Estimate rush hour commute time.
Rush hour is measured as the gap between two journey times over the same route, one at the congested average speed and one at the free-flow speed. Expressing congestion as a delay rather than a duration is what makes it actionable: a fourteen-mile commute at 19 mph instead of 38 loses about twenty-two minutes each way, and doubling for the return leg and multiplying by the working week turns that into nearly four hours. The delay is floored at zero, since a rush-hour speed above the off-peak figure means the inputs are reversed.
Journey times
Minutes = miles / average speed x 60, computed at both the rush-hour and off-peak speeds
Weekly loss
Hours lost = delay per leg x 2 legs x commuting days / 60
Often more than any route change. Average speeds on urban arterials commonly recover 30 to 50 percent in the half hour either side of the peak, because congestion is sharply peaked rather than spread across the morning.
Divide your own door-to-door distance by the times you actually experience, once at peak and once at a quiet hour. Navigation apps report the same figures if you compare a live estimate against an off-peak one.