Multiple nuclei model
The model describes the layout of a city. It says that even though a city may have begun with a Central business district, or CBD, other smaller CBDs develop on the outskirts of the city near the more valuable housing areas to allow shorter commutes from the outskirts of the city. This creates nodes or nuclei in other parts of the city besides the CBD thus the name multiple nuclei model. Their aim was to produce a more realistic, if more complicated, model. Their main goals in this were to:
- Move away from the concentric zone model
- To better reflect the complex nature of urban areas, especially those of larger size
Effects of multiple nuclei on Industry
As multiple nuclei develop, transportation hubs such as airports are constructed which allow industries to be established with reduced transportation costs. These transportation hubs have negative externalities such as noise pollution and lower land values, making land around the hub cheaper. Hotels are also constructed near airports because people who travel tend to want to stay near the source of travel. Housing develops in wedges and gets more expensive the farther it is from the CBD. 
Reasons for the model
Harris and Ullman argued that cities do not grow a single nucleus but several separate nuclei. Each nucleus acts like a growth point.
The theory was formed based on the idea that people have greater movement due to increased car ownership. This increase of movement allows for the specialization of regional centers (e.g. heavy industry, business parks, retail areas). The model is suitable for the large, expanding cities. The number of nuclei around which the city expands depends upon situational as well as historical factors. Multiple nuclei develop because:
- Certain industrial activities require transportation facilities e.g. ports, railway stations, etc. to lower transportation costs.
- Various combinations of activities tend to be kept apart e.g. residential areas and airports, factories and parks, etc.
- Other activities are found together to their mutual advantage e.g. universities, bookstores and coffee shops, etc.
- Some facilities need to be set in specific areas in a city - for example the CBD requires convenient traffic systems, and many factories need an abundant source of water
- Certain events benefit from the adjacent distance like the positions of factories and residence.
- In some cases, some constructions are located in less-than-ideal locations, often due to outside factors such as rent.
- Land is Flat
- Even Distribution of Resources
- Even Distribution of people in Residential areas
- Even Transportation Costs