The coverage of any core motorable network must be defined with respect to a required service quality, generally expressed in terms of an objective measure of the average time or effort of users put into getting around. Resources available dictate the stringency of the measure. The richer the society the more it can pay to avoid wasting time and effort in travel. Again, the index of service quality could require that certain key facilities be accessible to all within a given maximal travel time (for example, to cover medical emergencies). To satisfy it would involve not only improving roads, but also improving the transport services using them Such an access criterion is better in that it can take account of the entire journey, including travel time, vehicle availability, and walking distance but it can be complicated to apply.
A simpler one is preferable for the rural roads which interest us. Here, service quality can be defined in terms of a maximum walking distance to a year-round motorable road, in turn linked to the road network. Once there, assuming motor transport is available (and the money to pay the fare), services become accessible since they are normally situated on the motorable network. This walking distance will ideally be based on a nationally defined maximum distance. In the absence of a national benchmark, future users should be involved in setting it. For example, it could be agreed upon that all rural villages be within 2km (25 to 40 minutes walking time) of a motorable road and the motorable road network must be improved or extended to meet this. The distance can vary, depending essentially on resources available to extend or repair the network. Remember that the network of roads required to bring roads closer to people grows as the walking distance is shortened or populations are more sparsely distributed. Levels of income and population density are important in fixing the distance. Topography and climate must also be considered, for obvious reasons. A 2km stroll over flat and dry terrain is not the same as a scramble over steep mountain paths in torrential rain.
Since there can be a number of alternative routes, depending on the density of the network, a simple criterion for choosing among road improvements or extensions must be used, such as the lifetime cost per km for each link (or most direct path to the core motorable network) relative to population served. When reliable data exists it can be modified to to favour specific areas by incorporating, for example, indicators of the spatial distribution of poverty.
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