Mountain highways have low traffic volume. However, their maintenances that require huge resources and cost are typically necessary due to the needs of social development, land conservation and natural disaster prevention and response. Recognizing the vulnerable sections of a mountainous highway and associate factors that influence their usual operation is conducive in the development of appropriate maintenances strategies and related methods. Taking the Dasha River section of the Central Cross-island Highway (Tai-8 line) as an example, this study collects historical remote sensing images for more than 60 years, processing images and interpreting highway routes, landslides and gullies at different times to identify the sections that have been rerouted or have highway structural type modifications, in order to investigate the environmental and engineering geological factors that affect the highway maintenance. Results of images interpretation reveal that, in the 9.4 km long mountainous highway studied and since the completion of the highway in 1960, four local sections have been rerouted toward hillside, even went through by tunnels, and one section has structural type modification to mitigate the influence of landslides and/or lateral erosion of the river. The locations of landslides and gullies development that are related to roads rerouted and structural type modification are all neighboring to some sharp turns of the Dasha River and adjacent to boundaries of geological strata. The differential erosion rate caused by different lithological properties and significant changes in engineering characteristics of rock masses in the vicinity of strata boundaries account for the relatively intense lateral erosion along the river and rapid development of gullies under the impact of runoff, which are also important geological factors related with the reroutes and structural modification of highway, in association with the long-term influences of rainfalls and earthquakes.
Taiwan is a mountainous area with high population density, so that traffic construction in mountain area become important in social development, territorial conservation and disaster prevention. Although the traffic volume in mountain area is low, the mountain highways are usually close to the slopes so that disasters occur frequently. The maintenance costs in mountain highways are often higher than the city roads due to reasons such as the instability of the slopes along the highway, the pavement damage caused by climate change, and the maintenance of basic traffic safety signs, communications and manpower (Hearn, 2011). Mountain highways are not as convenient and accessible as urban roads, if the highway is blocked, it is easy to make mountain settlements isolate from the outside world, form a “land island” state, and the communication will interrupted and rescue materials will not easy to reach, so the existence of mountain highways for mountain residents is very important. Usually after the recurrence occurs frequently, one of the reasons is that the background of the slope along the mountain highway and the cause of landslide are being collected and investigated without systematically and efficiently.