A standard no-parking sign in Paris: The “not” symbol is understood to mean “no parking,” and “jour et nuit” translates to “day and night.” No Parking, Day or Night.

This system relies on interpretation and conscious action. People must follow the rules, must heed the signs, if the signs are to be effective. This is an open-choice system: We may follow the sign’s instructions, or we may not. The physical environment does not constrain us in any way. Contrast this with a constrained-choice no-parking system, which simply prevents users from parking in certain places or certain ways. Open-choice systems are significantly more flexible than constrained-choice systems, leading to their deployment in places like no-parking zones.