Odd-even rationing

Odd–even rationing is a method of rationing in which access to some resource is restricted to half the population on any given day. In a common example, private vehicles may be allowed to drive, park, or purchase gasoline on alternating days, according to whether the last digit in their license plate is even or odd. Similarly, during a drought, houses can be restricted from using water outdoors according to the parity of the house number.

Typically a day is "odd" or "even" depending on the day of the month. An issue with this approach is that two "odd" days in a row occur whenever a month ends on an odd-numbered day. Sometimes odd or even may be based on day of the week, with Sundays excluded or included for everyone.

The efficacy of odd–even rationing is debated. In a case like gasoline, it does not actually reduce consumption, since people prevented from filling up one day will just fill up the day before, and vice versa; the total number of people in line on each day is unchanged. Some propose it has psychological effects like reducing panic buying, discouraging people from making small purchases on a daily basis, or emphasizing the shortage and further discouraging unnecessary trips.

Rationing access, rather than gasoline, based on number plate parity can reduce traffic congestion. In some areas, wealthier people purposely own two cars with opposite-parity number plates, to circumvent any restrictions.Vanity plates which do not contain any digits may be arbitrarily classed as odd or even.

Zero is an even number; indeed, half of the numbers in a given range end in 0, 2, 4, 6, 8 and the other half in 1, 3, 5, 7, 9, so it makes sense to include 0 with the other even digits for rationing. However, many people are unaware that zero is even, and this ignorance can cause confusion. The relevant legislation sometimes stipulates that zero is even. In fact, an odd–even restriction on driving in 1977 Paris did lead to confusion when the rules were unclear. On an odd-only day, the police avoided fining drivers whose plates ended in 0, because they did not know whether 0 was even.

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