demand (fronthaul) cannot find return trips (backhaul)<sup>2</sup> . At the national level, the median country has a physical trade imbalance of about 50 percent, indicating that it exports twice as many kilograms of goods as it imports, or vice versa. The imbalance can grow larger when one focuses on only neighbor-level or lane-level trade. Second, search and matching frictions can generate empty miles as carriers are unable to find a job and would prefer to travel empty to another market in search of jobs. Empirical research points to cases where improved matching technology lowered empty trip shares by 4 to 5 percentage points, while also cautioning that the effect of matching on empty trips may not be monotonic. Third, regulatory or capital restrictions may create barriers that prevent certain carriers from serving particular products or lanes. In a World Bank survey, the majority of countries restrict foreign carriers from at least some types of jobs, with cabotage trips being the most commonly restricted. This translates into significant empty miles; in the European context, carriers from countries with cabotage rights have 20 percentage point lower rates of empty miles than carriers from countries without cabotage rights.

Diving deeper into the mechanisms, I build a simple model that captures the effect of the three causes on empty trips. Shippers search for carriers to carry fronthaul and backhaul jobs. Carriers search for a fronthaul job; if they find a job, they travel to the destination and search for a backhaul job. There are fewer backhaul shippers than fronthaul shippers, capturing the role of demand imbalance. Search frictions cause fewer shipper-carrier matches to be produced than the number of searching shippers and carriers. Some carriers are ineligible to search for or accept a backhaul job, reflecting the presence of regulatory barriers to cabotage. In equilibrium, all three forces contribute to depress backhaul probabilities and raise fronthaul prices. However, search frictions behave differently from demand imbalances and regulatory restrictions, as they also affect the number of fronthaul jobs. As a result, reducing search frictions, for example through better shipper-carrier matching platforms, may have ambiguous effects on the overall number of empty miles in the economy. I conclude by examining several cases, both hypothetical and historical, from the lens of the conceptual framework as an example of thinking about when each friction may be most important.

This paper contributes to our understanding of transportation economics by synthesizing the empirical evidence on empty trips and empty miles. Compared to the existing literature, which typically focuses on single-country or region-level studies, my meta-analysis provides estimates of the level and dispersion of empty miles across different settings. This paper

> 2In this paper, I will focus on empty trips and truck underutilization in the short-run, where I hold the geographic distribution of freight demand (i.e., the location of factories, households, natural resources, ports, etc.) and the population of trucking firms fixed. In the long run, firms may enter and exit or households may migrate, in ways that alleviate or exacerbate truck underutilization. I treat these as outside the scope of this paper.

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