The creation of the National Freight Network as part of the Moving Ahead for Progress in the 21st Century Act (MAP-21) requires states to designate critical rural freight corridors (CRFCs). The MAP-21 act lists the criteria for designating CRFCs but does not specify the method. This paper proposes mileage-based, segment-based, and weighted average approaches to determine whether a corridor meets one of the CRFC criteria: a rural principal arterial that has a minimum 25% of truck traffic. The three approaches are explained, assessed, and mapped for the reader to compare the resulting networks. The paper then uses policy analysis techniques to assess the advantages and disadvantages of each approach. The analysis compares the approaches on the basis of network connectivity of the resulting CRFCs, the mileage of non-Interstate rural principal arterials, and robustness. Results indicate that state departments of transportation should use the mileage-based approach for computing truck traffic percentages when determining whether a corridor meets the minimum threshold of 25% truck traffic. The mileage approach combines the highest network connectivity, designates the highest number of non-Interstate rural principal arterial miles, and is a robust technique that controls for variable segment lengths.