This paper proposes a concept of critical post-encroachment time (PET) for all-red clearance interval design at signalized intersections that aims at achieving the optimum performance in both safety and mobility. The critical PET is defined as the minimum accepted PET at the conflict point by the first entering drivers. Variability of the accepted PETs in the case of late exit was analyzed at a study intersection in order to estimate the critical PET. Results showed that they follow a two-parameter Weibull distribution and tend to increase as the entering distance rises. Its 15th percentile value, approximately 2.0s, was then used as the critical PET to discuss the implication in the design of all-red time through numerical calculations. Conclusions supported that the calculated all-red time based on the critical PET could achieve significant operational and safety benefits, as compared with the all-red times based on the existing methods.