Concrete agglomeration benefits: do roads improve urban connections or just attract more people?
研究发现，美国都市区的高速公路密度在控制人口规模后并不直接影响生产率，但利用人口外生变化时，高速公路密度能提升集聚效益，使生产率对人口的弹性从2%增至4%。
Cities with more roads are more productive. However, it can be unclear whether roads increase productivity directly, through improved intra-urban connections, or indirectly, by attracting more people. Our theory suggests that population responses may obscure the direct connectivity effects of roads. Indeed, conditional on population size, highway density does not affect productivity in a sample of US metropolitan areas. However, when exploiting exogenous variation in urban populations, we find that highway density improves agglomeration benefits: moving from the 50th to the 75th percentile of highway density increases the productivity-to-population elasticity from 2% to 4%. Moreover, travel-based measures outperform population size as a measure of agglomeration externalities.