The period during which the described developments in econometric modelling were taking place was one of increasing government involvement in transport planning. Having noted the failure of the TRRL model to take into account changes in income and vehicle costs in the estimation of saturation level, or any allowance for regional variation in saturation level, the report concluded that the method is unsatisfactory. Three more problem areas are saturation levels, treatment of multiple car ownership, and the nature of future income distribution. Although the procedure for estimation of saturation levels remained largely unaltered, attempts were made to introduce several additional controls. The estimation of saturation level involved a more complicated procedure than that employed in earlier TRRL work. Since the substitution of the power growth curve reduced the influence of saturation level on immediate predictions there has been less discussion of the earlier problem of estimation of saturation levels.