self modification
Growth coefficients do not necessarily remain static throughout an application. In response to rapid or depressed growth rates, the coefficients may be increased or decreased to further encourage system wide growth rate trends.

A second level of growth rules, termed self-modification rules, is prompted by an unusually high or low growth rate. The growth rate is the sum of the four different types of growth defined by the model for each model growth cycle, or "year." The limits CRITICAL_HIGH and CRITICAL_LOW (defined in the scenario_file) kick off an increase or decrease in three of the growth control parameters: dispersion, breed, and spread. If the growth rate exceeds the CRITICAL_HIGH, the coefficients are increased by a multiplier greater than one: BOOM. This increase imitates the tendency of an expanding system to grow ever more rapidly. If the growth rate falls below the CRITICAL_LOW, the coefficients are decreased by a multiplier less than one, BUST, causing growth to taper off as it does in a depressed or saturated system.

Under self-modification the coefficient values increase most rapidly at the beginning of a growth simulation when there are still many cells available to become urbanized, and while the growth rate exceeds the CRITICAL_HIGH. The coefficients are decreased as expansion levels off and the growth rate falls below the CRITICAL_LOW. Without self-modification the model produces linear or exponential growth; self-modification was essential for modeling the typical S-curve growth rate of urban expansion.