Barring cars from the road on set days by the last digit of their plate — the cheapest way to ration road space, and the least reliably effective.
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Plate-number rationing keeps part of the car fleet off the road on fixed days according to the last digit of its licence plate — odd plates one day and even the next, or one pair of digits banned per weekday. It is the cheapest demand measure a city can pass: no gantries, no accounts, no billing, just a calendar.
Cities reach for it when pollution or congestion becomes an emergency and congestion pricing is politically out of reach. Mexico City's Hoy No Circula has rationed by digit since 1989, Metro Manila since December 1995. Santiago refines the design: how many digits you lose depends on your car's vintage and emissions seal, and a bad air-quality forecast adds more.
The evidence is unusually mixed. Peer-reviewed evaluations of Mexico City going back to 2008 have not found cleaner air; households bought a second, usually older car within eleven months. A peer-reviewed study of Jakarta's ganjil-genap, enforced by plate cameras, found no significant effect on pollutants. Santiago is the exception: peak car trips fell 6-9%, with drivers shifting onto transit that had spare capacity. Manila, the longest-running of them, has no impact figure at all. What separates them is whether public transport can absorb the displaced trips, and whether anyone is really checking. Delhi's odd-even scheme is deliberately absent from this atlas: police and magistrates checked plates by eye, which makes it a traffic rule rather than a smart-city entry.
Added 2026-08-01 · Part of the growing Smart City Glossary