Washington following an ashfall that deposited between 1 and 5 cm (0.4-2 in) of ash from the 18 May 1980 eruption of Mt. St. Helens:
Ashfall may also result in the clogging of roadside ditches and culverts. During rainfall, poor or nonexistent drainage along a highway or road can cause erosion of the shoulder and road surface. During ash cleanup operations, ash should be prevented from accumulating in such drainages or entering underground wastewater or storm-drain systems.