Road marking coverage by volcanic ash: an experimental approach

Coverage of road markings by volcanic ash is one of the most commonly reported impacts to surface transportation networks during volcanic ashfall. Even minimal accumulation can obscure markings, leading to driver disorientation, diminished flow capacity and an increase in accidents. Such impacts may recur due to repeated direct ashfall (i.e. during prolonged eruptions) and/or due to the re-suspension of ash by wind, water, traffic or other human activities, and subsequent secondary deposition on the road surface. Cleaning is thus required to restore and maintain road network functionality. Previous studies have not constrained ash accumulation measurements to inform road cleaning initiation or plans for safe road operations in environments containing ash. This study uses a laboratory approach with digital image analysis to quantify the percentage of white road marking coverage by three types of volcanic ash with coarse, medium and fine particle size distributions. We find that very small accumulations of ash are responsible for road marking coverage and suggest that around 8 % visible white paint or less would result in the road markings being hidden. Road markings are more easily covered by fine-grained ash, with ash area densities of ~30 g m⁻² (estimated at <0.1 mm surface thickness) potentially causing markings to be obscured. For the coarse ash in our study, road marking coverage occurs at area densities of ~1000–2200 g m⁻² (~1.0–2.5 mm depth) with ash colour and line paint characteristics causing some of the variation. We suggest that risk management measures such as vehicle speed reduction and the initiation of road cleaning activities should be taken at or before the lower thresholds as our experiments are conducted at a relatively short horizontal distance and the ability to observe road markings when driving will be comparatively reduced.