Impedance is the reduction in the capacity of lower-priority movements, caused by the congestion or queueing of higher-priority movements at a stop-controlled approach.

The impedance factor recognizes that gaps that are theoretically available to vehicles on the minor street are often not available in practice, because other minor street movements having a higher rank (such as the major street left-turning movement) may use these gaps instead.

The HCM computes impedance effects based on the probability of a queue-free state for each of the higher-ranked minor movements. Major traffic streams of rank 1 and rank 2 are assumed to be unimpeded by any of the minor traffic stream movements.