Implications of modeling commercial vehicle empty trips are discussed, a theoretical derivation for parameter estimation is provided, and insight is given into the order of magnitude of estimation errors because of the improper modeling of commercial vehicle empty trips. A set of relatively simple cases was designed to illustrate the most important implications. Also addressed are estimation errors from using naive approaches to compensate for the lack of explicit modeling of empty trips and the errors associated with more advanced empty trip models. In the simplest simulation, directional errors for a basic complementary model were from three to six times fewer than those for the naive models. In the more complex case, a more sophisticated complementary model performed slightly better than the basic model and both complementary models were considerably better than the naive approaches. The directional errors for the naive models were four to seven times greater than those for the complementary models. Moreover, an analysis of the statistical distributions of the errors indicated that the complementary models had higher probabilities of producing accurate results, whereas the naive approaches had higher probabilities of producing very large errors. These analyses indicate that the naive approaches translate into significant errors in directional-traffic estimates. For that reason, their use should be discontinued in favor of the more advanced models presented.