and similar administrative units.<sup>19</sup> Travel times as a proxy for trade costs between each pair of provinces are derived from two detailed GIS road network data sets—one without and one with the NEN—using a standard shortest-path algorithm. Travel times in the “before” network (i.e. without the NEN in place) will always be no larger (usually substantially lower) than in the “after” (i.e. with the NEN in place) network, because highways allow greater speeds and more direct connections. One simplification is that we assume that travel times within a prefecture are zero ( _t_ ii = 0).<sup>20</sup> The novel data that we use in this paper concern information on migration and some of its alleged determinants. Chinese migration statistics are notoriously difficult to interpret (Chan, 2013). This is especially the case for bilateral migration flows between locations. A virtue of our structural modeling approach is that we can infer the importance of different migration determinants by looking at migration stock only. This information is available<sup>21</sup> . Our data on the stock of migrants comes from a comprehensive data set for 2000 and 2010 derived from official Chinese census publications and other statistical data sets such as the provincial statistical yearbooks (Chreod Ltd., 2013). The data set includes information on “total population of migrants from the same county”, “total population of migrants from other counties in the same province” and “total population of migrants from other provinces”, each for 2000 and 2010. It also provides data on “total population with household registration”, i.e. with a local Hukou, for both years.

We complement it with data on geographic characteristics/amenities possibly influencing people’s migration decisions. Information on terrain ruggedness comes from Nunn and Puga (2012). Data on cooling and heating degree days are from NASA (2009). Annual cooling degree days measure the total number of degrees by which daily temperatures exceeded 18 ̊C. It is often used as a measure of the need for air conditioning. Similarly, heating degree days indicate the accumulation of degrees when the daily mean temperature is below 18°. Information on amenities (the share of households with access to tap water, toilets and natural gas supply) come from the same data set as our migration stocks.<sup>22</sup> Finally, we have collected the dominant language for each prefectural region from a GIS data set of linguistic regions called the World Language Mapping System (Ethnologue, 2004).

> 19 The 331 prefectures covers 280 of China’s 283 prefectural cities, 11 of its 17 prefectures, all 30 of its ethnic minority autonomous prefectures, and all three of its leagues.  Also included are the municipalities of Beijing, Shanghai and Tianjin.  Three areas that together comprise Chongqing are also included in the sample – the “One Hour Circle”, the Northeast wing and Southeast wing (see Roberts _et al_ , p 590, for further details).

> 20 This means that we abstract from any difference in the quality of infrastructure within prefectures. See BaumSnow and Turner (2013) and Baum-Snow _et al._ (2013) for detailed studies on inter-prefectural infrastructure. 21 Its quality can still be debated. The official statistics that we use are likely to underreport actual migration stocks in some cities given the many unofficial migrants.

> 22 We use the 2010 figures in most of our estimations. Results are very similar when using estimates for 2007 instead (the same year for which we have our NEN-travel time-information), obtained by linearly interpolating the figures from 2000 and 2010.

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