Finally, as a thought experiment, we also assessed the cost of providing regular food and medicine supply by drones to remote areas in Sierra Leone. We found the cost is dramatically lower than the cost of paving the required roads in the same areas. While we do not claim that drones can replace roads, we argue they can be a useful short-term solution to improve the welfare of remote populations.

For future developments, measures of rural accessibility must be used with caution when it comes to planning investment in rural roads at the country level. Upgrading rural roads to all weather roads does not guarantee welfare returns if connectivity with the rest of the network and with economic opportunities is not ensured. Second, the investments can have significant negative impacts on biodiversity. This means that the RAI alone cannot be used in the absence of a broader network model to prioritize investments based on a broader list of criteria. Avenues for improvement include using more sophisticated measures of accessibility based on travel time to nodes of interest for social or economic goals, using criticality analysis that will prioritize roads with the highest impact on travel cost or economic activity (Espinet Alegre et al., 2018), and adding penalties when roads have negative impacts on forests and biodiversity.

# **References**

Asher, S.E., Garg, T., and Novosad, P.M. (2018). The ecological impact of transportation infrastructure (The World Bank).

Damania, R., Russ, J., Wheeler, D., and Barra, A.F. (2018). The Road to Growth: Measuring the Tradeoffs between Economic Growth and Ecological Destruction. World Development _101_ , 351–376.

Dellink, R., Chateau, J., Lanzi, E., and Magné, B. (2017). Long-term economic growth projections in the Shared Socioeconomic Pathways. Global Environmental Change _42_ , 200–214.

Dercon, S., Gilligan, D., Hoddinott, J., and Woldehanna, T. (2008). The Impact of Agricultural Extension and Roads on Poverty and Consumption Growth in Fifteen Ethiopian Villages. American Journal of Agricultural Economics 91 (2009) 4 _91_ .

Espinet Alegre, X., Rozenberg, J., Rao, K.S., and Ogita, S. (2018). Piloting the use of network analysis and decision-making under uncertainty in transport operations : preparation and appraisal of a rural roads project in Mozambique under changing flood risk and other deep uncertainties (The World Bank).

Fay, M., Han, S., Lee, H.I., Mastruzzi, M., and Cho, M. (2019). Hitting the Trillion Mark -- A Look at How Much Countries Are Spending on Infrastructure (The World Bank).

Gao, J. (2017). Downscaling Global Spatial Population Projections from 1/8-degree to 1-km Grid Cells.

Iimi, A., and Rao, K. (2018). Spatial Analysis of Liberia’s Transport Connectivity and Potential Growth (Washington DC, USA: The World Bank).

Iimi, A., Ahmed, A.K.F., Anderson, E.C., Diehl, A.S., Maiyo, L., Peralta Quiros, T., and Rao, K.S. (2016). New rural access index : main determinants and correlation to poverty (The World Bank).

Khandker, S.R., and Koolwal, G.B. (2011). Estimating the Long-Term Impacts of Rural Roads: A Dynamic Panel Approach (Rochester, NY: Social Science Research Network).

Laurance, W.F., Clements, G.R., Sloan, S., O’Connell, C.S., Mueller, N.D., Goosem, M., Venter, O., Edwards, D.P., Phalan, B., Balmford, A., et al. (2014). A global strategy for road building. Nature _513_ , 229–232.

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