30km/h (20mph) zones are a collection of 30km/h streets intended for local access traffic. Size of a zone is typically limited to within 1/2 a square mile to reduce traffic impact to the surrounding through streets.
Zones are useful in reducing the amount of signage required. As the default urban speed is 50km/h all streets at other speeds must be signed. Zones only require signage at entrance and exit points of a zone, or gateways. Instead of putting up 30km/h signs on every street in a zone, the Dutch can get away with posting them only at gateways.
As 30km/h streets are typically vastly different in feel and design than their through street counterparts, these gateways at intersections with through streets must catch driver attention. Consisting of signage and pavement treatments that force motorists to adjust to the change in speed, whether they are exiting or entering. For zones to be successful gateways must provide the cue to drivers that behavioral expectations have changed.
Treatment options are things like raised intersections, crossings, road narrowings, while intersections themselves can be signalized, unsignalized, or roundabouts. These physical features serve to imply the desired changes in road speed.