Understanding the Concept and the Potential of Roads for Water | 17
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almost for free, especially when site-selection criteria and safety measures are considered in upfront planning (van Steenbergen 2017). Roads in flood-prone areas may be built with lower embankments and equipped with controlled overflow “floodway” structures instead of high embankments (see chapter 7). This reduces costs enormously because the expenditure on the embankments is considerably less, and roads do not wash out in unpredictable locations. Culvert-less, “nonvented” drifts may be used as road crossings. Such drifts cost the same as road drifts with culverts but prevent the scouring of rivers and encourage the buildup of sand-water storage immediately upstream of the drift, effectively combining the function of a road crossing with that of a sand dam (Neal 2012). Based on a calculation of costs and benefits, Excellent Development (2018) estimates that maintenance costs on culvert-less drifts are only 13 percent of the maintenance costs of vented drifts. Water levels in the coastal lowlands, such as the polders in Bangladesh (see chapter 5), may be managed. The only option for doing so is to make use of the road network in these areas, expand it wisely, and equip it with appropriate cross-drainage structures. Road embankments may be used for water storage, as in countries as diverse as Burkina Faso, Portugal, Turkmenistan, Uganda, and the Republic of Yemen. Because road embankments are a sunk cost, reservoirs can be developed with comparatively minimal additional expense. Low-cost measures, such as drainage dips, water bars, and infiltration bunds, may be used on unpaved roads to guide water to productive uses and prevent the kind of damage to those roads that is usually not repaired (see chapter 10).
ROAD SAFETY CONSIDERATIONS Numerous issues must be carefully considered in the design, construction, and ongoing maintenance and rehabilitation of roads, many of which cannot be fully addressed in these guidelines. Road safety requirements are among the many issues that are not fully addressed but deserve mention. Attention to the road context is a first-order principle in road safety. Certain environments call for lower vehicle operating speeds, whereas others can be adapted to mitigate some of the risks posed by higher operating speeds, enabling faster movement. In general terms, measures to reduce speed are necessary in villages and towns and in other contexts in which nonmotorized road users are frequently in close proximity to high-speed traffic. Higher travel speeds can be accommodated more easily in rural areas where these risk factors are not present. In these environments, roadside trees and structures pose significant and often underappreciated risks to vehicle occupants. Road-safety decision-making also requires proactive identification of risks. For instance, operation and maintenance of certain road-water management facilities may also introduce people to high-risk areas of the roadside. In addition, motor vehicle operators often underestimate road-safety risks and overestimate their capacity to manage those risks. This overconfidence often results in higher and less safe operating speeds. For instance, removal of roadside objects may give vehicle operators a greater feeling of safety, resulting in higher speeds that are not suitable for the road surface conditions. Road user
