noun
30 km/h zone
A designated area where the speed limit is set to 30 kilometers per hour, often in residential or school zones.
この住宅街はゾーン３０に指定されています。
This residential area is designated as a 30 km/h zone.
ゾーン３０では、歩行者優先のルールが強化されている。
In 30 km/h zones, pedestrian-priority rules are strengthened.
速度制限 is a general term for speed limit, while ゾーン３０ specifically refers to a designated area with a 30 km/h limit.
Borrowed from English 'zone 30', a traffic engineering term adopted in Japan for residential speed management.