Alternate side parking is a traffic law that dictates on which side of a street cars can be parked on a given day. The law is intended to promote efficient flow of traffic, as well as to allow street sweepers and snowplows to reach the curb without parked cars impeding their progress. Some proponents also regard the law, which can be quite inconvenient for drivers, as a way to encourage the use of public transportation. From the beginning, the New York City law was "assailed" by opponents as actually impeding the efficient flow of traffic.  The system was created by Isidore Cohen, former superintendent for the department of sanitation in Queens, New York.
In many towns and cities, alternate side parking is reserved for certain times of year. In other places, such as New York City, it is a year-round rule suspended only for holidays and certain events. Signs are posted with the scheduled street sweeping times, and motorists must make sure their vehicles are on the correct side of the street or risk being ticketed or towed. The law can be confusing to visitors, who often choose to park in high-priced parking garages or use valet parking rather than risking fines. Even for locals, parking tickets are common; working late or oversleeping may cause a car to be left for too long on the wrong side of the street. Avoiding a ticket can consume a great deal of time, as drivers must search for other available spaces or sit double parked until the designated time, regardless of when street sweepers actually pass.
In Sweden, alternate side parking (datumparkering) is applied in zones covering an entire city, with signs indicating this at the city perimeter. Inside such date zones (datumzon), parking is prohibited on the morning of odd dates on the side of the street where houses have odd numbers. The drivers must think of what date it is the next morning if they leave the car in the evening. Inspired by Stockholm, more and more Swedish cities are abandoning such confusing zones and instead provide permanent parking on one or both sides of the street, with the exception for one day per week during December through May, when snowplowing and sweeping of sand can be required. The day when parking is prohibited is posted on a sign for each street.
In Denmark (datoparkering), the rules are exactly the opposite of those in Sweden, with parking prohibited on the morning of odd dates on the side of the street where houses have even numbers.
Belgium allows a half-monthly parking rule (Dutch: Halfmaandelijks beurtelings, French: Stationnement alterné semi-mensuel). When the entrance of the town is marked by road sign E11, alternate-side parking applies to the whole town agglomeration. Parking on the road from the 1st till the 15th of each month is only allowed on the side of the road with odd house numbers; from the 16th till the end of the month, parking on the road is only allowed on the side of the road with even house numbers. At the end of each period, cars should changes sides between 19:30 and 20:00. The rule doesn't apply on parking spots outside the roadway or on dedicated spots marked by other parking rules.
Similar parking regulations exist in France.
On a street running east to west, cars must be moved from the south side of the street for a few hours a day every Monday, Wednesday and Friday. On Tuesday, Thursday, and Saturday they must be moved from the north side. On Sunday and certain holidays, they can be left where they are. The specific times will vary from street to street. The days on which the rules are suspended may also vary from city to city and even from neighborhood to neighborhood.
- In the Seinfeld episode "The Alternate Side," George Costanza gets a job moving cars from one side of the street to the other.
- The novel Tepper Isn't Going Out is based on the quest for parking in New York City.
- In the Golden Girls episode "An Illegitimate Conception", Rose explains how the inhabitants of her hometown of Saint Olaf were confused by opposite side of the street parking as, 'It doesn't matter which side of the street you park on, there's ALWAYS an opposite SIDE...'
- In The Order of the Stick, goblins allegedly invented alternate-side parking, in addition to the oboe and guacamole, thus securing their place among the damned.
- Gail Robinson. "Parking". Gotham Gazette. Retrieved October 16, 2006.
- The New York Times (November 9, 1953). "AUTO CLUB ASSAILS CITY PARKING PLAN; Alternate-Side-of-Street Bans Called Inconsistent Burdens to Motorists -- Revisions Urged", November 9, 1953".
- "Parking". Madison Wisconsin Isthmus. Retrieved October 16, 2006.
- Frank Pomeroy. "Community Connection Alternate-Side Parking". Archived from the original on September 1, 2006. Retrieved October 16, 2006.
- New York City Department of Transportation. "Alternate Side Parking Regulations Suspension Calendar". Archived from the original on 28 September 2006. Retrieved October 16, 2006.
- David Wallis. "The Zen of Alternate-Side Parking". Retrieved October 16, 2006.
- Calvin Trillin (2003). Tepper Isn't Going Out. Random House. ISBN 0-375-75851-8.