by the World Bank, other multilateral donors, bilateral donors, and governments.

The data collection exercise was first conducted in five pilot countries, Bangladesh, India, Thailand, Viet Nam, and the Philippines; in 2002 a second set of countries was added including Ghana, Uganda, Poland, and Armenia; in 2004, Lao, Kyrgyz, Kazakhstan, Ethiopia, Nigeria, and Serbia and Montenegro were added. As table 1 shows, the current version of the database contains data from 3,322 work activities in ninety-nine low and middle income countries out of which 23% are located in low income countries.<sup>9</sup> Contracts date between 1984 and 2008, with 82% of contracts taking place between 1996 and 2006. Table S.1 in the appendix shows the distribution of projects by country over time. The ROCKS database is based on five concepts (World Bank 2006). First, to allow for comparability of similar activities, road works are classified into categories: road development works and road preservation works. Within these two categories, projects are further divided into work class, work type, and work activity. Second, comparisons are made possible through unit costs which are defined either as costs per square meter or costs per km. Third, the ROCKS database defines a minimum data requirement<sup>10</sup> which is required to make the data comparable. Fourth, to add flexibility, road agencies are able to enter highly recommended data and optional data.<sup>11</sup> Unit costs include civil works costs such as mobilization, pavement drainage, major structures, and line markings; they exclude agency costs such as design, land acquisition, resettlement, and supervision. Fifth, these costs are deflated to the year 2000 using the domestic consumer price index, and then converted into US$ using the exchange rate from 2000.<sup>12</sup> Bringing unit costs back to the same reference year and the 9We exclude duplicates of thirty-one contracts for which we have the same entry for country, date, cost per km, cost type, work activity, length, width, shoulder and lanes. We also drop two contracts for Reconstruction Bituminous (one in India and one in Bangladesh) for which the recorded costs were US$218 and US$2,289; the median cost of the 595 Reconstruction Bituminous work activities of our database is US$195,516 per km so these two entries are likely to be incorrect.

> 10For example, country, date, project or source name, currency, unit cost, work type, cost type.

> 11Highly recommended data include the predominant work activity, total cost, length and duration, carriage width, and terrain type. Optional data include number of bidders, value of individual bids, and unit costs of asphalt concrete and Portland cement concrete.

> 12As the database does not contain consumer price indices and exchange rates after 2004, we recalculate all conversions using the official exchange rate (LCU per US$, period average) and the consumer price index from the World Development Indicators 2012. Projects denominated in currencies other than US$ were first deflated or inflated to the reference year 2000, and then converted into US$. Projects denominated in US$ were deflated to the reference year 2000 using the consumer price index of the United States. The costs are very similar to the ones provided in the database, with 93% (91%) of costs per km (square meter) lying within 2% of the original data provided. Azerbaijan, Ghana and Venezuela devalued their currencies since 2000, so

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