it's increasingly common for a city to have several centres performing different functions.

D'Acci's new model is designed to cope with this increased complexity. His idea is to calculate the benefit of a given location to a resident, taking into account the effect of all the city's various amenities.Having done that, he calculates locations of equal benefit, connecting them with so-called "isobenefit lines".

That gives a simple and immediate visual representation of the structure of the city in terms of the benefits it offers.

D'Acci's approach is clearly a step forward. He points out that there is a strong correlation between isobenefit lines and property prices. That's a good indication that the model captures some important elements of human behaviour.