According to Washington State statute RCW 46.61.055: Traffic control signal legend. “Vehicle operators facing a steady circular red signal alone shall stop at a clearly marked stop line, but if none, before entering the crosswalk on the near side of the intersection or, if none, then before entering the intersection control area and shall remain standing until an indication to proceed is shown.” California’s law has almost identical wording.
Suppose you approach a red light and slow down your car, but the light turns from red to green before you reach the intersection. Clearly, you needn’t stop the vehicle because “an indication to proceed” was shown. No policeman would give you a ticket for continuing to move slowly, as long as your vehicle doesn’t pass the stop line or crosswalk while the signal is still red.
In a sense, you don’t need to stop at red lights. Just slow down sufficiently.
On the other hand, if you approach a stop sign, you must, by law, stop the vehicle.
This is a paradox. It is always possible to legally get away with not stopping at a red light: just slow down the vehicle and make sure you don’t pass through the red light. But at a stop sign — which is generally considered weaker — you must stop.