Eric Jaffe, reporting at the Atlantic Cities:
Since the pairs of counties are about the same size and distance apart, the “gravity law” treats them as equals. Sure enough, it predicts that only one commuter will make the trek in each place. The “radiation model,” on the other hand, is able to take into account the fact that population density is very low in Utah compared to the national average, which means work opportunities close to home may be harder to come by. As a result it predicts a considerably more accurate 66 commuters in Utah and two in Alabama.