Compared to 50 km/h roads, a 30 km/h zone has a positive effect on road safety (see also the question What are the road safety effects of 30 km/h zones?) and also on the quality of life: the noise level of the traffic is lower, the street can be crossed more easily, exhaust emissions are lower.
A 30 km/h zone has, by definition, a negative effect on the traffic flow, travel time and access for motorized traffic. The travel comfort also is negatively affected by road humps, plateaus, road narrowings and chicanes. The question is how heavily these disadvantages should weigh. After all, a 30 km/h zone is a residential area and by definition not intended to facilitate traffic flow, accessibility and short travel times. But if the area is too large, there is too much traffic on the entry and exit streets of the zone. Van Minnen  recommends a maximum size of 200 ha.
Whether an area is designed as a 30 km/h zone and the size of that area is the decision of the road authority. Different aspects must be weighed: wishes regarding traffic flow and accessibility, current traffic volumes, the presence of alternative routes, wishes of local residents, requirements for response times of emergency services, public transport routes (buses), and supplying shops and businesses. Of course the costs also play a role. In 2000, SWOV estimated the cost of measures to give 30 km/h zones a sober layout at 22,000 euro per kilometrer ; at the 2016 price level this is about 29,350 euro per kilometre [i].