During the development of the Strategic Highway Research Program low-temperature binder specifications, in an effort to propose practical laboratory tests that require less time to perform, the time-temperature superposition principle was used to show that the stiffness after 2 h of loading at the performance-graded (PG) low temperature can be approximated by the stiffness after 60 s of loading at 10°C above the PG low temperature. This equivalence principle was developed on the basis of test results from the eight core asphalts and is widely accepted today. However, actual 2-h tests were not performed to experimentally validate this equivalence. Furthermore, the effect of physical hardening on time-temperature superposition was not considered. The validity of the time-temperature equivalence factor used in the low-temperature specification criterion and the ways in which the deviations could affect the current specification are evaluated.