30 km/h zones (30 kilometres per hour zones) and the similar 20 mph zones (20 miles per hour zones) are forms of speed management used across areas of urban roads in some jurisdictions as an alternative to normal speed limits. The nominal maximum speed limits in these zones are 30 kilometres per hour (19 mph) and 20 miles per hour (32 km/h) respectively. Although these zones do have the nominal speed limit posted, speeds are generally ensured by the use of traffic calming (physical or psychological) measures, though limits with signs and lines only are increasingly used in the UK.
These zones are generally introduced in areas, particularly residential areas, in an attempt to keep road traffic speeds down to a safe level. The philosophy behind such zones is that the streets in the zone are public space, and they seek to help strike a balance between the realities of an urban area bustling with pedestrian activity and the circulatory function of the roadways. Streets in these zones are considered to be a space for people who live, work, play and study in the area, not for people who cross the zone to get somewhere else. The theory is to reduce rat running while improving the safety and quality of life in the area.
Research has shown that reducing driver speeds in built-up areas reduce injuries for all road users, including motorists, bicyclists, and pedestrians. The link between vehicle speed and pedestrian crash severity has been established by research studies, with crash severity increasing as a function of motor vehicle speeds. If a vehicle hits a pedestrian while traveling 15 miles per hour (24 km/h) most pedestrians will survive a crash, often sustaining only minor injuries. Minor increases in impact speed have been shown to have a profound effect on crash severity. At 25 miles per hour (40 km/h), almost all crashes result in severe injuries and roughly half are fatal; and at 40 miles per hour (64 km/h), fully 90% of crashes are fatal. The dramatic differences in fatality rates are a key part of the theory behind 20 mph and 30 km/h zones. Other studies have revealed that lower speeds reduce community severance caused by high speed roads in neighbourhoods, i.e. there is more neighborhood interaction and community cohesion when speeds are reduced to 30 km/h.
The objectives of the implementation of zones are to help:
Compared to normal signed but unenforced speed limits, these type of speed zone generally deliver the required traffic speeds because of their traffic calming aspects.
Physical calming measures can be uncomfortable for motor vehicle occupants and can impede emergency service vehicles. Compared to normal signed limits, zones can be expensive because of the engineering of traffic calming measures they may require. Noise, vibration and pollution can be caused by traffic slowing down and speeding up between the calming measures, though with the installation of average speed cameras in some areas, this is far less of an issue.
In European countries 30 km/h zones have been used widely. On September 1, 1992, the city of Graz, Austria, became the first European city to implement a city-wide 30 km/h limit on all roads except its largest. Significant 30 km/h zones are ubiquitous across the Netherlands. In Switzerland 30 km/h zones have been allowed by law since 1989 and they were first established in Zürich in 1991.
A network of 67 European NGOs organised a European Citizen's Initiative (ECI) "30kmh - making streets liveable" collected signatures of support for a 30 km/h speed limit as the normal limit for the European Union. 50 km/h speed limits shall become exceptions. Local authorities shall be able to decide on these exceptions and set other speed limits on their street network. 
In Munich 80% of the 2,300 kilometers of urban road network are in zone 30, remaining roads are limited at 50 km/h.
In the United Kingdom, 20 mph speed limits are gaining popularity. There is significant action across the UK, both by organisations and local councils, to implement more 20 mph limits and zones in local communities.20's Plenty for Us list populations in UK local authorities committed to wide-area 20mph limits at over 15 million people at March 2016 with over half of the largest UK 40 urban authorities having agreed a Total 20mph policy.
Some towns define th 20 mph zone as the general speed limit across the city, with a higher speed for main roads<re name="racfoundation" />.
In the US, 20 mph (32 km/h) speed limits exist along linear routes, but are slow to catch on for area-wide implementation. New York City is leading the way with neighborhood-scale 20 mph zones and is currently re-engineering 60 mi (100 km) of streets per year for conversion to 20 mph zones.
Ten US states already allow 15 mph (24 km/h) or 20 mph (32 km/h) speed limits for linear routes, as follows:
Mexican cities which have established 30 km/h (19 mph) zones (Zonas 30):